Biology

<h2 id="history">History</h2>
Main article: <a href="/facts/History_of_biology/hpWUBuml">History of biology</a>

The earliest of roots of science, which included medicine, can be traced to <a href="/facts/Ancient_Egypt/BDsEGK4g">ancient Egypt</a> and <a href="/facts/Mesopotamia/f0mmbhTK">Mesopotamia</a> in around 3000 to 1200 <a href="/facts/Common_Era/e5DpqUoe">BCE</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a><a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> Their contributions shaped ancient Greek <a href="/facts/Natural_philosophy/CUAsDWdk">natural philosophy</a>.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a><a class="footnote-ref" id="fnref:7" href="#fn:7">7</a><a class="footnote-ref" id="fnref:8" href="#fn:8">8</a><a class="footnote-ref" id="fnref:9" href="#fn:9">9</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> <a href="/facts/Ancient_Greece/PTVBrWXq">Ancient Greek</a> philosophers such as <a href="/facts/Aristotle/U2bDlBFW">Aristotle</a> (384–322 BCE) contributed extensively to the development of biological knowledge.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> He explored biological causation and the diversity of life. His successor, <a href="/facts/Theophrastus/YEeQeInW">Theophrastus</a>, began the scientific study of plants.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> Scholars of the <a href="/facts/Science_in_the_medieval_Islamic_world/ajn0zRSc">medieval Islamic world</a> who wrote on biology included <a href="/facts/Al-Jahiz/nWx3G4vY">al-Jahiz</a> (781–869), <a href="/facts/Al-D%25C4%25ABnawar%25C4%25AB/QI88GIm3">Al-Dīnawarī</a> (828–896), who wrote on botany,<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> and <a href="/facts/Muhammad_ibn_Zakar%25C4%25ABya_R%25C4%2581zi/PC32Rzes">Rhazes</a> (865–925) who wrote on <a href="/facts/Anatomy/p8TCWJuV">anatomy</a> and <a href="/facts/Physiology/fh25y2pQ">physiology</a>. Medicine was especially well studied by <a href="/facts/Islamic_scholars/7vMcAgBV">Islamic scholars</a> working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought.
Biology began to quickly develop with <a href="/facts/Anton_van_Leeuwenhoek/HQ9dH9oo">Anton van Leeuwenhoek</a>'s dramatic improvement of the <a href="/facts/Microscope/ldJp0CI3">microscope</a>. It was then that scholars discovered <a href="/facts/Spermatozoa/sTG4GWc0">spermatozoa</a>, bacteria, <a href="/facts/Infusoria/oLTsxd6c">infusoria</a> and the diversity of microscopic life. Investigations by <a href="/facts/Jan_Swammerdam/qPyydP1d">Jan Swammerdam</a> led to new interest in <a href="/facts/Entomology/zTtUNTWm">entomology</a> and helped to develop techniques of microscopic <a href="/facts/Dissection/7Tbbx3rr">dissection</a> and <a href="/facts/Staining/jqGqWEE1">staining</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> Advances in microscopy had a profound impact on biological thinking. In the early 19th century, biologists pointed to the central importance of the <a href="/facts/Cell_(biology)/bLM9UQvy">cell</a>. In 1838, <a href="/facts/Matthias_Jakob_Schleiden/UvKv4hBV">Schleiden</a> and <a href="/facts/Theodor_Schwann/zG3Cb08t">Schwann</a> began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells, continuing to support <a href="/facts/Spontaneous_generation/bUVSjNam">spontaneous generation</a>. However, <a href="/facts/Robert_Remak/k89nTu46">Robert Remak</a> and <a href="/facts/Rudolf_Virchow/T9Pqp63S">Rudolf Virchow</a> were able to reify the third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into <a href="/facts/Cell_theory/71xtwL7f">cell theory</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a><a class="footnote-ref" id="fnref:16" href="#fn:16">16</a>
Meanwhile, taxonomy and classification became the focus of natural historians. <a href="/facts/Carl_Linnaeus/gy5HWkHS">Carl Linnaeus</a> published a basic <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">taxonomy</a> for the natural world in 1735, and in the 1750s introduced <a href="/facts/Binomial_nomenclature/jgBqAhPv">scientific names</a> for all his species.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a> <a href="/facts/Georges-Louis_Leclerc%2c_Comte_de_Buffon/9LqPhlul">Georges-Louis Leclerc, Comte de Buffon</a>, treated species as artificial categories and living forms as malleable—even suggesting the possibility of <a href="/facts/Common_descent/bwEgMu7F">common descent</a>.<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a>

Serious evolutionary thinking originated with the works of <a href="/facts/Jean-Baptiste_Lamarck/3lxaPmqM">Jean-Baptiste Lamarck</a>, who presented a coherent theory of evolution.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> The British <a href="/facts/Natural_history/pSlAvbTX">naturalist</a> <a href="/facts/Charles_Darwin/uwDLGItY">Charles Darwin</a>, combining the biogeographical approach of <a href="/facts/Alexander_von_Humboldt/5AuxyDbh">Humboldt</a>, the uniformitarian geology of <a href="/facts/Charles_Lyell/6cAM8yRE">Lyell</a>, <a href="/facts/Thomas_Malthus/fNoDNncG">Malthus's</a> writings on population growth, and his own morphological expertise and extensive natural observations, forged a more successful evolutionary theory based on <a href="/facts/Natural_selection/JXTte6en">natural selection</a>; similar reasoning and evidence led <a href="/facts/Alfred_Russel_Wallace/bMvvalao">Alfred Russel Wallace</a> to independently reach the same conclusions.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a><a class="footnote-ref" id="fnref:21" href="#fn:21">21</a>
The basis for modern genetics began with the work of <a href="/facts/Gregor_Mendel/4UpILWAz">Gregor Mendel</a> in 1865.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a> This outlined the principles of biological inheritance.<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a> However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the <a href="/facts/Modern_synthesis_(20th_century)/anJdPq34">modern synthesis</a> reconciled Darwinian evolution with <a href="/facts/Classical_genetics/SwtEeqBW">classical genetics</a>.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a> In the 1940s and early 1950s, a <a href="/facts/Hershey%25E2%2580%2593Chase_experiment/kJJpIHIE">series of experiments</a> by <a href="/facts/Alfred_Hershey/lAEvkfBe">Alfred Hershey</a> and <a href="/facts/Martha_Chase/DxNpGKp8">Martha Chase</a> pointed to <a href="/facts/DNA/nB89Iauz">DNA</a> as the component of <a href="/facts/Chromosome/FRGf11FC">chromosomes</a> that held the trait-carrying units that had become known as <a href="/facts/Gene/e1swwPY6">genes</a>. A focus on new kinds of model organisms such as <a href="/facts/Virus/mf88Bcbl">viruses</a> and bacteria, along with the discovery of the double-helical structure of DNA by <a href="/facts/James_Watson/TnXJ7GCp">James Watson</a> and <a href="/facts/Francis_Crick/AG05HNKI">Francis Crick</a> in 1953, marked the transition to the era of <a href="/facts/Molecular_genetics/Q8wsm0FE">molecular genetics</a>. From the 1950s onwards, biology has been vastly extended in the <a href="/facts/Molecular_Biology/YgqU35ub">molecular</a> domain. The <a href="/facts/Genetic_code/zUK0bY1B">genetic code</a> was cracked by <a href="/facts/Har_Gobind_Khorana/nJmTdzap">Har Gobind Khorana</a>, <a href="/facts/Robert_W._Holley/h9LwT5j5">Robert W. Holley</a> and <a href="/facts/Marshall_Warren_Nirenberg/W7CreYHM">Marshall Warren Nirenberg</a> after DNA was understood to contain <a href="/facts/Codon/zUK0bY1B">codons</a>. The <a href="/facts/Human_Genome_Project/eQDHgHTP">Human Genome Project</a> was launched in 1990 to map the human <a href="/facts/Genome/31Sm08JT">genome</a>.<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a>

<h2 id="chemical-basis">Chemical basis</h2>
<h3>Atoms and molecules</h3>
Further information: <a href="/facts/Chemistry/SrNqkGVm">Chemistry</a>
All organisms are made up of <a href="/facts/Chemical_element/0jducgWD">chemical elements</a>;<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a> <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, <a href="/facts/Carbon/ukrgQexo">carbon</a>, <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, and <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> account for most (96%) of the mass of all organisms, with <a href="/facts/Calcium/hf252CC0">calcium</a>, <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a>, <a href="/facts/Sulfur/IK0amTfM">sulfur</a>, <a href="/facts/Sodium/eYuSPu2V">sodium</a>, <a href="/facts/Chlorine/y27UwYBM">chlorine</a>, and <a href="/facts/Magnesium/XyIhd2FG">magnesium</a> constituting essentially all the remainder. Different elements can combine to form <a href="/facts/Chemical_compound/37FXqqtv">compounds</a> such as water, which is fundamental to life.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a> <a href="/facts/Biochemistry/fa2pWEBp">Biochemistry</a> is the study of <a href="/facts/Chemical_process/EBSGtK3y">chemical processes</a> within and relating to living <a href="/facts/Organism/j8JkL09u">organisms</a>. <a href="/facts/Molecular_biology/YgqU35ub">Molecular biology</a> is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including <a href="/facts/Biomolecule/DGvFzN7R">molecular</a> synthesis, modification, mechanisms, and interactions.

<h3>Water</h3>
See also: <a href="/facts/Planetary_habitability/n8A0ehR8">Planetary habitability</a>, <a href="/facts/Circumstellar_habitable_zone/bjgaUkfB">Circumstellar habitable zone</a>, and <a href="/facts/Water_distribution_on_Earth/s6NmX34d">Water distribution on Earth</a>

Life arose from the Earth's first ocean, which formed some 3.8 billion years ago.<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a> Since then, water continues to be the most abundant molecule in every organism. Water is important to life because it is an effective <a href="/facts/Solvent/LoMPMz8S">solvent</a>, capable of dissolving solutes such as sodium and <a href="/facts/Chloride/m9NUaUDR">chloride</a> ions or other small molecules to form an <a href="/facts/Aqueous_solution/eRNr3MH2">aqueous</a> <a href="/facts/Solution_(chemistry)/MGchhMQV">solution</a>. Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reactions</a> that sustain life.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a> In terms of its <a href="/facts/Properties_of_water/FFsl65l1">molecular structure</a>, water is a small <a href="/facts/Chemical_polarity/KlIJ0zjB">polar molecule</a> with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H2O).<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a> Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge.<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a> This polar <a href="/facts/Properties_of_water/FFsl65l1">property</a> of water allows it to attract other water molecules via hydrogen bonds, which makes water <a href="/facts/Cohesion_(chemistry)/cthgtKKj">cohesive</a>.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a> <a href="/facts/Surface_tension/EuluYmSM">Surface tension</a> results from the cohesive force due to the attraction between molecules at the surface of the liquid.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a> Water is also <a href="/facts/Adhesion/NuVQPk4z">adhesive</a> as it is able to adhere to the surface of any polar or charged non-water molecules.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a> Water is <a href="/facts/Density/92p6hh9I">denser</a> as a <a href="/facts/Water/0lkXnJPK">liquid</a> than it is as a solid (or ice).<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a> This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby <a href="/facts/Thermal_insulation/j7tVT4uv">insulating</a> the liquid below from the cold air above.<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a> Water has the capacity to absorb energy, giving it a higher <a href="/facts/Specific_heat_capacity/9P0U6YgI">specific heat capacity</a> than other solvents such as <a href="/facts/Ethanol/3lCLfBSP">ethanol</a>.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a> Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into <a href="/facts/Water_vapor/DX5IUFG0">water vapor</a>.<a class="footnote-ref" id="fnref:38" href="#fn:38">38</a> As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen and <a href="/facts/Hydroxyl/1Hs2QAEv">hydroxyl</a> ions before reforming into a water molecule again.<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a> In <a href="/facts/Purified_water/r0mpReV1">pure water</a>, the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a <a href="/facts/PH/RnSv8D7y">pH</a> that is neutral.

<h3>Organic compounds</h3>
Further information: <a href="/facts/Organic_chemistry/djxhxaNg">Organic chemistry</a>

<a href="/facts/Organic_compound/gqCNaN45">Organic compounds</a> are molecules that contain carbon bonded to another element such as hydrogen.<a class="footnote-ref" id="fnref:40" href="#fn:40">40</a> With the exception of water, nearly all the molecules that make up each organism contain carbon.<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a><a class="footnote-ref" id="fnref:42" href="#fn:42">42</a> Carbon can form <a href="/facts/Covalent_bond/EzMLf3Pz">covalent bonds</a> with up to four other atoms, enabling it to form diverse, large, and complex molecules.<a class="footnote-ref" id="fnref:43" href="#fn:43">43</a><a class="footnote-ref" id="fnref:44" href="#fn:44">44</a> For example, a single carbon atom can form four single covalent bonds such as in <a href="/facts/Methane/kBHneYnh">methane</a>, two <a href="/facts/Double_bond/qDemMn0c">double covalent bonds</a> such as in <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> (CO2), or a <a href="/facts/Triple_bond/poDWRayX">triple covalent bond</a> such as in <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> (CO). Moreover, carbon can form very long chains of interconnecting <a href="/facts/Carbon%25E2%2580%2593carbon_bond/nLE4z5BK">carbon–carbon bonds</a> such as <a href="/facts/Octane/3E91j6Fe">octane</a> or ring-like structures such as <a href="/facts/Glucose/oUSkoLCD">glucose</a>.
The simplest form of an organic molecule is the <a href="/facts/Hydrocarbon/Dcg95Ju5">hydrocarbon</a>, which is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other elements such as oxygen (O), hydrogen (H), phosphorus (P), and sulfur (S), which can change the chemical behavior of that compound.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a> Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called <a href="/facts/Functional_group/GXkSPsjM">functional groups</a>.<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a> There are six prominent functional groups that can be found in organisms: <a href="/facts/Amine/JvfYJfP6">amino group</a>, <a href="/facts/Carboxylic_acid/U4xuMosy">carboxyl group</a>, <a href="/facts/Carbonyl_group/KJ71bo3X">carbonyl group</a>, <a href="/facts/Hydroxy_group/1Hs2QAEv">hydroxyl group</a>, <a href="/facts/Phosphate/FDGVCxUG">phosphate group</a>, and <a href="/facts/Thiol/VemKEDP9">sulfhydryl group</a>.<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a>
In 1953, the <a href="/facts/Miller%25E2%2580%2593Urey_experiment/SYTLzGJu">Miller–Urey experiment</a> showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions of <a href="/facts/Early_Earth/e0nCkeqE">early Earth</a>, thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (see <a href="/facts/Abiogenesis/BvAmLPQP">abiogenesis</a>).<a class="footnote-ref" id="fnref:48" href="#fn:48">48</a><a class="footnote-ref" id="fnref:49" href="#fn:49">49</a>

<h3>Macromolecules</h3>
Main article: <a href="/facts/Macromolecule/039ULSLg">Macromolecule</a>

<a href="/facts/Macromolecule/039ULSLg">Macromolecules</a> are large molecules made up of smaller subunits or <a href="/facts/Monomer/Gw3CltMb">monomers</a>.<a class="footnote-ref" id="fnref:50" href="#fn:50">50</a> Monomers include sugars, amino acids, and nucleotides.<a class="footnote-ref" id="fnref:51" href="#fn:51">51</a> <a href="/facts/Carbohydrate/Geuuzxsf">Carbohydrates</a> include monomers and polymers of sugars.<a class="footnote-ref" id="fnref:52" href="#fn:52">52</a>
Lipids are the only class of macromolecules that are not made up of polymers. They include <a href="/facts/Steroid/4AhMbgXt">steroids</a>, <a href="/facts/Phospholipid/u6GhNCF5">phospholipids</a>, and fats,<a class="footnote-ref" id="fnref:53" href="#fn:53">53</a> largely nonpolar and hydrophobic (water-repelling) substances.<a class="footnote-ref" id="fnref:54" href="#fn:54">54</a>
Proteins are the most diverse of the macromolecules. They include <a href="/facts/Enzyme/DSI1Fh7y">enzymes</a>, <a href="/facts/Transport_protein/kVVhlTAK">transport proteins</a>, large <a href="/facts/Cell_signal/6oYAVP9I">signaling</a> molecules, <a href="/facts/Antibodies/MX8pdTdP">antibodies</a>, and <a href="/facts/Structural_proteins/Jxmagu3E">structural proteins</a>. The basic unit (or monomer) of a protein is an <a href="/facts/Amino_acid/WDxYwBny">amino acid</a>.<a class="footnote-ref" id="fnref:55" href="#fn:55">55</a> Twenty amino acids are used in proteins.<a class="footnote-ref" id="fnref:56" href="#fn:56">56</a>
Nucleic acids are polymers of <a href="/facts/Nucleotide/pQKEwlJI">nucleotides</a>.<a class="footnote-ref" id="fnref:57" href="#fn:57">57</a> Their function is to store, transmit, and express hereditary information.<a class="footnote-ref" id="fnref:58" href="#fn:58">58</a>

<h2 id="cells">Cells</h2>
Main article: <a href="/facts/Cell_(biology)/bLM9UQvy">Cell (biology)</a>
<a href="/facts/Cell_theory/71xtwL7f">Cell theory</a> states that <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through <a href="/facts/Cell_division/n5MrunWj">cell division</a>.<a class="footnote-ref" id="fnref:59" href="#fn:59">59</a> Most cells are very small, with diameters ranging from 1 to 100 <a href="/facts/Micrometre/7eDFxD2L">micrometers</a> and are therefore only visible under a <a href="/facts/Light_microscope/5700vcna">light</a> or <a href="/facts/Electron_microscope/XtaBktwF">electron microscope</a>.<a class="footnote-ref" id="fnref:60" href="#fn:60">60</a> There are generally two types of cells: <a href="/facts/Eukaryotic/SkwyPLMA">eukaryotic</a> cells, which contain a <a href="/facts/Cell_nucleus/D7sFRa3c">nucleus</a>, and <a href="/facts/Prokaryotic/oYyvaDDH">prokaryotic</a> cells, which do not. Prokaryotes are <a href="/facts/Unicellular_organism/9PQsRuHU">single-celled organisms</a> such as <a href="/facts/Bacterium/wC8xSCsU">bacteria</a>, whereas eukaryotes can be single-celled or multicellular. In <a href="/facts/Multicellular_organisms/rk8QBF7o">multicellular organisms</a>, every cell in the organism's body is derived ultimately from a <a href="/facts/Zygote/mSu2xMWA">single cell</a> in a fertilized <a href="/facts/Egg_(biology)/KyMSx9v4">egg</a>.

<h3>Cell structure</h3>

Every cell is enclosed within a <a href="/facts/Cell_membrane/vAXSD0DT">cell membrane</a> that separates its <a href="/facts/Cytoplasm/Wa0sE6xf">cytoplasm</a> from the <a href="/facts/Extracellular_space/E1Vaq60Y">extracellular space</a>.<a class="footnote-ref" id="fnref:61" href="#fn:61">61</a> A cell membrane consists of a <a href="/facts/Lipid_bilayer/CQKQ9l3g">lipid bilayer</a>, including <a href="/facts/Cholesterol/TnfPlYc1">cholesterols</a> that sit between phospholipids to maintain their <a href="/facts/Membrane_fluidity/Q19FXbJM">fluidity</a> at various temperatures. Cell membranes are <a href="/facts/Semipermeable_membrane/YxENKzn5">semipermeable</a>, allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as <a href="/facts/Ion/3bePpDna">ions</a>.<a class="footnote-ref" id="fnref:62" href="#fn:62">62</a> Cell membranes also contain <a href="/facts/Membrane_protein/FboTeM8Q">membrane proteins</a>, including <a href="/facts/Integral_membrane_protein/Vztpmlu1">integral membrane proteins</a> that go across the membrane serving as <a href="/facts/Membrane_transporter/h1aczVxg">membrane transporters</a>, and <a href="/facts/Peripheral_protein/EGsWYg2e">peripheral proteins</a> that loosely attach to the outer side of the cell membrane, acting as enzymes shaping the cell.<a class="footnote-ref" id="fnref:63" href="#fn:63">63</a> Cell membranes are involved in various cellular processes such as <a href="/facts/Cell_adhesion/Vg4alzxb">cell adhesion</a>, <a href="/facts/Membrane_potential/DpNG14c7">storing electrical energy</a>, and <a href="/facts/Cell_signalling/6oYAVP9I">cell signalling</a> and serve as the attachment surface for several extracellular structures such as a <a href="/facts/Cell_wall/DYXJVrnw">cell wall</a>, <a href="/facts/Glycocalyx/mozTP4zq">glycocalyx</a>, and <a href="/facts/Cytoskeleton/1sWlJRX9">cytoskeleton</a>.

Within the cytoplasm of a cell, there are many biomolecules such as <a href="/facts/Protein/Jxmagu3E">proteins</a> and <a href="/facts/Nucleic_acid/2tTgcI0v">nucleic acids</a>.<a class="footnote-ref" id="fnref:64" href="#fn:64">64</a> In addition to biomolecules, eukaryotic cells have specialized structures called <a href="/facts/Organelle/vhDFnGbs">organelles</a> that have their own lipid bilayers or are spatially units.<a class="footnote-ref" id="fnref:65" href="#fn:65">65</a> These organelles include the <a href="/facts/Cell_nucleus/D7sFRa3c">cell nucleus</a>, which contains most of the cell's DNA, or <a href="/facts/Mitochondrion/ErHqrdJ1">mitochondria</a>, which generate <a href="/facts/Adenosine_triphosphate/nB9bwgcU">adenosine triphosphate</a> (ATP) to power cellular processes. Other organelles such as <a href="/facts/Endoplasmic_reticulum/DVbLA7GE">endoplasmic reticulum</a> and <a href="/facts/Golgi_apparatus/F91pGb3t">Golgi apparatus</a> play a role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by <a href="/facts/Lysosome/0mcrzJU9">lysosomes</a>, another specialized organelle. <a href="/facts/Plant_cell/T4RqyI2f">Plant cells</a> have additional organelles that distinguish them from <a href="/facts/Animal_cell/SkwyPLMA">animal cells</a> such as a cell wall that provides support for the plant cell, <a href="/facts/Chloroplast/fo8U3yTx">chloroplasts</a> that harvest sunlight energy to produce sugar, and <a href="/facts/Vacuole/9QjBVb7j">vacuoles</a> that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds.<a class="footnote-ref" id="fnref:66" href="#fn:66">66</a> Eukaryotic cells also have cytoskeleton that is made up of <a href="/facts/Microtubule/ENL1TxSe">microtubules</a>, <a href="/facts/Intermediate_filament/ocjFHTvJ">intermediate filaments</a>, and <a href="/facts/Microfilament/TR7qU9Jv">microfilaments</a>, all of which provide support for the cell and are involved in the movement of the cell and its organelles.<a class="footnote-ref" id="fnref:67" href="#fn:67">67</a> In terms of their structural composition, the microtubules are made up of <a href="/facts/Tubulin/j4Bk2wcL">tubulin</a> (e.g., <a href="/facts/Tubulin/j4Bk2wcL">α-tubulin</a> and <a href="/facts/Tubulin/j4Bk2wcL">β-tubulin</a>) whereas intermediate filaments are made up of fibrous proteins.<a class="footnote-ref" id="fnref:68" href="#fn:68">68</a> Microfilaments are made up of <a href="/facts/Actin/Hu3qIUL9">actin</a> molecules that interact with other strands of proteins.<a class="footnote-ref" id="fnref:69" href="#fn:69">69</a>

<h3>Metabolism</h3>
Further information: <a href="/facts/Bioenergetics/oslNljbx">Bioenergetics</a>

All cells require energy to sustain cellular processes. <a href="/facts/Metabolism/WxDrm8k5">Metabolism</a> is the set of <a href="/facts/Chemical_reactions/GpqWlKrz">chemical reactions</a> in an organism. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food/fuel to monomer building blocks; and the elimination of <a href="/facts/Metabolic_waste/qGH5b3od">metabolic wastes</a>. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as <a href="/facts/Catabolic/CFWUdphX">catabolic</a>—the breaking down of compounds (for example, the breaking down of glucose to pyruvate by <a href="/facts/Cellular_respiration/kb6gpsvL">cellular respiration</a>); or <a href="/facts/Anabolic/eH3Q5AHF">anabolic</a>—the building up (<a href="/facts/Biosynthesis/aFk7Tk7f">synthesis</a>) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized into <a href="/facts/Metabolic_pathway/8Py8HKJd">metabolic pathways</a>, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by <a href="/facts/Coupling_(physics)/Pg2Yg0Tz">coupling</a> them to <a href="/facts/Spontaneous_process/xyGhdUG0">spontaneous reactions</a> that release energy. Enzymes act as <a href="/facts/Catalysis/LPBS7TQC">catalysts</a>—they allow a reaction to proceed more rapidly without being consumed by it—by reducing the amount of <a href="/facts/Activation_energy/GqkgPvo0">activation energy</a> needed to convert <a href="/facts/Reactant/c3GLInCu">reactants</a> into <a href="/facts/Product_(chemistry)/8uCoDUdv">products</a>. Enzymes also allow the <a href="/facts/Metabolic_pathway/8Py8HKJd">regulation</a> of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.

<h3>Cellular respiration</h3>
Main article: <a href="/facts/Cellular_respiration/kb6gpsvL">Cellular respiration</a>

Cellular respiration is a set of metabolic reactions and processes that take place in cells to convert <a href="/facts/Energy/wd8PrQM7">chemical energy</a> from <a href="/facts/Nutrients/34bn4skK">nutrients</a> into adenosine triphosphate (ATP), and then release waste products.<a class="footnote-ref" id="fnref:70" href="#fn:70">70</a> The reactions involved in respiration are <a href="/facts/Catabolism/CFWUdphX">catabolic reactions</a>, which break large molecules into smaller ones, releasing energy. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are <a href="/facts/Redox/Pyd5RVcj">redox</a> reactions. Although cellular respiration is technically a <a href="/facts/Combustion_reaction/ohQoWj8z">combustion reaction</a>, it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions.
Sugar in the form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: <a href="/facts/Glycolysis/a5ApxLoG">glycolysis</a>, <a href="/facts/Citric_acid_cycle/EGJL9NXP">citric acid cycle</a> (or Krebs cycle), <a href="/facts/Electron_transport_chain/QjcIkgeR">electron transport chain</a>, and <a href="/facts/Oxidative_phosphorylation/en2l08P9">oxidative phosphorylation</a>.<a class="footnote-ref" id="fnref:71" href="#fn:71">71</a> Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two <a href="/facts/Pyruvic_acid/qJTgl4J9">pyruvates</a>, with two net molecules of ATP being produced at the same time.<a class="footnote-ref" id="fnref:72" href="#fn:72">72</a> Each pyruvate is then oxidized into <a href="/facts/Acetyl-CoA/XyxFfC84">acetyl-CoA</a> by the <a href="/facts/Pyruvate_dehydrogenase_complex/6Cmv35f6">pyruvate dehydrogenase complex</a>, which also generates <a href="/facts/Nicotinamide_adenine_dinucleotide/BHdLW925">NADH</a> and carbon dioxide. Acetyl-CoA enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH2, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the <a href="/facts/Crista/2PUrXT28">mitochondrial cristae</a>. Oxidative phosphorylation comprises the electron transport chain, which is a series of four <a href="/facts/Protein_complex/GW11Ee34">protein complexes</a> that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH2 that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane (<a href="/facts/Chemiosmosis/2clDS3bT">chemiosmosis</a>), which generates a <a href="/facts/Chemiosmosis/2clDS3bT">proton motive force</a>.<a class="footnote-ref" id="fnref:73" href="#fn:73">73</a> Energy from the proton motive force drives the enzyme <a href="/facts/ATP_synthase/5NBMxsVX">ATP synthase</a> to synthesize more ATPs by <a href="/facts/Phosphorylation/LXUEEeIL">phosphorylating</a> <a href="/facts/Adenosine_diphosphate/JEDnsCjn">ADPs</a>. The transfer of electrons terminates with molecular oxygen being the final <a href="/facts/Electron_acceptor/sRvhMXqH">electron acceptor</a>.
If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of <a href="/facts/Fermentation/rxaDr28b">fermentation</a>. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to <a href="/facts/Cellular_waste_product/HJ6raT9X">waste products</a> that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD+ so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD+ for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is <a href="/facts/Lactic_acid/BsVRFx7s">lactic acid</a>. This type of fermentation is called <a href="/facts/Lactic_acid_fermentation/8eQ5uBDq">lactic acid fermentation</a>. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD+ regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD+ attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or <a href="/facts/Ethanol_fermentation/lpY5qenQ">ethanol fermentation</a>. The ATP generated in this process is made by <a href="/facts/Substrate-level_phosphorylation/1hxqqgBS">substrate-level phosphorylation</a>, which does not require oxygen.

<h3>Photosynthesis</h3>
Main article: <a href="/facts/Photosynthesis/TMV7w693">Photosynthesis</a>

Photosynthesis is a process used by plants and other organisms to <a href="/facts/Energy_transformation/8g4seWcj">convert</a> <a href="/facts/Light_energy/8QHpm6KY">light energy</a> into <a href="/facts/Chemical_energy/Y3VP3Ifq">chemical energy</a> that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water.<a class="footnote-ref" id="fnref:74" href="#fn:74">74</a><a class="footnote-ref" id="fnref:75" href="#fn:75">75</a><a class="footnote-ref" id="fnref:76" href="#fn:76">76</a> In most cases, oxygen is released as a waste product. Most plants, <a href="/facts/Algae/U1ftVKU3">algae</a>, and <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a> perform photosynthesis, which is largely responsible for producing and maintaining the <a href="/facts/Atmospheric_oxygen/EbmISm33">oxygen content</a> of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.<a class="footnote-ref" id="fnref:77" href="#fn:77">77</a>
Photosynthesis has four stages: <a href="/facts/Light_absorption/xBYKt7yS">Light absorption</a>, electron transport, ATP synthesis, and <a href="/facts/Carbon_fixation/JqvfmCdH">carbon fixation</a>.<a class="footnote-ref" id="fnref:78" href="#fn:78">78</a> Light absorption is the initial step of photosynthesis whereby light energy is absorbed by <a href="/facts/Chlorophyll/1Sb0NyyM">chlorophyll</a> pigments attached to proteins in the <a href="/facts/Thylakoid/d9PEe649">thylakoid membranes</a>. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a <a href="/facts/Quinone/RY00GqrL">quinone</a> designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP+, which is reduced to NADPH, a process that takes place in a protein complex called <a href="/facts/Photosystem_I/z9hRH0kt">photosystem I</a> (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration.<a class="footnote-ref" id="fnref:79" href="#fn:79">79</a>
During the third stage of photosynthesis, the movement of protons down their <a href="/facts/Concentration_gradient/lcIz28qY">concentration gradients</a> from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase.<a class="footnote-ref" id="fnref:80" href="#fn:80">80</a> The NADPH and ATPs generated by the <a href="/facts/Light-dependent_reaction/kBnR8Lcp">light-dependent reactions</a> in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as <a href="/facts/Ribulose_bisphosphate/1DwB2uHE">ribulose bisphosphate</a> (RuBP) in a sequence of light-independent (or dark) reactions called the <a href="/facts/Calvin_cycle/M1bIgQb9">Calvin cycle</a>.<a class="footnote-ref" id="fnref:81" href="#fn:81">81</a>

<h3>Cell signaling</h3>
Main article: <a href="/facts/Cell_signaling/6oYAVP9I">Cell signaling</a>
Cell signaling (or communication) is the ability of <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> to receive, process, and transmit signals with its environment and with itself.<a class="footnote-ref" id="fnref:82" href="#fn:82">82</a><a class="footnote-ref" id="fnref:83" href="#fn:83">83</a> Signals can be non-chemical such as light, <a href="/facts/Action_potential/Sz8yCRqx">electrical impulses</a>, and heat, or chemical signals (or <a href="/facts/Ligand/6etB3qeW">ligands</a>) that interact with <a href="/facts/Receptor_(biochemistry)/tjTAC76a">receptors</a>, which can be found <a href="/facts/Cell_surface_receptor/0YGSjDdL">embedded</a> in the <a href="/facts/Cell_membrane/vAXSD0DT">cell membrane</a> of another cell or <a href="/facts/Intracellular_receptor/Up8SbQIX">located deep inside</a> a cell.<a class="footnote-ref" id="fnref:84" href="#fn:84">84</a><a class="footnote-ref" id="fnref:85" href="#fn:85">85</a> There are generally four types of chemical signals: <a href="/facts/Autocrine_signaling/EwfQEYzG">autocrine</a>, <a href="/facts/Paracrine_signaling/V8eVwG9z">paracrine</a>, <a href="/facts/Juxtacrine_signalling/dBTsF87M">juxtacrine</a>, and <a href="/facts/Hormone/WFnBnINJ">hormones</a>.<a class="footnote-ref" id="fnref:86" href="#fn:86">86</a> In autocrine signaling, the ligand affects the same cell that releases it. <a href="/facts/Neoplasm/K0ytzmPz">Tumor</a> cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affects them. For example, brain cells called <a href="/facts/Neuron/G7CyTVdH">neurons</a> release ligands called <a href="/facts/Neurotransmitter/LlpYFSU3">neurotransmitters</a> that diffuse across a <a href="/facts/Chemical_synapse/M1x5rhz7">synaptic cleft</a> to bind with a receptor on an adjacent cell such as another neuron or <a href="/facts/Muscle_cell/cP7Jea3q">muscle cell</a>. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through the <a href="/facts/Circulatory_system/WXwudzzc">circulatory systems</a> of animals or <a href="/facts/Vascular_system/WXwudzzc">vascular systems</a> of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an <a href="/facts/Ligand-gated_ion_channel/gN9RkNC2">inotropic receptor</a> can alter the <a href="/facts/Cell_excitability/DpNG14c7">excitability</a> of a target cell. Other types of receptors include <a href="/facts/Protein_kinase/U4DQVhW3">protein kinase</a> receptors (e.g., <a href="/facts/Insulin_receptor/PCEbQj6k">receptor</a> for the hormone <a href="/facts/Insulin/oXG0myt1">insulin</a>) and <a href="/facts/G_protein-coupled_receptor/0arUMkQc">G protein-coupled receptors</a>. Activation of G protein-coupled receptors can initiate <a href="/facts/Second_messenger_system/J0eEENxQ">second messenger</a> cascades. The process by which a chemical or physical signal is transmitted through a cell as a <a href="/facts/Biochemical_cascade/bCsA7XXG">series of molecular events</a> is called <a href="/facts/Signal_transduction/1bs0Me6w">signal transduction</a>.

<h3>Cell cycle</h3>
Main article: <a href="/facts/Cell_cycle/nLMT7K4r">Cell cycle</a>

The cell cycle is a series of events that take place in a <a href="/facts/Cell_(biology)/bLM9UQvy">cell</a> that cause it to divide into two daughter cells. These events include the <a href="/facts/DNA_replication/wrU6RuR3">duplication of its DNA</a> and some of its <a href="/facts/Organelle/vhDFnGbs">organelles</a>, and the subsequent partitioning of its cytoplasm into two daughter cells in a process called <a href="/facts/Cell_division/n5MrunWj">cell division</a>.<a class="footnote-ref" id="fnref:87" href="#fn:87">87</a> In <a href="/facts/Eukaryote/SkwyPLMA">eukaryotes</a> (i.e., animal, plant, <a href="/facts/Fungal/BHo5DP59">fungal</a>, and <a href="/facts/Protist/NYVfaGVz">protist</a> cells), there are two distinct types of cell division: <a href="/facts/Mitosis/ExxrSUs8">mitosis</a> and <a href="/facts/Meiosis/Pj5Q6nGN">meiosis</a>.<a class="footnote-ref" id="fnref:88" href="#fn:88">88</a> Mitosis is part of the cell cycle, in which replicated <a href="/facts/Chromosomes/FRGf11FC">chromosomes</a> are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of <a href="/facts/Interphase/c37v07K4">interphase</a> (during which the DNA is replicated) and is often followed by <a href="/facts/Telophase/c4eAwvmD">telophase</a> and <a href="/facts/Cytokinesis/hhrnmTwY">cytokinesis</a>; which divides the <a href="/facts/Cytoplasm/Wa0sE6xf">cytoplasm</a>, <a href="/facts/Organelle/vhDFnGbs">organelles</a> and <a href="/facts/Cell_membrane/vAXSD0DT">cell membrane</a> of one cell into two new <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.<a class="footnote-ref" id="fnref:89" href="#fn:89">89</a> The cell cycle is a vital process by which a single-celled <a href="/facts/Fertilized_egg/mSu2xMWA">fertilized egg</a> develops into a mature organism, as well as the process by which hair, skin, <a href="/facts/Blood_cell/lf9GA0Dj">blood cells</a>, and some <a href="/facts/Viscus/FUYsLaxz">internal organs</a> are renewed. After cell division, each of the daughter cells begin the <a href="/facts/Interphase/c37v07K4">interphase</a> of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions.<a class="footnote-ref" id="fnref:90" href="#fn:90">90</a> <a href="/facts/Homologous_chromosome/hktsBhyx">Homologous chromosomes</a> are separated in the first division (<a href="/facts/Meiosis/Pj5Q6nGN">meiosis I</a>), and sister chromatids are separated in the second division (<a href="/facts/Meiosis/Pj5Q6nGN">meiosis II</a>). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.

<a href="/facts/Prokaryote/oYyvaDDH">Prokaryotes</a> (i.e., <a href="/facts/Archaea/Adxh0aHw">archaea</a> and bacteria) can also undergo cell division (or <a href="/facts/Binary_fission/9Ed5vUr6">binary fission</a>). Unlike the processes of <a href="/facts/Mitosis/ExxrSUs8">mitosis</a> and <a href="/facts/Meiosis/Pj5Q6nGN">meiosis</a> in eukaryotes, binary fission in prokaryotes takes place without the formation of a <a href="/facts/Spindle_apparatus/FMFd1GM5">spindle apparatus</a> on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by <a href="/facts/FtsZ/tKkb9buQ">FtsZ</a> polymerization and "Z-ring" formation).<a class="footnote-ref" id="fnref:91" href="#fn:91">91</a> The new cell wall (<a href="/facts/Septum_(cell_biology)/XAVGxGNB">septum</a>) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, <a href="/facts/Ribosome/CF2u2gtg">ribosomes</a>, and <a href="/facts/Plasmid/yf59JVlA">plasmids</a>.

<h3>Sexual reproduction and meiosis</h3>
Meiosis is a central feature of sexual reproduction in eukaryotes, and the most fundamental function of <a href="/facts/Meiosis/Pj5Q6nGN">meiosis</a> appears to be conservation of the integrity of the <a href="/facts/Genome/31Sm08JT">genome</a> that is passed on to progeny by parents.<a class="footnote-ref" id="fnref:92" href="#fn:92">92</a><a class="footnote-ref" id="fnref:93" href="#fn:93">93</a> Two aspects of <a href="/facts/Sexual_reproduction/hmsXy53s">sexual reproduction</a>, <a href="/facts/Genetic_recombination/uocNH0m7">meiotic recombination</a> and <a href="/facts/Outcrossing/6sAJ4Gh7">outcrossing</a>, are likely maintained respectively by the adaptive advantages of recombinational repair of genomic <a href="/facts/DNA_damage_(naturally_occurring)/CjGTLKGl">DNA damage</a> and genetic <a href="/facts/Complementation_(genetics)/4YJL6RGW">complementation</a> which masks the expression of deleterious recessive <a href="/facts/Mutation/Ee4NnWbY">mutations</a>.<a class="footnote-ref" id="fnref:94" href="#fn:94">94</a>
The beneficial effect of genetic complementation, derived from outcrossing (cross-fertilization) is also referred to as hybrid vigor or heterosis. Charles Darwin in his 1878 book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom<a class="footnote-ref" id="fnref:95" href="#fn:95">95</a> at the start of chapter XII noted “The first and most important of the conclusions which may be drawn from the observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented.” <a href="/facts/Genetic_variation/f2wzYLbI">Genetic variation</a>, often produced as a byproduct of sexual reproduction, may provide long-term advantages to those sexual lineages that engage in <a href="/facts/Outcrossing/6sAJ4Gh7">outcrossing</a>.<a class="footnote-ref" id="fnref:96" href="#fn:96">96</a>

<h2 id="genetics">Genetics</h2>
<h3>Inheritance</h3>
Main article: <a href="/facts/Classical_genetics/SwtEeqBW">Classical genetics</a>

<a href="/facts/Genetics/bu8wne0A">Genetics</a> is the scientific study of inheritance.<a class="footnote-ref" id="fnref:97" href="#fn:97">97</a><a class="footnote-ref" id="fnref:98" href="#fn:98">98</a><a class="footnote-ref" id="fnref:99" href="#fn:99">99</a> <a href="/facts/Mendelian_inheritance/v2038wkl">Mendelian inheritance</a>, specifically, is the process by which genes and traits are passed on from parents to offspring.<a class="footnote-ref" id="fnref:100" href="#fn:100">100</a> It has several principles. The first is that genetic characteristics, <a href="/facts/Allele/8FxCS9KA">alleles</a>, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on the <a href="/facts/Mendelian_inheritance/v2038wkl">law of dominance and uniformity</a>, which states that some alleles are <a href="/facts/Dominance_(genetics)/DsvtPyQU">dominant</a> while others are <a href="/facts/Dominance_(genetics)/DsvtPyQU">recessive</a>; an organism with at least one dominant allele will display the <a href="/facts/Phenotype/vxWXKGLj">phenotype</a> of that dominant allele. During gamete formation, the alleles for each gene segregate, so that each gamete carries only one allele for each gene. <a href="/facts/Heterozygous/XzEJ5gPp">Heterozygotic</a> individuals produce gametes with an equal frequency of two alleles. Finally, the <a href="/facts/Mendelian_inheritance/v2038wkl">law of independent assortment</a>, states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are <a href="/facts/Sex_linkage/buXJkWJg">sex-linked</a>. <a href="/facts/Test_cross/wTbb96F1">Test crosses</a> can be performed to experimentally determine the underlying <a href="/facts/Genotype/MlzK1Mfz">genotype</a> of an organism with a dominant phenotype.<a class="footnote-ref" id="fnref:101" href="#fn:101">101</a> A <a href="/facts/Punnett_square/2CFbg0XX">Punnett square</a> can be used to predict the results of a test cross. The <a href="/facts/Boveri%25E2%2580%2593Sutton_chromosome_theory/sXfg8KsS">chromosome theory of inheritance</a>, which states that genes are found on chromosomes, was supported by <a href="/facts/Thomas_Hunt_Morgan/4wswVmWC">Thomas Morgans</a>'s experiments with <a href="/facts/Drosophila_melanogaster/KtnHVCcB">fruit flies</a>, which established the <a href="/facts/Sex_linkage/buXJkWJg">sex linkage</a> between eye color and sex in these insects.<a class="footnote-ref" id="fnref:102" href="#fn:102">102</a>

<h3>Genes and DNA</h3>
Further information: <a href="/facts/Gene/e1swwPY6">Gene</a> and <a href="/facts/DNA/nB89Iauz">DNA</a>

A gene is a unit of <a href="/facts/Heredity/t0aMmRWx">heredity</a> that corresponds to a region of deoxyribonucleic acid (DNA) that carries genetic information that controls form or function of an organism. DNA is composed of two <a href="/facts/Polynucleotide/eWr1V5Pg">polynucleotide</a> chains that coil around each other to form a <a href="/facts/Nucleic_acid_double_helix/XHeMuslB">double helix</a>.<a class="footnote-ref" id="fnref:103" href="#fn:103">103</a> It is found as linear <a href="/facts/Chromosome/FRGf11FC">chromosomes</a> in <a href="/facts/Eukaryote/SkwyPLMA">eukaryotes</a>, and circular chromosomes in <a href="/facts/Prokaryote/oYyvaDDH">prokaryotes</a>. The set of chromosomes in a cell is collectively known as its <a href="/facts/Genome/31Sm08JT">genome</a>. In eukaryotes, DNA is mainly in the <a href="/facts/Cell_nucleus/D7sFRa3c">cell nucleus</a>.<a class="footnote-ref" id="fnref:104" href="#fn:104">104</a> In prokaryotes, the DNA is held within the <a href="/facts/Nucleoid/nVJNKJDS">nucleoid</a>.<a class="footnote-ref" id="fnref:105" href="#fn:105">105</a> The genetic information is held within genes, and the complete assemblage in an organism is called its <a href="/facts/Genotype/MlzK1Mfz">genotype</a>.<a class="footnote-ref" id="fnref:106" href="#fn:106">106</a>
<a href="/facts/DNA_replication/wrU6RuR3">DNA replication</a> is a <a href="/facts/Semiconservative_replication/rJJAglA3">semiconservative</a> process whereby each strand serves as a template for a new strand of DNA.<a class="footnote-ref" id="fnref:107" href="#fn:107">107</a> Mutations are heritable changes in DNA.<a class="footnote-ref" id="fnref:108" href="#fn:108">108</a> They can arise <a href="/facts/Mutation/Ee4NnWbY">spontaneously</a> as a result of replication errors that were not corrected by proofreading or can be <a href="/facts/Mutation/Ee4NnWbY">induced</a> by an environmental <a href="/facts/Mutagen/xI5lKvmp">mutagen</a> such as a chemical (e.g., <a href="/facts/Nitrous_acid/xaVd1w2D">nitrous acid</a>, <a href="/facts/Benzopyrene/KMkKf04r">benzopyrene</a>) or radiation (e.g., <a href="/facts/X-ray/wseP79cN">x-ray</a>, <a href="/facts/Gamma_ray/5Drf913J">gamma ray</a>, <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet radiation</a>, particles emitted by unstable isotopes).<a class="footnote-ref" id="fnref:109" href="#fn:109">109</a> Mutations can lead to phenotypic effects such as loss-of-function, <a href="/facts/Gain-of-function_research/X7aBzabH">gain-of-function</a>, and conditional mutations.<a class="footnote-ref" id="fnref:110" href="#fn:110">110</a>
Some mutations are beneficial, as they are a source of <a href="/facts/Genetic_variation/f2wzYLbI">genetic variation</a> for evolution.<a class="footnote-ref" id="fnref:111" href="#fn:111">111</a> Others are harmful if they were to result in a loss of function of genes needed for survival.<a class="footnote-ref" id="fnref:112" href="#fn:112">112</a>

<h3>Gene expression</h3>

Main article: <a href="/facts/Gene_expression/alCPohLR">Gene expression</a>
Gene expression is the molecular process by which a <a href="/facts/Genotype/MlzK1Mfz">genotype</a> encoded in DNA gives rise to an observable <a href="/facts/Phenotype/vxWXKGLj">phenotype</a> in the proteins of an organism's body. This process is summarized by the <a href="/facts/Central_dogma_of_molecular_biology/Q0T52Dk6">central dogma of molecular biology</a>, which was formulated by <a href="/facts/Francis_Crick/AG05HNKI">Francis Crick</a> in 1958.<a class="footnote-ref" id="fnref:113" href="#fn:113">113</a><a class="footnote-ref" id="fnref:114" href="#fn:114">114</a><a class="footnote-ref" id="fnref:115" href="#fn:115">115</a> According to the Central Dogma, genetic information flows from DNA to RNA to protein. There are two gene expression processes: <a href="/facts/Transcription_(genetics)/bDgp9HDN">transcription</a> (DNA to RNA) and <a href="/facts/Translation_(genetics)/JcPC1rth">translation</a> (RNA to protein).<a class="footnote-ref" id="fnref:116" href="#fn:116">116</a>

<h3>Gene regulation</h3>
Main article: <a href="/facts/Regulation_of_gene_expression/9N4G341I">Regulation of gene expression</a>
The regulation of gene expression by environmental factors and during different stages of <a href="/facts/Developmental_biology/9dLJhCCv">development</a> can occur at each step of the process such as <a href="/facts/Transcription_(biology)/bDgp9HDN">transcription</a>, <a href="/facts/RNA_splicing/a2QEvCRI">RNA splicing</a>, <a href="/facts/Translation_(biology)/JcPC1rth">translation</a>, and <a href="/facts/Post-translational_modification/KMUayTIc">post-translational modification</a> of a protein.<a class="footnote-ref" id="fnref:117" href="#fn:117">117</a> Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins called <a href="/facts/Transcription_factor/wro0DPHe">transcription factors</a> bind to the DNA sequence close to or at a promoter.<a class="footnote-ref" id="fnref:118" href="#fn:118">118</a> A cluster of genes that share the same promoter is called an <a href="/facts/Operon/0m7pJKWn">operon</a>, found mainly in prokaryotes and some lower eukaryotes (e.g., <a href="/facts/Caenorhabditis_elegans/XJVJzPn2">Caenorhabditis elegans</a>).<a class="footnote-ref" id="fnref:119" href="#fn:119">119</a><a class="footnote-ref" id="fnref:120" href="#fn:120">120</a> In positive regulation of gene expression, the <a href="/facts/Activator_(genetics)/VF1xqnrv">activator</a> is the transcription factor that stimulates transcription when it binds to the sequence near or at the promoter. Negative regulation occurs when another transcription factor called a <a href="/facts/Repressor/qGBBQwqS">repressor</a> binds to a DNA sequence called an <a href="/facts/Operon/0m7pJKWn">operator</a>, which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds called <a href="/facts/Inducer/WjE8RGKS">inducers</a> (e.g., <a href="/facts/Allolactose/yWq1hFcU">allolactose</a>), thereby allowing transcription to occur.<a class="footnote-ref" id="fnref:121" href="#fn:121">121</a> Specific genes that can be activated by inducers are called <a href="/facts/Gene_expression/alCPohLR">inducible genes</a>, in contrast to <a href="/facts/Gene_expression/alCPohLR">constitutive genes</a> that are almost constantly active.<a class="footnote-ref" id="fnref:122" href="#fn:122">122</a> In contrast to both, <a href="/facts/Structural_gene/dhTA2nkS">structural genes</a> encode proteins that are not involved in gene regulation.<a class="footnote-ref" id="fnref:123" href="#fn:123">123</a> In addition to regulatory events involving the promoter, gene expression can also be regulated by <a href="/facts/Epigenetics/UfAAERcn">epigenetic</a> changes to <a href="/facts/Chromatin/ha0TJheJ">chromatin</a>, which is a complex of DNA and protein found in eukaryotic cells.<a class="footnote-ref" id="fnref:124" href="#fn:124">124</a>

<h3>Genes, development, and evolution</h3>
Main article: <a href="/facts/Evolutionary_developmental_biology/gl4ysPCa">Evolutionary developmental biology</a>
<a href="/facts/Developmental_biology/9dLJhCCv">Development</a> is the process by which a <a href="/facts/Multicellular_organism/rk8QBF7o">multicellular organism</a> (plant or animal) goes through a series of changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle.<a class="footnote-ref" id="fnref:125" href="#fn:125">125</a> There are four key processes that underlie development: <a href="/facts/Cell_fate_determination/5QeX65un">Determination</a>, <a href="/facts/Cellular_differentiation/bCVqUG7w">differentiation</a>, <a href="/facts/Morphogenesis/zHJh8Eae">morphogenesis</a>, and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells arise from less specialized cells such as <a href="/facts/Stem_cell/qgB9dFrI">stem cells</a>.<a class="footnote-ref" id="fnref:126" href="#fn:126">126</a><a class="footnote-ref" id="fnref:127" href="#fn:127">127</a> Stem cells are <a href="/facts/Cellular_differentiation/bCVqUG7w">undifferentiated</a> or partially differentiated <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> that can differentiate into various <a href="/facts/Types_of_cells/IjWXx31l">types of cells</a> and <a href="/facts/Cell_proliferation/Jp3hWo2t">proliferate</a> indefinitely to produce more of the same stem cell.<a class="footnote-ref" id="fnref:128" href="#fn:128">128</a> Cellular differentiation dramatically changes a cell's size, shape, <a href="/facts/Membrane_potential/DpNG14c7">membrane potential</a>, <a href="/facts/Metabolism/WxDrm8k5">metabolic activity</a>, and responsiveness to signals, which are largely due to highly controlled modifications in <a href="/facts/Gene_expression/alCPohLR">gene expression</a> and <a href="/facts/Epigenetics/UfAAERcn">epigenetics</a>. With a few exceptions, cellular differentiation almost never involves a change in the <a href="/facts/DNA/nB89Iauz">DNA</a> sequence itself.<a class="footnote-ref" id="fnref:129" href="#fn:129">129</a> Thus, different cells can have very different physical characteristics despite having the same <a href="/facts/Genome/31Sm08JT">genome</a>. Morphogenesis, or the development of body form, is the result of spatial differences in gene expression.<a class="footnote-ref" id="fnref:130" href="#fn:130">130</a> A small fraction of the genes in an organism's genome called the <a href="/facts/Evo-devo_gene_toolkit/wRe0E6BN">developmental-genetic toolkit</a> control the development of that organism. These toolkit genes are highly conserved among <a href="/facts/Phylum/ELT3Jyax">phyla</a>, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the <a href="/facts/Hox_gene/kP1yVyDY">Hox genes</a>. Hox genes determine where repeating parts, such as the many <a href="/facts/Vertebra/qAdLhD5H">vertebrae</a> of snakes, will grow in a developing embryo or larva.<a class="footnote-ref" id="fnref:131" href="#fn:131">131</a>

<h2 id="evolution">Evolution</h2>
<h3>Evolutionary processes</h3>
Main article: <a href="/facts/Evolutionary_biology/8tBoG55U">Evolutionary biology</a>

<a href="/facts/Evolution/Hj0qAaBk">Evolution</a> is a central organizing concept in biology. It is the change in <a href="/facts/Heredity/t0aMmRWx">heritable</a> <a href="/facts/Phenotypic_trait/seYSUTul">characteristics</a> of populations over successive <a href="/facts/Generation/sjgIbwW0">generations</a>.<a class="footnote-ref" id="fnref:132" href="#fn:132">132</a><a class="footnote-ref" id="fnref:133" href="#fn:133">133</a> In <a href="/facts/Artificial_selection/CAkkM2UG">artificial selection</a>, animals were selectively bred for specific traits.
<a class="footnote-ref" id="fnref:134" href="#fn:134">134</a> Given that traits are inherited, populations contain a varied mix of traits, and reproduction is able to increase any population, Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits.<a class="footnote-ref" id="fnref:135" href="#fn:135">135</a> Darwin inferred that individuals who possessed heritable traits better adapted to their environments are more likely to survive and produce more offspring than other individuals.<a class="footnote-ref" id="fnref:136" href="#fn:136">136</a> He further inferred that this would lead to the accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment.<a class="footnote-ref" id="fnref:137" href="#fn:137">137</a><a class="footnote-ref" id="fnref:138" href="#fn:138">138</a><a class="footnote-ref" id="fnref:139" href="#fn:139">139</a><a class="footnote-ref" id="fnref:140" href="#fn:140">140</a><a class="footnote-ref" id="fnref:141" href="#fn:141">141</a>

<h3>Speciation</h3>
Main article: <a href="/facts/Speciation/pmRTQbTK">Speciation</a>
A species is a group of organisms that mate with one another and speciation is the process by which one lineage splits into two lineages as a result of having evolved independently from each other.<a class="footnote-ref" id="fnref:142" href="#fn:142">142</a> For speciation to occur, there has to be <a href="/facts/Reproductive_isolation/cVB4s7by">reproductive isolation</a>.<a class="footnote-ref" id="fnref:143" href="#fn:143">143</a> Reproductive isolation can result from incompatibilities between genes as described by <a href="/facts/Bateson%25E2%2580%2593Dobzhansky%25E2%2580%2593Muller_model/SSvg3LCG">Bateson–Dobzhansky–Muller model</a>. Reproductive isolation also tends to increase with <a href="/facts/Genetic_divergence/s4Hl2oe2">genetic divergence</a>. Speciation can occur when there are physical barriers that divide an ancestral species, a process known as <a href="/facts/Allopatric_speciation/H6K8mSDf">allopatric speciation</a>.<a class="footnote-ref" id="fnref:144" href="#fn:144">144</a>

<h3>Phylogeny</h3>
Main article: <a href="/facts/Phylogenetics/bw4rsovt">Phylogenetics</a>

A phylogeny is an evolutionary history of a specific group of organisms or their genes.<a class="footnote-ref" id="fnref:145" href="#fn:145">145</a> It can be represented using a <a href="/facts/Phylogenetic_tree/f5vVEpeY">phylogenetic tree</a>, a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a <a href="/facts/Lineage_(evolution)/wIChE4CL">lineage</a> of descendants of a particular species or population. When a lineage divides into two, it is represented as a fork or split on the phylogenetic tree.<a class="footnote-ref" id="fnref:146" href="#fn:146">146</a> Phylogenetic trees are the basis for comparing and grouping different species.<a class="footnote-ref" id="fnref:147" href="#fn:147">147</a> Different species that share a feature inherited from a common ancestor are described as having <a href="/facts/Homology_(biology)/muctKlyT">homologous</a> features (or <a href="/facts/Apomorphy_and_synapomorphy/t21GWRl1">synapomorphy</a>).<a class="footnote-ref" id="fnref:148" href="#fn:148">148</a><a class="footnote-ref" id="fnref:149" href="#fn:149">149</a><a class="footnote-ref" id="fnref:150" href="#fn:150">150</a> Phylogeny provides the basis of biological classification.<a class="footnote-ref" id="fnref:151" href="#fn:151">151</a> This classification system is rank-based, with the highest rank being the <a href="/facts/Domain_(biology)/RpP6V7n6">domain</a> followed by <a href="/facts/Kingdom_(biology)/QGRKbHos">kingdom</a>, <a href="/facts/Phylum/ELT3Jyax">phylum</a>, <a href="/facts/Class_(biology)/a7MrgFu3">class</a>, <a href="/facts/Order_(biology)/GKLJvIpt">order</a>, <a href="/facts/Family_(biology)/qTrn6E8E">family</a>, <a href="/facts/Genus/m8EFjSms">genus</a>, and <a href="/facts/Species/oDg6b96r">species</a>.<a class="footnote-ref" id="fnref:152" href="#fn:152">152</a> All organisms can be classified as belonging to one of <a href="/facts/Three-domain_system/np75fPUv">three domains</a>: Archaea (originally Archaebacteria), Bacteria (originally eubacteria), or Eukarya (includes the fungi, plant, and animal kingdoms).<a class="footnote-ref" id="fnref:153" href="#fn:153">153</a>

<h3>History of life</h3>
Main article: <a href="/facts/History_of_life/Ss6eB0EX">History of life</a>
The history of life on Earth traces how organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct, descended from a <a href="/facts/Last_universal_common_ancestor/pnpExNAU">last universal common ancestor</a> that lived about <a href="/facts/Timeline_of_evolution/ehf7lh2p">3.5 billion years ago</a>.<a class="footnote-ref" id="fnref:154" href="#fn:154">154</a><a class="footnote-ref" id="fnref:155" href="#fn:155">155</a> Geologists have developed a <a href="/facts/Geologic_time_scale/vBAm0D8Q">geologic time scale</a> that divides the history of the Earth into major divisions, starting with four eons (<a href="/facts/Hadean/j7LWS4ah">Hadean</a>, <a href="/facts/Archean/PHt8Sm8n">Archean</a>, <a href="/facts/Proterozoic/sSGXxqUU">Proterozoic</a>, and <a href="/facts/Phanerozoic/EKKuYnwl">Phanerozoic</a>), the first three of which are collectively known as the <a href="/facts/Precambrian/zjF3N3Rx">Precambrian</a>, which lasted approximately 4 billion years.<a class="footnote-ref" id="fnref:156" href="#fn:156">156</a> Each eon can be divided into eras, with the Phanerozoic eon that began 539 million years ago<a class="footnote-ref" id="fnref:157" href="#fn:157">157</a> being subdivided into <a href="/facts/Paleozoic/5qe2wz7U">Paleozoic</a>, <a href="/facts/Mesozoic/yYsvXGPa">Mesozoic</a>, and <a href="/facts/Cenozoic/yFslcz6n">Cenozoic</a> eras.<a class="footnote-ref" id="fnref:158" href="#fn:158">158</a> These three eras together comprise eleven <a href="/facts/Geologic_time_scale/vBAm0D8Q">periods</a> (<a href="/facts/Cambrian/jIfzKtod">Cambrian</a>, <a href="/facts/Ordovician/BjMrLIEA">Ordovician</a>, <a href="/facts/Silurian/1qx4epPg">Silurian</a>, <a href="/facts/Devonian/nWgakqzk">Devonian</a>, <a href="/facts/Carboniferous/BRKlkYyd">Carboniferous</a>, <a href="/facts/Permian/CYgLgu5u">Permian</a>, <a href="/facts/Triassic/r6CNxKXC">Triassic</a>, <a href="/facts/Jurassic/o5Cy4KzX">Jurassic</a>, <a href="/facts/Cretaceous/nwKtFtKx">Cretaceous</a>, <a href="/facts/Tertiary/Krcz3olj">Tertiary</a>, and <a href="/facts/Quaternary/3cEgHAqc">Quaternary</a>).<a class="footnote-ref" id="fnref:159" href="#fn:159">159</a>
The similarities among all known present-day <a href="/facts/Species/oDg6b96r">species</a> indicate that they have diverged through the process of <a href="/facts/Evolution/Hj0qAaBk">evolution</a> from their common ancestor.<a class="footnote-ref" id="fnref:160" href="#fn:160">160</a> Biologists regard the ubiquity of the <a href="/facts/Genetic_code/zUK0bY1B">genetic code</a> as evidence of universal <a href="/facts/Common_descent/bwEgMu7F">common descent</a> for all <a href="/facts/Bacterium/wC8xSCsU">bacteria</a>, <a href="/facts/Archaea/Adxh0aHw">archaea</a>, and <a href="/facts/Eukaryote/SkwyPLMA">eukaryotes</a>.<a class="footnote-ref" id="fnref:161" href="#fn:161">161</a><a class="footnote-ref" id="fnref:162" href="#fn:162">162</a><a class="footnote-ref" id="fnref:163" href="#fn:163">163</a><a class="footnote-ref" id="fnref:164" href="#fn:164">164</a> <a href="/facts/Microbial_mat/qVfYYzvW">Microbial mats</a> of coexisting bacteria and archaea were the dominant form of life in the early <a href="/facts/Archean/PHt8Sm8n">Archean</a> eon and many of the major steps in early evolution are thought to have taken place in this environment.<a class="footnote-ref" id="fnref:165" href="#fn:165">165</a> The earliest evidence of <a href="/facts/Eukaryote/SkwyPLMA">eukaryotes</a> dates from 1.85 billion years ago,<a class="footnote-ref" id="fnref:166" href="#fn:166">166</a><a class="footnote-ref" id="fnref:167" href="#fn:167">167</a> and while they may have been present earlier, their diversification accelerated when they started using oxygen in their <a href="/facts/Metabolism/WxDrm8k5">metabolism</a>. Later, around 1.7 billion years ago, <a href="/facts/Multicellular_organism/rk8QBF7o">multicellular organisms</a> began to appear, with <a href="/facts/Cellular_differentiation/bCVqUG7w">differentiated cells</a> performing specialised functions.<a class="footnote-ref" id="fnref:168" href="#fn:168">168</a>
Algae-like multicellular land plants are dated back to about 1 billion years ago,<a class="footnote-ref" id="fnref:169" href="#fn:169">169</a> although evidence suggests that <a href="/facts/Microorganism/pysJYcE2">microorganisms</a> formed the earliest <a href="/facts/Terrestrial_ecosystem/CsE4XjgP">terrestrial ecosystems</a>, at least 2.7 billion years ago.<a class="footnote-ref" id="fnref:170" href="#fn:170">170</a> Microorganisms are thought to have paved the way for the inception of land plants in the <a href="/facts/Ordovician/BjMrLIEA">Ordovician</a> period. Land plants were so successful that they are thought to have contributed to the <a href="/facts/Late_Devonian_extinction/gkThyMzz">Late Devonian extinction event</a>.<a class="footnote-ref" id="fnref:171" href="#fn:171">171</a>
<a href="/facts/Ediacara_biota/1lx1CXxT">Ediacara biota</a> appear during the <a href="/facts/Ediacaran/z8jSUITT">Ediacaran</a> period,<a class="footnote-ref" id="fnref:172" href="#fn:172">172</a> while <a href="/facts/Vertebrate/tT54FSTT">vertebrates</a>, along with most other modern <a href="/facts/Phylum/ELT3Jyax">phyla</a> originated about 525 million years ago during the <a href="/facts/Cambrian_explosion/Wg29M7PN">Cambrian explosion</a>.<a class="footnote-ref" id="fnref:173" href="#fn:173">173</a> During the Permian period, <a href="/facts/Synapsid/uzYVtB0F">synapsids</a>, including the ancestors of <a href="/facts/Mammal/phSwTzBo">mammals</a>, dominated the land,<a class="footnote-ref" id="fnref:174" href="#fn:174">174</a> but most of this group became extinct in the <a href="/facts/Permian%25E2%2580%2593Triassic_extinction_event/d8kI8Mfn">Permian–Triassic extinction event</a> 252 million years ago.<a class="footnote-ref" id="fnref:175" href="#fn:175">175</a> During the recovery from this catastrophe, <a href="/facts/Archosaur/Urd4MHfp">archosaurs</a> became the most abundant land vertebrates;<a class="footnote-ref" id="fnref:176" href="#fn:176">176</a> one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods.<a class="footnote-ref" id="fnref:177" href="#fn:177">177</a> After the <a href="/facts/Cretaceous%25E2%2580%2593Paleogene_extinction_event/HNyDuEcx">Cretaceous–Paleogene extinction event</a> 66 million years ago killed off the non-avian dinosaurs,<a class="footnote-ref" id="fnref:178" href="#fn:178">178</a> mammals <a href="/facts/Adaptive_radiation/NdoH2o1f">increased rapidly in size and diversity</a>.<a class="footnote-ref" id="fnref:179" href="#fn:179">179</a> Such <a href="/facts/Extinction_event/HloJubtR">mass extinctions</a> may have accelerated evolution by providing opportunities for new groups of organisms to diversify.<a class="footnote-ref" id="fnref:180" href="#fn:180">180</a>

<h2 id="diversity">Diversity</h2>
<h3>Bacteria and Archaea</h3>
Further information: <a href="/facts/Microbiology/k7udsq81">Microbiology</a>

Bacteria are a type of <a href="/facts/Cell_(biology)/bLM9UQvy">cell</a> that constitute a large <a href="/facts/Domain_(biology)/RpP6V7n6">domain</a> of <a href="/facts/Prokaryotic/oYyvaDDH">prokaryotic</a> <a href="/facts/Microorganism/pysJYcE2">microorganisms</a>. Typically a few <a href="/facts/Micrometre/7eDFxD2L">micrometers</a> in length, bacteria have a <a href="/facts/Bacterial_cell_structure/0UHPLKKF">number of shapes</a>, ranging from <a href="/facts/Coccus/51QVlEDC">spheres</a> to <a href="/facts/Bacillus_(shape)/0rGR5vvU">rods</a> and <a href="/facts/Spiral_bacteria/aK3qN2Xa">spirals</a>. Bacteria were among the first life forms to appear on Earth, and are present in most of its <a href="/facts/Habitat/zo1DN2gA">habitats</a>. Bacteria inhabit soil, water, <a href="/facts/Hot_spring/YrzFykH1">acidic hot springs</a>, <a href="/facts/Radioactive_waste/MjYh5tlu">radioactive waste</a>,<a class="footnote-ref" id="fnref:181" href="#fn:181">181</a> and the <a href="/facts/Deep_biosphere/K8wlFPzV">deep biosphere</a> of the <a href="/facts/Earth%2527s_crust/VSy2u31r">Earth's crust</a>. Bacteria also live in <a href="/facts/Symbiotic/MrBArRGa">symbiotic</a> and <a href="/facts/Parasitic/HWPcgky7">parasitic</a> relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the <a href="/facts/Bacterial_phyla/ChzCDhYl">bacterial phyla</a> have species that can be <a href="/facts/Microbiological_culture/y3VaXn9Q">grown</a> in the laboratory.<a class="footnote-ref" id="fnref:182" href="#fn:182">182</a>
<a href="/facts/Archaea/Adxh0aHw">Archaea</a> constitute the other domain of prokaryotic cells and were initially <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">classified</a> as bacteria, receiving the name archaebacteria (in the Archaebacteria <a href="/facts/Kingdom_(biology)/QGRKbHos">kingdom</a>), a term that has fallen out of use.<a class="footnote-ref" id="fnref:183" href="#fn:183">183</a> Archaeal cells have unique properties separating them from the other <a href="/facts/Three-domain_system/np75fPUv">two domains</a>, Bacteria and <a href="/facts/Eukaryote/SkwyPLMA">Eukaryota</a>. Archaea are further divided into multiple recognized <a href="/facts/Phylum/ELT3Jyax">phyla</a>. Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat and square cells of <a href="/facts/Haloquadratum_walsbyi/Wubz4Ubv">Haloquadratum walsbyi</a>.<a class="footnote-ref" id="fnref:184" href="#fn:184">184</a> Despite this <a href="/facts/Morphology_(biology)/Nl3B0myy">morphological</a> similarity to bacteria, archaea possess <a href="/facts/Gene/e1swwPY6">genes</a> and several <a href="/facts/Metabolic_pathway/8Py8HKJd">metabolic pathways</a> that are more closely related to those of eukaryotes, notably for the <a href="/facts/Enzyme/DSI1Fh7y">enzymes</a> involved in <a href="/facts/Transcription_(genetics)/bDgp9HDN">transcription</a> and <a href="/facts/Translation_(biology)/JcPC1rth">translation</a>. Other aspects of archaeal biochemistry are unique, such as their reliance on <a href="/facts/Ether_lipid/5oMHla5m">ether lipids</a> in their <a href="/facts/Cell_membrane/vAXSD0DT">cell membranes</a>,<a class="footnote-ref" id="fnref:185" href="#fn:185">185</a> including <a href="/facts/Archaeol/Wh30jfYG">archaeols</a>. Archaea use more energy sources than eukaryotes: these range from <a href="/facts/Organic_compounds/gqCNaN45">organic compounds</a>, such as sugars, to <a href="/facts/Ammonia/MtLtJFtz">ammonia</a>, <a href="/facts/Ion/3bePpDna">metal ions</a> or even <a href="/facts/Hydrogen/BoYHjl0J">hydrogen gas</a>. <a href="/facts/Halophile/UKBTAPls">Salt-tolerant</a> archaea (the <a href="/facts/Haloarchaea/8x5CYcB3">Haloarchaea</a>) use sunlight as an energy source, and other species of archaea <a href="/facts/Carbon_fixation/JqvfmCdH">fix carbon</a>, but unlike plants and <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a>, no known species of archaea does both. Archaea <a href="/facts/Asexual_reproduction/HpovW0VS">reproduce asexually</a> by <a href="/facts/Binary_fission/9Ed5vUr6">binary fission</a>, <a href="/facts/Fragmentation_(reproduction)/f0941CX2">fragmentation</a>, or <a href="/facts/Budding/TEB3zXxB">budding</a>; unlike bacteria, no known species of Archaea form <a href="/facts/Endospore/UKWvt6Qu">endospores</a>.
The first observed archaea were <a href="/facts/Extremophile/JGk1eYCk">extremophiles</a>, living in extreme environments, such as <a href="/facts/Hot_spring/YrzFykH1">hot springs</a> and <a href="/facts/Salt_lake/gEtjt4Rl">salt lakes</a> with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every <a href="/facts/Habitat/zo1DN2gA">habitat</a>, including soil, oceans, and <a href="/facts/Marshland/AG0WLJ2K">marshlands</a>. Archaea are particularly numerous in the oceans, and the archaea in <a href="/facts/Plankton/WGo3LU9T">plankton</a> may be one of the most abundant groups of organisms on the planet.
Archaea are a major part of <a href="/facts/Life/DNw7NUId">Earth's life</a>. They are part of the <a href="/facts/Microbiota/V4ymrawz">microbiota</a> of all organisms. In the <a href="/facts/Human_microbiome/xJIRMUAY">human microbiome</a>, they are important in the <a href="/facts/Colon_(anatomy)/vQtlNBv9">gut</a>, mouth, and on the skin.<a class="footnote-ref" id="fnref:186" href="#fn:186">186</a> Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and <a href="/facts/Syntrophy/WUHtX5sa">syntrophic</a> communities, for example.<a class="footnote-ref" id="fnref:187" href="#fn:187">187</a>

<h3>Eukaryotes</h3>
Main article: <a href="/facts/Eukaryote/SkwyPLMA">Eukaryote</a>

Eukaryotes are hypothesized to have split from archaea, which was followed by their <a href="/facts/Endosymbiont/eWvSS8ID">endosymbioses</a> with bacteria (or <a href="/facts/Symbiogenesis/2gHlgj4j">symbiogenesis</a>) that gave rise to mitochondria and chloroplasts, both of which are now part of modern-day eukaryotic cells.<a class="footnote-ref" id="fnref:188" href="#fn:188">188</a> The major lineages of eukaryotes diversified in the <a href="/facts/Precambrian/zjF3N3Rx">Precambrian</a> about 1.5 billion years ago and can be classified into eight major <a href="/facts/Clade/XefyYmWn">clades</a>: <a href="/facts/Alveolate/3TrXju5S">alveolates</a>, <a href="/facts/Excavata/qnRADFMd">excavates</a>, <a href="/facts/Stramenopile/6zmwHPEU">stramenopiles</a>, plants, <a href="/facts/Rhizaria/Jc0DxKyp">rhizarians</a>, <a href="/facts/Amoebozoa/6wBSrR9h">amoebozoans</a>, <a href="/facts/Fungus/BHo5DP59">fungi</a>, and animals.<a class="footnote-ref" id="fnref:189" href="#fn:189">189</a> Five of these clades are collectively known as <a href="/facts/Protist/NYVfaGVz">protists</a>, which are mostly microscopic <a href="/facts/Eukaryotic/SkwyPLMA">eukaryotic</a> organisms that are not plants, fungi, or animals.<a class="footnote-ref" id="fnref:190" href="#fn:190">190</a> While it is likely that protists share a <a href="/facts/Common_descent/bwEgMu7F">common ancestor</a> (the <a href="/facts/Last_eukaryotic_common_ancestor/SkwyPLMA">last eukaryotic common ancestor</a>),<a class="footnote-ref" id="fnref:191" href="#fn:191">191</a> protists by themselves do not constitute a separate clade as some protists may be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such as <a href="/facts/Algae/U1ftVKU3">algae</a>, <a href="/facts/Invertebrate/3CyqCVnN">invertebrates</a>, or <a href="/facts/Protozoa/JczTf25H">protozoans</a>, the protist grouping is not a formal taxonomic group but is used for convenience.<a class="footnote-ref" id="fnref:192" href="#fn:192">192</a><a class="footnote-ref" id="fnref:193" href="#fn:193">193</a> Most protists are unicellular; these are called microbial eukaryotes.<a class="footnote-ref" id="fnref:194" href="#fn:194">194</a>
Plants are mainly multicellular <a href="/facts/Organism/j8JkL09u">organisms</a>, predominantly <a href="/facts/Photosynthetic/TMV7w693">photosynthetic</a> eukaryotes of the <a href="/facts/Kingdom_(biology)/QGRKbHos">kingdom</a> Plantae, which would exclude fungi and some <a href="/facts/Algae/U1ftVKU3">algae</a>. Plant cells were derived by endosymbiosis of a <a href="/facts/Cyanobacterium/S8bKxIU9">cyanobacterium</a> into an early eukaryote about one billion years ago, which gave rise to chloroplasts.<a class="footnote-ref" id="fnref:195" href="#fn:195">195</a> The first several clades that emerged following primary endosymbiosis were aquatic and most of the aquatic photosynthetic eukaryotic organisms are collectively described as algae, which is a term of convenience as not all algae are closely related.<a class="footnote-ref" id="fnref:196" href="#fn:196">196</a> Algae comprise several distinct clades such as <a href="/facts/Glaucophyte/gTuqKfI3">glaucophytes</a>, which are microscopic freshwater algae that may have resembled in form to the early unicellular ancestor of Plantae.<a class="footnote-ref" id="fnref:197" href="#fn:197">197</a> Unlike glaucophytes, the other algal clades such as <a href="/facts/Red_algae/jCeIc7Kq">red</a> and <a href="/facts/Green_algae/3NlQ6pKF">green algae</a> are multicellular. Green algae comprise three major clades: <a href="/facts/Chlorophyte/hRL4moDT">chlorophytes</a>, <a href="/facts/Coleochaetophyceae/BHwwkhzX">coleochaetophytes</a>, and <a href="/facts/Stonewort/rr9rLEbV">stoneworts</a>.<a class="footnote-ref" id="fnref:198" href="#fn:198">198</a>
<a href="/facts/Fungus/BHo5DP59">Fungi</a> are eukaryotes that digest foods outside their bodies,<a class="footnote-ref" id="fnref:199" href="#fn:199">199</a> secreting digestive enzymes that break down large food molecules before absorbing them through their cell membranes. Many fungi are also <a href="/facts/Saprobe/sJkDfrJW">saprobes</a>, feeding on dead organic matter, making them important <a href="/facts/Decomposer/QufPBSGu">decomposers</a> in ecological systems.<a class="footnote-ref" id="fnref:200" href="#fn:200">200</a>
Animals are multicellular eukaryotes. With few exceptions, animals <a href="/facts/Heterotroph/wtzlgBtz">consume organic material</a>, <a href="/facts/Cellular_respiration/kb6gpsvL">breathe oxygen</a>, are <a href="/facts/Motility/Df5BECHL">able to move</a>, can <a href="/facts/Sexual_reproduction/hmsXy53s">reproduce sexually</a>, and grow from a hollow sphere of <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a>, the <a href="/facts/Blastula/sJVSBmbH">blastula</a>, during <a href="/facts/Embryogenesis/1lFY4EoW">embryonic development</a>. Over 1.5 million <a href="/facts/Extant_taxon/YK8uj7qU">living</a> animal <a href="/facts/Species/oDg6b96r">species</a> have been <a href="/facts/Species_description/JyFpxSvJ">described</a>—of which around 1 million are <a href="/facts/Insecta/IgDNvnUL">insects</a>—but it has been estimated there are over 7 million animal species in total. They have <a href="/facts/Ecology/YE8upuJU">complex interactions</a> with each other and their environments, forming intricate <a href="/facts/Food_web/YaakLo08">food webs</a>.<a class="footnote-ref" id="fnref:201" href="#fn:201">201</a>

<h3>Viruses</h3>
Main article: <a href="/facts/Virus/mf88Bcbl">Virus</a>

Viruses are submicroscopic <a href="/facts/Infectious_agent/axMXtV1M">infectious agents</a> that <a href="/facts/Viral_replication/btE3fu1w">replicate</a> inside the <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> of <a href="/facts/Organism/j8JkL09u">organisms</a>.<a class="footnote-ref" id="fnref:202" href="#fn:202">202</a> Viruses infect all types of <a href="/facts/Life_forms/AtpDM11f">life forms</a>, from animals and plants to <a href="/facts/Microorganism/pysJYcE2">microorganisms</a>, including bacteria and <a href="/facts/Archaea/Adxh0aHw">archaea</a>.<a class="footnote-ref" id="fnref:203" href="#fn:203">203</a><a class="footnote-ref" id="fnref:204" href="#fn:204">204</a> More than 6,000 <a href="/facts/Virus_species/oLn3H1Na">virus species</a> have been described in detail.<a class="footnote-ref" id="fnref:205" href="#fn:205">205</a> Viruses are found in almost every <a href="/facts/Ecosystem/kVE8mSmR">ecosystem</a> on Earth and are the most numerous type of biological entity.<a class="footnote-ref" id="fnref:206" href="#fn:206">206</a><a class="footnote-ref" id="fnref:207" href="#fn:207">207</a>
The origins of viruses in the evolutionary <a href="/facts/History_of_life/Ss6eB0EX">history of life</a> are unclear: some may have <a href="/facts/Evolution/Hj0qAaBk">evolved</a> from <a href="/facts/Plasmid/yf59JVlA">plasmids</a>—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of <a href="/facts/Horizontal_gene_transfer/FPyUIsf2">horizontal gene transfer</a>, which increases <a href="/facts/Genetic_diversity/GIymbUr8">genetic diversity</a> in a way analogous to <a href="/facts/Sexual_reproduction/hmsXy53s">sexual reproduction</a>.<a class="footnote-ref" id="fnref:208" href="#fn:208">208</a> Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life",<a class="footnote-ref" id="fnref:209" href="#fn:209">209</a> and as <a href="/facts/Self-replication/NbsfzT4l">self-replicators</a>.<a class="footnote-ref" id="fnref:210" href="#fn:210">210</a>

<h2 id="ecology">Ecology</h2>
Main article: <a href="/facts/Ecology/YE8upuJU">Ecology</a>
Ecology is the study of the distribution and abundance of life, the interaction between organisms and their <a href="/facts/Natural_environment/NzGY148M">environment</a>.<a class="footnote-ref" id="fnref:211" href="#fn:211">211</a>

<h3>Ecosystems</h3>
Main article: <a href="/facts/Ecosystem/kVE8mSmR">Ecosystem</a>
The <a href="/facts/Community_(ecology)/HnQwVCdz">community</a> of living (<a href="/facts/Biotic_component/fzEc9c8k">biotic</a>) organisms in conjunction with the nonliving (<a href="/facts/Abiotic_component/CA3Dwtta">abiotic</a>) components (e.g., water, light, radiation, temperature, <a href="/facts/Humidity/Nwb28VlT">humidity</a>, <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a>, <a href="/facts/Acidity/FySU18n1">acidity</a>, and soil) of their environment is called an <a href="/facts/Ecosystem/kVE8mSmR">ecosystem</a>.<a class="footnote-ref" id="fnref:212" href="#fn:212">212</a><a class="footnote-ref" id="fnref:213" href="#fn:213">213</a><a class="footnote-ref" id="fnref:214" href="#fn:214">214</a> These biotic and abiotic components are linked together through <a href="/facts/Nutrient_cycle/S57nuGbQ">nutrient cycles</a> and energy flows.<a class="footnote-ref" id="fnref:215" href="#fn:215">215</a> Energy from the sun enters the system through <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a> and is incorporated into plant tissue. By feeding on plants and on one another, animals move <a href="/facts/Matter/osyc04UN">matter</a> and energy through the system. They also influence the quantity of plant and <a href="/facts/Microbe/pysJYcE2">microbial</a> <a href="/facts/Biomass_(ecology)/A5rDsUax">biomass</a> present. By breaking down dead <a href="/facts/Organic_matter/t17tVCvk">organic matter</a>, <a href="/facts/Decomposer/QufPBSGu">decomposers</a> release <a href="/facts/Carbon/ukrgQexo">carbon</a> back to the atmosphere and facilitate <a href="/facts/Nutrient_cycling/S57nuGbQ">nutrient cycling</a> by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.<a class="footnote-ref" id="fnref:216" href="#fn:216">216</a>

<h3>Populations</h3>
Main article: <a href="/facts/Population_ecology/D6Sy5QfY">Population ecology</a>

A population is the group of <a href="/facts/Organism/j8JkL09u">organisms</a> of the same <a href="/facts/Species/oDg6b96r">species</a> that occupies an <a href="/facts/Geographical_area/RkjfcY6k">area</a> and <a href="/facts/Sexual_reproduction/hmsXy53s">reproduce</a> from generation to generation.<a class="footnote-ref" id="fnref:217" href="#fn:217">217</a><a class="footnote-ref" id="fnref:218" href="#fn:218">218</a><a class="footnote-ref" id="fnref:219" href="#fn:219">219</a><a class="footnote-ref" id="fnref:220" href="#fn:220">220</a><a class="footnote-ref" id="fnref:221" href="#fn:221">221</a> <a href="/facts/Population_size/euKMjN9h">Population size</a> can be estimated by multiplying population density by the area or volume. The <a href="/facts/Carrying_capacity/7KNgGmIv">carrying capacity</a> of an <a href="/facts/Natural_environment/NzGY148M">environment</a> is the maximum population size of a <a href="/facts/Species/oDg6b96r">species</a> that can be sustained by that specific environment, given the food, <a href="/facts/Habitat/zo1DN2gA">habitat</a>, <a href="/facts/Drinking_water/FmSJcKGG">water</a>, and other <a href="/facts/Resource/NM1retdD">resources</a> that are available.<a class="footnote-ref" id="fnref:222" href="#fn:222">222</a> The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability of resources and the cost of maintaining them. In <a href="/facts/Human_population/tzYcheuD">human populations</a>, new <a href="/facts/Technology/9Uav5WVR">technologies</a> such as the <a href="/facts/Green_revolution/sB1gbDxN">Green revolution</a> have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the most famous of which was by <a href="/facts/Thomas_Robert_Malthus/fNoDNncG">Thomas Malthus</a> in the 18th century.<a class="footnote-ref" id="fnref:223" href="#fn:223">223</a>

<h3>Communities</h3>
Main article: <a href="/facts/Community_(ecology)/HnQwVCdz">Community (ecology)</a>

A community is a group of populations of species occupying the same geographical area at the same time.<a class="footnote-ref" id="fnref:224" href="#fn:224">224</a> A <a href="/facts/Biological_interaction/uhcg3EaM">biological interaction</a> is the effect that a pair of <a href="/facts/Organism/j8JkL09u">organisms</a> living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, like <a href="/facts/Pollination/u383g7XJ">pollination</a> and <a href="/facts/Predation/JEGdrX3f">predation</a>, or long-term; both often strongly influence the <a href="/facts/Evolution/Hj0qAaBk">evolution</a> of the species involved. A long-term interaction is called a <a href="/facts/Symbiosis/MrBArRGa">symbiosis</a>. Symbioses range from <a href="/facts/Mutualism_(biology)/29hCDDS2">mutualism</a>, beneficial to both partners, to <a href="/facts/Competition_(biology)/3xN0IynL">competition</a>, harmful to both partners.<a class="footnote-ref" id="fnref:225" href="#fn:225">225</a> Every species participates as a consumer, resource, or both in <a href="/facts/Consumer%25E2%2580%2593resource_interactions/vy6f9kBb">consumer–resource interactions</a>, which form the core of <a href="/facts/Food_chain/mvDKQTGu">food chains</a> or <a href="/facts/Food_web/YaakLo08">food webs</a>.<a class="footnote-ref" id="fnref:226" href="#fn:226">226</a> There are different <a href="/facts/Trophic_level/tHMpgkUC">trophic levels</a> within any food web, with the lowest level being the primary producers (or <a href="/facts/Autotroph/UDc8bzqD">autotrophs</a>) such as plants and algae that convert energy and inorganic material into <a href="/facts/Organic_compound/gqCNaN45">organic compounds</a>, which can then be used by the rest of the community.<a class="footnote-ref" id="fnref:227" href="#fn:227">227</a><a class="footnote-ref" id="fnref:228" href="#fn:228">228</a><a class="footnote-ref" id="fnref:229" href="#fn:229">229</a> At the next level are the <a href="/facts/Heterotroph/wtzlgBtz">heterotrophs</a>, which are the species that obtain energy by breaking apart organic compounds from other organisms.<a class="footnote-ref" id="fnref:230" href="#fn:230">230</a> Heterotrophs that consume plants are primary consumers (or <a href="/facts/Herbivore/me4YFdQA">herbivores</a>) whereas heterotrophs that consume herbivores are secondary consumers (or <a href="/facts/Carnivore/q6h6aSqe">carnivores</a>). And those that eat secondary consumers are tertiary consumers and so on. <a href="/facts/Omnivore/b8zcsuJF">Omnivorous</a> heterotrophs are able to consume at multiple levels. Finally, there are <a href="/facts/Decomposer/QufPBSGu">decomposers</a> that feed on the waste products or dead bodies of organisms.<a class="footnote-ref" id="fnref:231" href="#fn:231">231</a>
On average, the total amount of energy incorporated into the <a href="/facts/Biomass/4VHjaya3">biomass</a> of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level.<a class="footnote-ref" id="fnref:232" href="#fn:232">232</a>

<h3>Biosphere</h3>
Main article: <a href="/facts/Biosphere/P1ltA4sb">Biosphere</a>

In the global ecosystem or biosphere, matter exists as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations.<a class="footnote-ref" id="fnref:233" href="#fn:233">233</a> For example, matter from terrestrial autotrophs are both biotic and accessible to other organisms whereas the matter in rocks and minerals are abiotic and inaccessible. A <a href="/facts/Biogeochemical_cycle/3vG5KBu4">biogeochemical cycle</a> is a pathway by which specific <a href="/facts/Chemical_element/0jducgWD">elements</a> of matter are turned over or moved through the biotic (<a href="/facts/Biosphere/P1ltA4sb">biosphere</a>) and the abiotic (<a href="/facts/Lithosphere/1mqaxASd">lithosphere</a>, <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a>, and <a href="/facts/Hydrosphere/RbJmcSX6">hydrosphere</a>) compartments of Earth. There are biogeochemical cycles for <a href="/facts/Nitrogen_cycle/0X1z74dN">nitrogen</a>, <a href="/facts/Carbon_cycle/gVusRaPF">carbon</a>, and <a href="/facts/Water_cycle/wALHdKjJ">water</a>.

<h3>Conservation</h3>
Main article: <a href="/facts/Conservation_biology/S2693mIF">Conservation biology</a>
Conservation biology is the study of the conservation of Earth's <a href="/facts/Biodiversity/GruakEzs">biodiversity</a> with the aim of protecting <a href="/facts/Species/oDg6b96r">species</a>, their <a href="/facts/Habitats/zo1DN2gA">habitats</a>, and <a href="/facts/Ecosystems/kVE8mSmR">ecosystems</a> from excessive rates of <a href="/facts/Extinction/24p5NP2q">extinction</a> and the erosion of biotic interactions.<a class="footnote-ref" id="fnref:234" href="#fn:234">234</a><a class="footnote-ref" id="fnref:235" href="#fn:235">235</a><a class="footnote-ref" id="fnref:236" href="#fn:236">236</a> It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engender <a href="/facts/Genetics/bu8wne0A">genetic</a>, population, <a href="/facts/Species/oDg6b96r">species</a>, and ecosystem diversity.<a class="footnote-ref" id="fnref:237" href="#fn:237">237</a><a class="footnote-ref" id="fnref:238" href="#fn:238">238</a><a class="footnote-ref" id="fnref:239" href="#fn:239">239</a><a class="footnote-ref" id="fnref:240" href="#fn:240">240</a> The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years,<a class="footnote-ref" id="fnref:241" href="#fn:241">241</a> which has contributed to poverty, starvation, and will reset the course of evolution on this planet.<a class="footnote-ref" id="fnref:242" href="#fn:242">242</a><a class="footnote-ref" id="fnref:243" href="#fn:243">243</a> <a href="/facts/Biodiversity/GruakEzs">Biodiversity</a> affects the functioning of ecosystems, which provide a variety of <a href="/facts/Ecosystem_services/Fm4owYGV">services</a> upon which people depend. Conservation biologists research and educate on the trends of <a href="/facts/Biodiversity_loss/p7xI1P0G">biodiversity loss</a>, species <a href="/facts/Extinction/24p5NP2q">extinctions</a>, and the negative effect these are having on our capabilities to <a href="/facts/Sustainability/9oYH9mAs">sustain</a> the well-being of human society. Organizations and citizens are responding to the <a href="/facts/Holocene_extinction/C1LzDDy7">current biodiversity crisis</a> through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales.<a class="footnote-ref" id="fnref:244" href="#fn:244">244</a><a class="footnote-ref" id="fnref:245" href="#fn:245">245</a><a class="footnote-ref" id="fnref:246" href="#fn:246">246</a><a class="footnote-ref" id="fnref:247" href="#fn:247">247</a>

<h2 id="see-also">See also</h2>

<ul><li><a href="/facts/Biology_in_fiction/UR0IGqsN">Biology in fiction</a></li>
<li><a href="/facts/Glossary_of_biology/UEPUhSN8">Glossary of biology</a></li>
<li><a href="/facts/Idiobiology/m859p1wN">Idiobiology</a></li>
<li><a href="/facts/List_of_biological_websites/rkLHkvWb">List of biological websites</a></li>
<li><a href="/facts/List_of_biologists/HsCGQLFI">List of biologists</a></li>
<li><a href="/facts/List_of_biology_journals/7YLnrGCw">List of biology journals</a></li>
<li><a href="/facts/List_of_biology_topics/pI02Mcpl">List of biology topics</a></li>
<li><a href="/facts/List_of_life_sciences/9HYkCgqe">List of life sciences</a></li>
<li><a href="/facts/List_of_omics_topics_in_biology/aNfaxHGB">List of omics topics in biology</a></li>
<li><a href="/facts/National_Association_of_Biology_Teachers/aKhb7Koo">National Association of Biology Teachers</a></li>
<li><a href="/facts/Outline_of_biology/pI02Mcpl">Outline of biology</a></li>
<li>Periodic table of life sciences in <a href="/facts/Tinbergen%2527s_four_questions/2yMa03CF">Tinbergen's four questions</a></li>
<li><a href="/facts/Science_tourism/lfCLGHtj">Science tourism</a></li>
<li>Terminology of biology</li></ul>

<h2 id="further-reading">Further reading</h2>
Further information: <a href="/facts/Bibliography_of_biology/Bh1kqqUf">Bibliography of biology</a>

<ul><li>Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walter, P. (2002). <a href="https://archive.org/details/molecularbiolog000wils">Molecular Biology of the Cell</a> (4th ed.). Garland. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-8153-3218-3. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/145080076">145080076</a>.</li>
<li>Begon, M.; Townsend, C. R.; Harper, J. L. (2005). <a href="/facts/Ecology%3a_From_Individuals_to_Ecosystems/KdYaWqD3">Ecology: From Individuals to Ecosystems</a> (4th ed.). Blackwell Publishing Limited. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-4051-1117-1. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/57639896">57639896</a>.</li>
<li><a href="/facts/Neil_Campbell_(scientist)/RnqTFrL4">Campbell, Neil</a> (2004). Biology (7th ed.). Benjamin-Cummings Publishing Company. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-8053-7146-8. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/71890442">71890442</a>.</li>
<li><a href="/facts/Paul_Colinvaux/CtC5Jsuh">Colinvaux, Paul</a> (1979). <a href="https://archive.org/details/whybigfierceanim00paul">Why Big Fierce Animals are Rare: An Ecologist's Perspective</a> (reissue ed.). Princeton University Press. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-691-02364-9. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/10081738">10081738</a>.</li>
<li>Mayr, Ernst (1982). <a href="https://books.google.com/books?id=pHThtE2R0UQC">The Growth of Biological Thought: Diversity, Evolution, and Inheritance</a>. Harvard University Press. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-674-36446-2. <a href="https://web.archive.org/web/20151003080726/https://books.google.com/books?id=pHThtE2R0UQC">Archived</a> from the original on 2015-10-03. Retrieved 2015-06-27.</li>
<li>Hoagland, Mahlon (2001). The Way Life Works. Jones and Bartlett Publishers inc. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-7637-1688-2. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/223090105">223090105</a>.</li>
<li>Janovy, John (2004). On Becoming a Biologist (2nd ed.). Bison Books. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-8032-7620-8. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/55138571">55138571</a>.</li>
<li><a href="/facts/Johnson_George_B./QkJwfcaP">Johnson, George B.</a> (2005). <a href="https://archive.org/details/holtbiologyvisua00john">Biology, Visualizing Life</a>. Holt, Rinehart, and Winston. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-03-016723-2. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/36306648">36306648</a>.</li>
<li>Tobin, Allan; Dusheck, Jennie (2005). Asking About Life (3rd ed.). Belmont, California: Wadsworth. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-534-40653-0.</li></ul>

<h2 id="external-links">External links</h2>

<ul><li><a href="http://phylonames.org/code/">OSU's Phylocode</a></li>
<li><a href="https://www.biologyonline.com/dictionary">Biology Online – Wiki Dictionary</a></li>
<li><a href="https://ocw.mit.edu/courses/biology/7-012-introduction-to-biology-fall-2004/">MIT video lecture series on biology</a></li>
<li><a href="https://www.onezoom.org/">OneZoom Tree of Life</a></li>
<li><a href="https://link.springer.com/journal/10739">Journal of the History of Biology (springer.com)</a></li></ul>
Journal links

<ul><li><a href="/facts/PLOS_One/fTQQQbTr">PLOS ONE</a></li>
<li><a href="https://journals.plos.org/plosbiology/">PLOS Biology</a> A peer-reviewed, open-access journal published by the <a href="/facts/Public_Library_of_Science/HycfC8Ph">Public Library of Science</a></li>
<li><a href="http://www.cell.com/current-biology/">Current Biology</a>: General journal publishing <a href="/facts/Original_research/mo8joeTL">original research</a> from all areas of biology</li>
<li><a href="https://royalsocietypublishing.org/journal/rsbl">Biology Letters</a>: A <a href="/facts/Impact_factor/CamGH5MQ">high-impact</a> <a href="/facts/Royal_Society/KI5yXCuV">Royal Society</a> journal publishing <a href="/facts/Peer-review/ly6yEgcW">peer-reviewed</a> biology papers of general interest</li>
<li><a href="https://www.science.org/collections">Science</a>: Internationally renowned <a href="/facts/American_Association_for_the_Advancement_of_Science/zfWYugCg">AAAS</a> science journal – see sections of the life sciences</li>
<li><a href="https://www.ijbs.com/">International Journal of Biological Sciences</a>: A biological journal publishing significant peer-reviewed scientific papers</li>
<li><a href="https://www.press.jhu.edu/journals/perspectives-biology-and-medicine">Perspectives in Biology and Medicine</a>: An <a href="/facts/Interdisciplinarity/HP0uaNUs">interdisciplinary</a> <a href="/facts/Scholarly_method/W4eDafUn">scholarly</a> journal publishing essays of broad relevance</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Modell, Harold; Cliff, William; Michael, Joel; McFarland, Jenny; Wenderoth, Mary Pat; Wright, Ann (December 2015). "A physiologist's view of homeostasis". Advances in Physiology Education. 39 (4): 259–266. doi:10.1152/advan.00107.2015. ISSN 1043-4046. PMC 4669363. PMID 26628646. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4669363" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4669363</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Davies, PC; Rieper, E; Tuszynski, JA (January 2013). "Self-organization and entropy reduction in a living cell". Bio Systems. 111 (1): 1–10. Bibcode:2013BiSys.111....1D. doi:10.1016/j.biosystems.2012.10.005. PMC 3712629. PMID 23159919. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712629" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712629</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Pearce, Ben K.D.; Tupper, Andrew S.; Pudritz, Ralph E.; et al. (March 1, 2018). "Constraining the Time Interval for the Origin of Life on Earth". Astrobiology. 18 (3): 343–364. arXiv:1808.09460. Bibcode:2018AsBio..18..343P. doi:10.1089/ast.2017.1674. PMID 29570409. S2CID 4419671. <a href="/wiki/Ralph_Pudritz" target="_blank">/wiki/Ralph_Pudritz</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Lindberg, David C. (2007). "Science before the Greeks". The beginnings of Western science: the European Scientific tradition in philosophical, religious, and institutional context (2nd ed.). Chicago, Illinois: University of Chicago Press. pp. 1–20. ISBN 978-0-226-48205-7. <a href="978-0-226-48205-7" target="_blank">978-0-226-48205-7</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Grant, Edward (2007). "Ancient Egypt to Plato". A History of Natural Philosophy: From the Ancient World to the Nineteenth Century. New York: Cambridge University Press. pp. 1–26. ISBN 978-052-1-68957-1. <a href="978-052-1-68957-1" target="_blank">978-052-1-68957-1</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Handbook of the Historiography of Biology. Historiographies of Science. 2021. doi:10.1007/978-3-319-90119-0. ISBN 978-3-319-90118-3. <a href="978-3-319-90118-3" target="_blank">978-3-319-90118-3</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Lindberg, David C. (2007). "Science before the Greeks". The beginnings of Western science: the European Scientific tradition in philosophical, religious, and institutional context (2nd ed.). Chicago, Illinois: University of Chicago Press. pp. 1–20. ISBN 978-0-226-48205-7. <a href="978-0-226-48205-7" target="_blank">978-0-226-48205-7</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Grant, Edward (2007). "Ancient Egypt to Plato". A History of Natural Philosophy: From the Ancient World to the Nineteenth Century. New York: Cambridge University Press. pp. 1–26. ISBN 978-052-1-68957-1. <a href="978-052-1-68957-1" target="_blank">978-052-1-68957-1</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Magner, Lois N. (2002). A History of the Life Sciences, Revised and Expanded. CRC Press. ISBN 978-0-203-91100-6. Archived from the original on 2015-03-24. <a href="978-0-203-91100-6" target="_blank">978-0-203-91100-6</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Serafini, Anthony (2013). The Epic History of Biology. Springer. ISBN 978-1-4899-6327-7. Archived from the original on 15 April 2021. Retrieved 14 July 2015. <a href="978-1-4899-6327-7" target="_blank">978-1-4899-6327-7</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Morange, Michel. 2021. A History of Biology. Princeton, NJ: Princeton University Press. Translated by Teresa Lavender Fagan and Joseph Muise. <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Theophrastus". Encyclopædia Britannica (11th ed.). Cambridge University Press. <a href="/wiki/Public_domain" target="_blank">/wiki/Public_domain</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Fahd, Toufic (1996). "Botany and agriculture". In Morelon, Régis; Rashed, Roshdi (eds.). Encyclopedia of the History of Arabic Science. Vol. 3. Routledge. p. 815. ISBN 978-0-415-12410-2. <a href="978-0-415-12410-2" target="_blank">978-0-415-12410-2</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Magner, Lois N. (2002). A History of the Life Sciences, Revised and Expanded. CRC Press. pp. 133–44. ISBN 978-0-203-91100-6. Archived from the original on 2015-03-24. <a href="978-0-203-91100-6" target="_blank">978-0-203-91100-6</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Sapp, Jan (2003). "7". Genesis: The Evolution of Biology. New York: Oxford University Press. ISBN 978-0-19-515618-8. <a href="978-0-19-515618-8" target="_blank">978-0-19-515618-8</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Coleman, William (1977). Biology in the Nineteenth Century: Problems of Form, Function, and Transformation. New York: Cambridge University Press. ISBN 978-0-521-29293-1. <a href="978-0-521-29293-1" target="_blank">978-0-521-29293-1</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Mayr, Ernst. The Growth of Biological Thought, chapter 4 <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">Mayr, Ernst. The Growth of Biological Thought, chapter 7 <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Gould, Stephen Jay. The Structure of Evolutionary Theory. The Belknap Press of Harvard University Press: Cambridge, 2002. ISBN 0-674-00613-5. p. 187. <a href="/wiki/Stephen_Jay_Gould" target="_blank">/wiki/Stephen_Jay_Gould</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Mayr, Ernst. The Growth of Biological Thought, chapter 10: "Darwin's evidence for evolution and common descent"; and chapter 11: "The causation of evolution: natural selection" <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Larson, Edward J. (2006). "Ch. 3". Evolution: The Remarkable History of a Scientific Theory. Random House Publishing Group. ISBN 978-1-58836-538-5. Archived from the original on 2015-03-24. <a href="978-1-58836-538-5" target="_blank">978-1-58836-538-5</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Henig (2000). Op. cit. pp. 134–138. <a href="http://archive.org/details/monkingardenlost00heni" target="_blank">http://archive.org/details/monkingardenlost00heni</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Miko, Ilona (2008). "Gregor Mendel's principles of inheritance form the cornerstone of modern genetics. So just what are they?". Nature Education. 1 (1): 134. Archived from the original on 2019-07-19. Retrieved 2021-05-13. <a href="https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/" target="_blank">https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">Futuyma, Douglas J.; Kirkpatrick, Mark (2017). "Evolutionary Biology". Evolution (4th ed.). Sunderland, Mass.: Sinauer Associates. pp. 3–26. <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">Noble, Ivan (2003-04-14). "Human genome finally complete". BBC News. Archived from the original on 2006-06-14. Retrieved 2006-07-22. <a href="https://news.bbc.co.uk/2/hi/science/nature/2940601.stm" target="_blank">https://news.bbc.co.uk/2/hi/science/nature/2940601.stm</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "The chemical context of life". Campbell Biology (11th ed.). New York: Pearson. pp. 28–43. ISBN 978-0134093413. <a href="978-0134093413" target="_blank">978-0134093413</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
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<li id="fn:28">Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily; Carmichael, Jeff (2017). "Water and carbon: The chemical basis of life". Biological Science (6th ed.). Hoboken, N.J.: Pearson. pp. 55–77. ISBN 978-0321976499. <a href="978-0321976499" target="_blank">978-0321976499</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
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Biology open-in-new

Biology