Biology

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Biology is the science of life. It spans multiple levels from biomolecules and cells to organisms and populations.
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Biology
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Applications

Biology is the

reproduce.[1][2][3] Finally, all organisms are able to regulate their own internal environments.[1][2][3][4][5]

Biologists are able to study life at multiple levels of organization,[1] from the molecular biology of a cell to the anatomy and physiology of plants and animals, and evolution of populations.[1][6] Hence, there are multiple subdisciplines within biology, each defined by the nature of their research questions and the tools that they use.[7][8][9] Like other scientists, biologists use the scientific method to make observations, pose questions, generate hypotheses, perform experiments, and form conclusions about the world around them.[1]

Life on Earth, which emerged more than 3.7 billion years ago,

biophysical environment
.

History

Main article: History of biology
A drawing of a fly from facing up, with wing detail
Diagram of a fly from Robert Hooke's innovative Micrographia, 1665

The earliest of roots of science, which included medicine, can be traced to

Islamic scholars
working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought.

Biology began to quickly develop with

spermatozoa, bacteria, infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop techniques of microscopic dissection and staining.[17] Advances in microscopy had a profound impact on biological thinking. In the early 19th century, biologists pointed to the central importance of the cell. In 1838, Schleiden and Schwann 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 spontaneous generation. However, Robert Remak and Rudolf Virchow were able to reify the third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into cell theory.[18][19]

Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735, and in the 1750s introduced scientific names for all his species.[20] Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent.[21]

In 1842, Charles Darwin penned his first sketch of On the Origin of Species.[22]

Serious evolutionary thinking originated with the works of

Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, forged a more successful evolutionary theory based on natural selection; similar reasoning and evidence led Alfred Russel Wallace to independently reach the same conclusions.[24][25]

The basis for modern genetics began with the work of

codons. The Human Genome Project was launched in 1990 to map the human genome.[29]

Chemical basis

Atoms and molecules

Further information: Chemistry

All organisms are made up of chemical elements;[30] oxygen, carbon, hydrogen, and nitrogen account for most (96%) of the mass of all organisms, with calcium, phosphorus, sulfur, sodium, chlorine, and magnesium constituting essentially all the remainder. Different elements can combine to form compounds such as water, which is fundamental to life.[30] Biochemistry is the study of chemical processes within and relating to living organisms. Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including molecular synthesis, modification, mechanisms, and interactions.

Water

See also:
Circumstellar habitable zone, and Water distribution on Earth
Model of hydrogen bonds (1) between molecules of water

Life arose from the Earth's first ocean, which formed some 3.8 billion years ago.

hydroxyl ions before reforming into a water molecule again.[31] In pure water, the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a pH
that is neutral.

Organic compounds

Further information: Organic chemistry
Organic compounds such as glucose are vital to organisms.

Organic compounds are molecules that contain carbon bonded to another element such as hydrogen.[31] With the exception of water, nearly all the molecules that make up each organism contain carbon.[31][32] Carbon can form covalent bonds with up to four other atoms, enabling it to form diverse, large, and complex molecules.[31][32] For example, a single carbon atom can form four single covalent bonds such as in methane, two double covalent bonds such as in carbon dioxide (CO2), or a triple covalent bond such as in carbon monoxide (CO). Moreover, carbon can form very long chains of interconnecting carbon–carbon bonds such as octane or ring-like structures such as glucose.

The simplest form of an organic molecule is the hydrocarbon, 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.[31] Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called functional groups.[31] There are six prominent functional groups that can be found in organisms: amino group, carboxyl group, carbonyl group, hydroxyl group, phosphate group, and sulfhydryl group.[31]

In 1953, the Miller–Urey experiment showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions of early Earth, thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (see abiogenesis).[33][31]

Macromolecules

Main article: Macromolecule
The (a) primary, (b) secondary, (c) tertiary, and (d) quaternary structures of a hemoglobin protein

Macromolecules are large molecules made up of smaller subunits or monomers.[34] Monomers include sugars, amino acids, and nucleotides.[35] Carbohydrates include monomers and polymers of sugars.[36] Lipids are the only class of macromolecules that are not made up of polymers. They include steroids, phospholipids, and fats,[35] largely nonpolar and hydrophobic (water-repelling) substances.[37] Proteins are the most diverse of the macromolecules. They include

structural proteins. The basic unit (or monomer) of a protein is an amino acid.[34] Twenty amino acids are used in proteins.[34]
Nucleic acids are polymers of nucleotides.[38] Their function is to store, transmit, and express hereditary information.[35]

Cells

Main article: Cell (biology)

egg
.

Cell structure

Structure of an animal cell depicting various organelles

Every cell is enclosed within a

cell signalling and serve as the attachment surface for several extracellular structures such as a cell wall, glycocalyx, and cytoskeleton
.

Structure of a plant cell

Within the cytoplasm of a cell, there are many biomolecules such as

animal cells such as a cell wall that provides support for the plant cell, chloroplasts that harvest sunlight energy to produce sugar, and vacuoles that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds.[46] Eukaryotic cells also have cytoskeleton that is made up of microtubules, intermediate filaments, and microfilaments, all of which provide support for the cell and are involved in the movement of the cell and its organelles.[46] In terms of their structural composition, the microtubules are made up of tubulin (e.g., α-tubulin and β-tubulin whereas intermediate filaments are made up of fibrous proteins.[46] Microfilaments are made up of actin molecules that interact with other strands of proteins.[46]

Metabolism

Further information: Bioenergetics
Example of an enzyme-catalysed exothermic reaction

All cells require energy to sustain cellular processes.

reactants into products. Enzymes also allow the regulation
of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.

Cellular respiration

Main article: Cellular respiration
Respiration in a eukaryotic cell

Cellular respiration is a set of metabolic reactions and processes that take place in cells to convert

combustion reaction
, 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: glycolysis, citric acid cycle (or Krebs cycle), electron transport chain, and oxidative phosphorylation.[48] Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates, with two net molecules of ATP being produced at the same time.[48] Each pyruvate is then oxidized into acetyl-CoA by the pyruvate dehydrogenase complex, which also generates NADH 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 mitochondrial cristae. Oxidative phosphorylation comprises the electron transport chain, which is a series of four protein complexes 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 (chemiosmosis), which generates a proton motive force.[48] Energy from the proton motive force drives the enzyme ATP synthase to synthesize more ATPs by phosphorylating ADPs. The transfer of electrons terminates with molecular oxygen being the final electron acceptor.

If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to waste products 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 lactic acid. This type of fermentation is called lactic acid fermentation. 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 ethanol fermentation. The ATP generated in this process is made by substrate-level phosphorylation, which does not require oxygen.

Photosynthesis

Main article: Photosynthesis
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.

Photosynthesis is a process used by plants and other organisms to

oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.[52]

Photosynthesis has four stages:

carbon fixation.[48] Light absorption is the initial step of photosynthesis whereby light energy is absorbed by chlorophyll pigments attached to proteins in the thylakoid membranes. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a quinone 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 photosystem I (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.[48]

During the third stage of photosynthesis, the movement of protons down their

ribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle.[53]

Cell signaling

Main article: Cell signaling

Cell signaling (or communication) is the ability of

excitability of a target cell. Other types of receptors include protein kinase receptors (e.g., receptor for the hormone insulin) and G protein-coupled receptors. Activation of G protein-coupled receptors can initiate second messenger cascades. The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events is called signal transduction

Cell cycle

In meiosis, the chromosomes duplicate and the homologous chromosomes exchange genetic information during meiosis I. The daughter cells divide again in meiosis II to form haploid gametes.
Main article: Cell cycle

The cell cycle is a series of events that take place in a

internal organs are renewed. After cell division, each of the daughter cells begin the interphase 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.[60] Homologous chromosomes are separated in the first division (meiosis I), and sister chromatids are separated in the second division (meiosis II
). 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.

binary fission). Unlike the processes of mitosis and meiosis in eukaryotes, binary fission takes in prokaryotes takes place without the formation of a spindle apparatus 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 FtsZ polymerization and "Z-ring" formation)[61] The new cell wall (septum) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes, and plasmids
.

Sexual reproduction and meiosis

Meiosis is a central feature of sexual reproduction in eukaryotes, and the most fundamental function of meiosis appears to be conservation of the integrity of the genome that is passed on to progeny by parents.[62][63] Two aspects of sexual reproduction, meiotic recombination and outcrossing, are likely maintained respectively by the adaptive advantages of recombinational repair of genomic DNA damage and genetic complementation which masks the expression of deleterious recessive mutations.[64]

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[65] 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.” Genetic variation, often produced as a byproduct of sexual reproduction, may provide long-term advantages to those sexual lineages that engage in outcrossing.[64]

Genetics

Inheritance

Main article: Classical genetics
Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms

Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, the law of independent assortment, 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 sex-linked. Test crosses can be performed to experimentally determine the underlying genotype of an organism with a dominant phenotype.[69] A Punnett square can be used to predict the results of a test cross. The chromosome theory of inheritance, which states that genes are found on chromosomes, was supported by Thomas Morgans's experiments with fruit flies, which established the sex linkage between eye color and sex in these insects.[70]

Genes and DNA

Further information: Gene and DNA
Bases lie between two spiraling DNA strands.

A gene is a unit of heredity 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 polynucleotide chains that coil around each other to form a double helix.[71] It is found as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell is collectively known as its genome. In eukaryotes, DNA is mainly in the cell nucleus.[72] In prokaryotes, the DNA is held within the nucleoid.[73] The genetic information is held within genes, and the complete assemblage in an organism is called its genotype.[74] DNA replication is a semiconservative process whereby each strand serves as a template for a new strand of DNA.[71] Mutations are heritable changes in DNA.[71] They can arise spontaneously as a result of replication errors that were not corrected by proofreading or can be induced by an environmental mutagen such as a chemical (e.g., nitrous acid, benzopyrene) or radiation (e.g., x-ray, gamma ray, ultraviolet radiation, particles emitted by unstable isotopes).[71] Mutations can lead to phenotypic effects such as loss-of-function, gain-of-function, and conditional mutations.[71] Some mutations are beneficial, as they are a source of genetic variation for evolution.[71] Others are harmful if they were to result in a loss of function of genes needed for survival.[71]

Gene expression

The extended central dogma of molecular biology includes all the processes involved in the flow of genetic information.
Main article: Gene expression

Gene expression is the molecular process by which a

translation (RNA to protein).[78]

Gene regulation

Main article: Regulation of gene expression

The regulation of gene expression by environmental factors and during different stages of development can occur at each step of the process such as transcription, RNA splicing, translation, and post-translational modification of a protein.[79] Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins called transcription factors bind to the DNA sequence close to or at a promoter.[79] A cluster of genes that share the same promoter is called an operon, found mainly in prokaryotes and some lower eukaryotes (e.g., Caenorhabditis elegans).[79][80] In positive regulation of gene expression, the activator 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 repressor binds to a DNA sequence called an operator, which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds called inducers (e.g., allolactose), thereby allowing transcription to occur.[79] Specific genes that can be activated by inducers are called inducible genes, in contrast to constitutive genes that are almost constantly active.[79] In contrast to both, structural genes encode proteins that are not involved in gene regulation.[79] In addition to regulatory events involving the promoter, gene expression can also be regulated by epigenetic changes to chromatin, which is a complex of DNA and protein found in eukaryotic cells.[79]

Genes, development, and evolution

Main article: Evolutionary developmental biology

types of cells and proliferate indefinitely to produce more of the same stem cell.[84] Cellular differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals, which are largely due to highly controlled modifications in gene expression and epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself.[85] Thus, different cells can have very different physical characteristics despite having the same genome. Morphogenesis, or the development of body form, is the result of spatial differences in gene expression.[81] A small fraction of the genes in an organism's genome called the developmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved among phyla, 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 Hox genes. Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva.[86]

Evolution

Evolutionary processes

Main article: Evolutionary biology
Natural selection for darker traits

artificial selection
, animals were selectively bred for specific traits. [89] 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.[89] 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.[89] 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.[90][91][92][89][93]

Speciation

Main article: Speciation

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.[94] For speciation to occur, there has to be reproductive isolation.[94] Reproductive isolation can result from incompatibilities between genes as described by Bateson–Dobzhansky–Muller model. Reproductive isolation also tends to increase with genetic divergence. Speciation can occur when there are physical barriers that divide an ancestral species, a process known as allopatric speciation.[94]

Phylogeny

Main article: Phylogenetics
BacteriaArchaeaEukaryotaAquifexThermotogaBacteroides–CytophagaPlanctomyces"Cyanobacteria"ProteobacteriaSpirochetesGram-positivesChloroflexiThermoproteus–PyrodictiumThermococcus celerMethanococcusMethanobacteriumMethanosarcinaHaloarchaeaEntamoebaeSlime moldsAnimalsFungiPlantsCiliatesFlagellatesTrichomonadsMicrosporidiaDiplomonads
Phylogenetic tree showing the domains of
bacteria, archaea, and eukaryotes


A phylogeny is an evolutionary history of a specific group of organisms or their genes.[95] It can be represented using a phylogenetic tree, a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a lineage 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.[95] Phylogenetic trees are the basis for comparing and grouping different species.[95] Different species that share a feature inherited from a common ancestor are described as having homologous features (or synapomorphy).[96][97][95] Phylogeny provides the basis of biological classification.[95] This classification system is rank-based, with the highest rank being the domain followed by kingdom, phylum, class, order, family, genus, and species.[95] All organisms can be classified as belonging to one of three domains: Archaea (originally Archaebacteria); bacteria (originally eubacteria), or eukarya (includes the protist, fungi, plant, and animal kingdoms).[98]

History of life

Main article: History of life

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

3.5 billion years ago.[99][100] Geologists have developed a geologic time scale that divides the history of the Earth into major divisions, starting with four eons (Hadean, Archean, Proterozoic, and Phanerozoic), the first three of which are collectively known as the Precambrian, which lasted approximately 4 billion years.[101] Each eon can be divided into eras, with the Phanerozoic eon that began 539 million years ago[102] being subdivided into Paleozoic, Mesozoic, and Cenozoic eras.[101] These three eras together comprise eleven periods (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, and Quaternary).[101]

The similarities among all known present-day

bacteria, archaea, and eukaryotes.[104][10][105][106] Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean epoch and many of the major steps in early evolution are thought to have taken place in this environment.[107] The earliest evidence of eukaryotes dates from 1.85 billion years ago,[108][109] and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions.[110]

Algae-like multicellular land plants are dated back even to about 1 billion years ago,[111] although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 billion years ago.[112] Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event.[113]

synapsids, including the ancestors of mammals, dominated the land,[116] but most of this group became extinct in the Permian–Triassic extinction event 252 million years ago.[117] During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates;[118] one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods.[119] After the Cretaceous–Paleogene extinction event 66 million years ago killed off the non-avian dinosaurs,[120] mammals increased rapidly in size and diversity.[121] Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.[122]

Diversity

Bacteria and Archaea

Further information: Microbiology
Bacteria – Gemmatimonas aurantiaca (-=1 Micrometer)

Bacteria are a type of

parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory.[124]

Halobacteria

binary fission, fragmentation, or budding; unlike bacteria, no known species of Archaea form endospores
.

The first observed archaea were

marshlands. Archaea are particularly numerous in the oceans, and the archaea in plankton
may be one of the most abundant groups of organisms on the planet.

Archaea are a major part of

gut, mouth, and on the skin.[128] 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 syntrophic communities, for example.[129]

Eukaryotes

Main article: Eukaryote
Euglena, a single-celled eukaryote that can both move and photosynthesize

Eukaryotes are hypothesized to have split from archaea, which was followed by their

last eukaryotic common ancestor),[131] 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 algae, invertebrates, or protozoans, the protist grouping is not a formal taxonomic group but is used for convenience.[130][132] Most protists are unicellular; these are called microbial eukaryotes.[130]

Plants are mainly multicellular

stoneworts.[133]

saprobes, feeding on dead organic matter, making them important decomposers in ecological systems.[134]

Animals are multicellular eukaryotes. With few exceptions, animals

insects—but it has been estimated there are over 7 million animal species in total. They have complex interactions with each other and their environments, forming intricate food webs.[135]

Viruses

Main article: Virus
Bacteriophages attached to a bacterial cell wall

Viruses are

virus species have been described in detail.[139] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.[140][141]

The origins of viruses in the

evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction.[142] Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life",[143] and as self-replicators.[144]

Ecology

Main article: Ecology

Ecology is the study of the distribution and abundance of life, the interaction between organisms and their environment.[145]

Ecosystems

Main article: Ecosystem

The

nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.[150]

Populations

Main article: Population ecology
Reaching carrying capacity through a logistic growth curve

A population is the group of

Green revolution 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 Thomas Malthus in the 18th century.[151]

Communities

Main article: Community (ecology)
A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.[157]

A community is a group of populations of species occupying the same geographical area at the same time. A biological interaction is the effect that a pair of organisms 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 pollination and predation, or long-term; both often strongly influence the evolution of the species involved. A long-term interaction is called a symbiosis. Symbioses range from mutualism, beneficial to both partners, to competition, harmful to both partners.[158] Every species participates as a consumer, resource, or both in consumer–resource interactions, which form the core of food chains or food webs.[159] There are different trophic levels within any food web, with the lowest level being the primary producers (or autotrophs) such as plants and algae that convert energy and inorganic material into organic compounds, which can then be used by the rest of the community.[52][160][161] At the next level are the heterotrophs, which are the species that obtain energy by breaking apart organic compounds from other organisms.[159] Heterotrophs that consume plants are primary consumers (or herbivores) whereas heterotrophs that consume herbivores are secondary consumers (or carnivores). And those that eat secondary consumers are tertiary consumers and so on. Omnivorous heterotrophs are able to consume at multiple levels. Finally, there are decomposers that feed on the waste products or dead bodies of organisms.[159] On average, the total amount of energy incorporated into the biomass 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.[162]

Biosphere

Main article: Biosphere
Fast carbon cycle showing the movement of carbon between land, atmosphere, and oceans in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. Effects of the slow carbon cycle, such as volcanic and tectonic activity, are not included.[163]

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.[164] 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 biogeochemical cycle is a pathway by which specific elements of matter are turned over or moved through the biotic (biosphere) and the abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. There are biogeochemical cycles for nitrogen, carbon, and water.

Conservation

Main article: Conservation biology

Conservation biology is the study of the conservation of Earth's

services upon which people depend. Conservation biologists research and educate on the trends of biodiversity loss, species extinctions, and the negative effect these are having on our capabilities to sustain the well-being of human society. Organizations and citizens are responding to the current biodiversity crisis through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales.[175][168][169][170]

See also

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Further reading

Further information: Bibliography of biology

External links

Journal links

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