Organisms acquire energy by two general methods: by light or by chemical oxidation. Productive organisms, called autotrophs, convert light or chemicals into energy-rich organic compounds beginning with energy-poor carbon dioxide (CO₂). These autotrophs provide energy for the other organisms, the heterotrophs. Heterotrophs are organisms that acquire their energy by the controlled breakdown of preexisting organic molecules, or food. Human beings, like most other animals, fungi, protists, and bacteria, are heterotrophs.

Autotrophic organisms are often primary producers in their ecosystems. They acquire their useful free energy from sources other than food: either from the energy of sunlight (photoautotrophs) or from oxidative chemical reactions (chemoautotrophs). The latter mode of metabolism refers to life-forms that use inorganic materials (ammonia [NH₃], methane [CH₄], or hydrogen sulfide [H₂S]) combined with oxygen to generate their energy. Only some bacteria are capable of obtaining energy by “burning” inorganic chemicals.

Green plants are typical photoautotrophs. Plants absorb sunlight to generate ATP and to disassociate water into oxygen and hydrogen. To break down the water molecule, H₂O, into hydrogen and oxygen requires much energy. The hydrogen from water is then combined in the “dark reactions” with carbon dioxide, CO₂. The result is the production of such energy-rich organic molecules as sugars, amino acids, and nucleotides. The oxygen becomes the gas O₂, which is released as waste back into the atmosphere. Animals, which are strictly heterotrophs, cannot live on carbon dioxide, sunlight, and water with a few salts like plants do. They must breathe in the atmospheric oxygen. Animals combine oxygen chemically with hydrogen atoms that they remove from their food—that is, from organic materials such as sugar, protein, and amino acids. Animals release water as a waste product from the oxygen respiration. Animals, like all heterotrophs, use organic materials as their sole source of carbon. This conversion of carbon provides an example of an aspect of an ecological cycle in which a required element flows through different types of organisms as it changes its oxidation state from CO₂ to (CH₂O)_n and back to CO₂.

Metabolic cycles in general—the extraction by organisms of useful energy and food molecules from environmental source material—can be described in terms of oxidation-reduction reactions. In the case of oxygen respiration, oxygen molecules from the air accept electrons ultimately from glucose or amino acids. The oxygen, which has a great affinity for electrons, is called an electron acceptor, whereas the glucose, or other sugar or organic molecules, is an electron donor. Animal respiration is the prototype of oxidation-reduction reactions, but certainly not all oxidation-reduction reactions (or “redox reactions,” as they are often called) involve oxygen. Many other inorganic compounds are respired, or “breathed,” at the cell level. Biological electron acceptors besides oxygen include nitrate, nitrite, sulfate, carbonate, elemental sulfur, and methanol. Biological electron donors (other than sugar and amino acids) include hydrogen, nitrogen compounds (as ammonia, nitrite), sulfide, and methane. For acceptor-donor transformations to be available to chemoautotrophs and heterotrophs over sustained periods of time, ecological cycles are required. For geologically short periods of time, organisms may live off a finite supply of material; however, for any long-term continuance of life, a dynamic cycling of matter involving complementary types of organisms must prevail. If life exists on other planets, the requisite elements and liquid water must cycle. A search for such transformations provides one method of detecting extraterrestrial life.

In addition to energy, all forms of life require carbon sources. Autotrophic organisms (chemosynthetic and photosynthetic bacteria, algae, and plants) derive this essential element from carbon dioxide. Heterotrophs use preformed organic compounds as their source of carbon. Among autotrophs many types of cells do not depend on light to generate ATP; those that do without light are the chemoautotrophic bacteria, including the methanogens, ammonia oxidizers, sulfide oxidizers, hydrogen oxidizers, and a few obscure others. Indeed, at least five metabolic pathways entirely different from each other have evolved to use carbon dioxide gas. One is the oxygenic pathway described above, which is used by plants, algae, and cyanobacteria: the Calvin-Benson dark reactions. Other, more obscure pathways include phosphoenolpyruvate (PEP), succinate, and methanogen pathways. They all need to bring energy-poor carbon dioxide into the energy-rich carbon-hydrogen compound metabolism of organisms. All life on Earth depends on these autotrophic reactions that begin with carbon dioxide or its equivalent. Equivalents as carbon sources in autotrophic metabolism include the carbonate ion, bicarbonate ion, and carbon monoxide. As usual, with respect to metabolic variation and virtuosity, the bacterial repertoire is vastly more diverse than that of eukaryotes—that is, plants, animals, and other organisms composed of cells with nuclei. In general, nucleated organisms, eukaryotes, are either photolithoautotrophs (i.e., algae and plants) that derive energy from light or minerals or chemo-organoheterotrophs (animals, fungi, and most protists) that derive energy and carbon from preformed organic compounds (food).

ATP

All ATP biological electron-transfer reactions lead to the net production of ATP molecules. Two of the three phosphates (PO₄) of this molecule are held by energy-rich bonds sufficiently stable to survive for long periods of time in the cell but not so strong that the cell cannot tap these bonds for energy when needed. ATP and similar molecules (such as guanosine triphosphate [GTP]) have a five-carbon sugar and three phosphates. As far as is known, such molecules are the general and unique energy currency of living systems on Earth.

No metabolic process occurs in a single step. The ordinary six-carbon sugar, glucose, does not oxidize to carbon dioxide and water in living cells in the same way that glucose in air burns. Any release of energy by burning would be too sudden and too concentrated in a small volume to happen safely inside the tiny cell. Instead, glucose is broken down at ambient (i.e., relatively cool) temperatures by a series of successive and coordinated steps. Each step is mediated by a particular and specific enzyme. In most cells that metabolize glucose, the sugar first breaks down in a set of steps that occur in the absence of oxygen. The total number of such steps in plants, animals, fungi, and protists (see below Hypotheses of origins) is about 11. Other organisms, primarily bacteria and obscure protists and fungi, are anaerobes: they do not utilize molecular oxygen in their metabolism. In anaerobes, glucose metabolism stops at compounds such as ethanol or lactic acid. Aerobic organisms, including all animals, carry the oxidation of glucose farther. They rapidly use anaerobic glucose breakdown products such as lactic acid, ethanol, or acetate with Krebs-cycle intermediates in the mitochondria. Aerobic oxidation of glucose requires an additional 60 enzyme-catalyzed steps. The anaerobic breakdown of glucose uses enzymes suspended freely in solution in the cells. The aerobic steps occur on enzymes localized in mitochondria, the “power packs” of cells where oxygen gas is used to make the energy compound ATP. The complete aerobic breakdown of sugar to carbon dioxide and water is about 10 times more efficient than the anaerobic in that 10 times as many ATP molecules are produced. (See metabolism.)

Energy made available to cells in the form of ATP is used in a variety of ways—for example, for motility. When an amoeba extends pseudopods or when a person walks, ATP molecules are tapped for their energy-rich phosphate bonds. ATP molecules are used for the synthesis of proteins that all cells require in their growth and division, amino acids, and five-carbon sugars of nucleic acids. Each synthetic process is controlled and enzymatically mediated. Each starts from an organic building-block compound available to the cell as food. The amino acid L-leucine, for example, is produced from pyruvic acid, which is itself the product of the anaerobic breakdown of glucose. Synthesis of L-leucine from pyruvic acid involves eight enzyme-mediated steps with an addition of acetic acid and water.

These exquisitely interlocked and controlled metabolic steps are not performed in a diffuse manner helter-skelter in the cell. Rather, a marvelously architectured cellular interior displays specialized regions visible at the electron-microscopic level. Particular chemical reactions are performed in association with specific structures. In aerobic eukaryotes the mitochondrion with its intricate cristate membrane (the folds in the membrane are called cristae) provides the site of pyruvate, acetate, and lactate metabolism. These molecules are transformed and passed on from one enzyme to another as through a conveyor belt in a factory. Similarly, in those eukaryotes capable of oxygenic photosynthesis (algae, plants), photosynthesis occurs only in an organelle (a cell part) called a chloroplast. Chlorophyll, carotenoids, and other pigments that absorb visible light, as well as the detailed enzymatic apparatus for the photosynthetic process, reside there.

Chloroplasts and mitochondria contain DNA. Moreover, this DNA has a sequence distribution that differs entirely from that of the nucleus and greatly resembles that of free-living photosynthetic and oxygen-respiring bacteria. The best explanation for these facts is that the ancestors to the oxygen-releasing chloroplasts and oxygen-respiring mitochondria were once free-living bacteria.

Prokaryotes and eukaryotes

All life is composed of cells of one of two types: prokaryotes (those that lack a nucleus) or eukaryotes (those with a nucleus). Even in one-celled organisms this distinction is very clear.

All bacteria are prokaryotic, even though many, probably most, are multicelled in nature. The only other single-celled organisms that exist are fungi (one-celled fungi are called yeasts). All nucleated organisms (cells with nuclei and chromosomes in their cells) that are not animals, fungi, or plants are Protista. This huge group includes the unicellular or few-celled protists and their multicellular descendants. The large kingdom of Protista has 250,000 estimated species alive today. Some are very large, such as red algae and the kelp Macrocystis. One-celled protists include the familiar amoebas, paramecia, and euglenas as well as 50,000 less-familiar types. Scientifically speaking, no such thing as a one-celled animal exists. All animals and plants are by definition multicellular, since they all develop from multicellular embryos. Accordingly, all “protozoans” are now classified as single-celled Protista, not animals. Nor are there any one-celled plants. Organisms formerly called one-celled plants are algae and, as such, are now classified with Protista as well. If a mature organism is determined to be one-celled, then it must be either a bacterium (prokaryotic) or a fungus or protist (eukaryotic). All animals and plants develop from embryos that by definition combine two complementary sets of chromosomes (i.e., they are diploids at some stage in their development). They are all multicellular eukaryotes. But though there are no one-celled plants or animals, there are indeed myriad many-celled protists. Multicellularity evolved not only in the ancestors to the plants and the animals but also in the bacteria, the protists, and the fungi.

All eukaryotic cells undergo some form of mitosis, a sequence of cell division events that occurs after chromosomal DNA protein replication. Mitosis ensures that chromosomal DNA and protein are equally distributed to the offspring cells. Mitosis is the most distinctive activity of eukaryotic cells, which have nucleoprotein chromosomes in their nuclei and a membrane that separates the nucleus from the cytoplasm. In mitosis, mitochondria, which are usually present in the cytoplasm as well as in the chloroplasts of algae and plants, are smoothly distributed with the chromosomes to offspring cells. The Golgi apparatus and endoplasmic reticulum (ER), an intricately convoluted structure, serve to anchor many cytoplasmic enzymes excluded from mitochondria or chloroplasts. They also divide and are distributed in mitosis.

Nuclei, chromosomes, mitochondria, chloroplasts, ER, and nuclear membranes are all absent in prokaryotes. Prokaryotic cells, which include all the cyanobacteria (formerly called blue-green algae), are bacteria in every way. Division is nonmitotic in all prokaryotes. Bacteria lack nucleoprotein and a nuclear membrane, and, when chromosome stain is applied, only fuzz or nothing is seen. Whereas all eukaryotic cells have more than one chromosome and sometimes over a thousand, the genes of prokaryotic cells are organized into a single “chromoneme” or “genophore.” (The term bacterial chromosome, while still in use, is, technically speaking, inaccurate.) The genes may or may not be concentrated enough to be seen, but in any case bacterial DNA floats freely in the cytoplasm. Prokaryote cell organization is less complex than that of eukaryotes. The basic question of the evolution of prokaryotes into eukaryotes—often rated the second major evolutionary mystery, after the origin of life—is thought to involve a complex series of partnerships in which distinct strains of bacteria entered each others’ bodies, merged symbiotically, and traded genes.