Cellular metabolism is an over-arching term for chemical reactions and transformations within cells. At a broad, conceptual level, much of cellular metabolism relates to procurement of organic compounds for sustenance and growth, and breakdown (through oxidation) of compounds to liberate energy (usually organic compounds). A basic understanding of this is important to astrobiology. It most particularly concerns the search for microbial life beyond Earth, but also impacts the search for intelligent life using radio SETI (since intelligent life presumably evolved from microbial life on the same celestial body). The search for microbial life beyond Earth often involves detection of chemical compounds beyond Earth, often from great distances, such as via astronomical methods. The search often does not permit (at least initially) the great luxury of physically sampling and “seeing” what is there and happening. As such, interpretation of the implications and significance of chemical compositions and changes at locations beyond Earth can be key in forming initial hypotheses about possibilities of life beyond Earth (and hence, where best to look for it). The “trail”, so to speak, often starts there. This can include use of astronomical methods to detect chemical compositions and changes on other bodies beyond Earth, such as planets and moons, as well is in space itself.
Cellular metabolism causes chemical reactions that, en masse, can lead to large-scale, observable changes. As a concrete example, early microbial life on Earth, over time and en masse, is believed to have drastically altered the composition of the Earth’s atmosphere, adding elemental oxygen as a significant percentage of the entire atmosphere.
Organic compounds are needed for known life. They are used to form the “stuff” of cells and life, and, generally, are also broken down (through oxidization) to yield life process-sustaining energy, among other things.
Procurement of complex organic compounds is generally accomplished in one of two ways: by obtaining them “prefabricated” from the environment (organisms that do this, including humans, are called heterotrophs), or by actually making, or synthesizing, them from inorganic compounds (most notably carbon dioxide (CO2)) or from simple organic compounds (most notably methane (CH4)).
Generally, there are two known sources of energy from which organisms initiate the synthesis of complex organic compounds: (1) sunlight; and (2) reactions of chemical compounds.
Photosynthesis is a process using sunlight, usually to drive synthesis of glucose (C6H12O6) from carbon dioxide (CO2).
Photosynthesis can be oxygenic (elemental oxygen (O2)-releasing), which is used by cyanobacteria, algae and plants, or anoxygenic (not elemental oxygen (O2)-releasing), which is used by some forms of bacteria that were prevalent very early in life’s history on Earth. A simplified chemical formula for oxygenic photosynthesis can be expressed as:
carbon dioxide (CO2) + water (H2O )+ energy (sunlight) → glucose (C6H12O6) + elemental oxygen (O2).
Chemosynthesis is the process of using reactions of chemical compounds (and not sunlight) for energy to drive synthesis of organic compounds. Chemosynthesis generally uses the breakdown (oxidation) of inorganic compounds or the simple organic compound methane (CH4) to drive synthesis of glucose (C6H12O6)from carbon dioxide (called carbon “fixing”) or methane (CH4) (note that methane (CH4) can therefore be on either “side” of the process).
In additional to procuring organic compounds, organisms must also liberate energy to drive life-sustaining processes. Cells use the molecule adenosine triphosphate (ATP) to transfer energy; it is essentially the energy “currency” within cells. However, energy must first be liberated to drive ATP synthesis.
Cellular respiration is the broad term that includes this liberation of energy and its use to form ATP, generally using glycolysis, which uses the breakdown of the sugar glucose (C6H12O6).
Cellular respiration includes the processes of aerobic respiration, fermentation and anaerobic respiration.
In aerobic respiration, glucose (C6H12O6) is broken down using elemental oxygen (O2), resulting in carbon dioxide (CO2) and water (H2O), plus energy. A simplified chemical formula for aerobic respiration can be expressed as:
glucose (C6H12O6) + elemental oxygen (O2) → carbon dioxide (CO2) + water (H2O) + energy
Fermentation is a metabolic process used by yeast and some bacteria to break down glucose and obtain energy. It does not require elemental oxygen, and it sometimes produces alcohol (which is why fermentation can be used in producing alcoholic beverages). For example, a simplified chemical formula for fermentation can be expressed as:
It is notable that plants use oxygenic photosynthesis, which produces elemental oxygen, but plants also use aerobic respiration, which uses elemental oxygen. The metabolic processes of plants result in a net production of elemental oxygen because plant photosynthesis produces more elemental oxygen than plant aerobic respiration uses.
Anaerobic respiration is a metabolic pathway that, while different than fermentation, also breaks down glucose (C6H12O6) to obtain energy, without elemental oxygen (O2). Anaerobic respiration is less efficient than aerobic respiration, but suffices to sustain anaerobic microbes. Some examples of simplified chemical formulas of forms of anaerobic respiration include:
Ex. (1): carbon dioxide (CO2) + elemental oxygen (O2) → methane (CH4)+ water (H2O) + energy [this is a form of methanogenesis];
Ex. (2): acetic acid (CH3COOH) → methane (CH4) + carbon dioxide (CO2) + energy [this is also a form of methanogenesis];
Ex. (3): methane (CH4) + sulfate → bicarbonate + hydrogen sulfide + water (H2O) + energy [this form is used by sulfate-reducing bacteria].
Chemolithotrophs are microbes (bacteria and archaea) that obtain energy not from sunlight, and not from oxidation of organic compounds (such as may be obtained from carbohydrates, proteins or fats), but instead from the oxidation of various inorganic compounds (via aerobic or anaerobic respiration), such as hydrogen sulfide (H2S), elemental sulfur ( S0) thiosulfate ( S2O32−), elemental hydrogen (H2), ferrous iron (Fe2+), nitrite NO2− and ammonia (NH3). Often, these microbes are chemosynthetic as well. Notably, some of these microbes live around hydrothermal vents in the dark depths of oceans, even in extremely hot, acidic conditions, deriving energy from the rich mineral emissions. Such microbes raise intriguing astrobiological speculation about the hypothetical possibility of microbes living around hydrothermal vents on moons in the solar system.
