Organisms that are not phototrophic capture energy from other sources, specifically by transforming thermodynamically unstable molecules into more stable species. Such organisms are known generically as chemotrophs. They can be divided into various groups, depending upon the types of food molecules (energy sources) they use: these include organotrophs, which use carbon-containing molecules (you yourself are an organotroph) and lithotrophs or rock eaters, which use various inorganic molecules. In the case of organisms that can “eat” H2, the electrons that result are delivered, along with accompanying H+ ions, to CO2 to form methane (CH4) following the reaction:
CO2 + 4H2 ⇌ CH4 + 2H2O.
Such organisms are referred to as methanogens (methane-producers)181. In the modern world methanogens (typically archaea) are found in environments with low levels of O2, such as your gut. In many cases reactions of this type can occur only in the absence of O2. In fact O2 is so reactive, that it can be thought of as a poison, particularly for organisms that cannot actively “detoxify” it. When we think about the origins and subsequent evolution of life, we have to consider how organisms that originally arose in the absence of molecular O2 adapted as significant levels of O2 began to appear in their environment. It is commonly assumed that modern strict obligate anaerobes might still have features common to the earliest organisms.
The amount of energy that an organism can capture is determined by the energy of the electrons that the electron acceptor(s) they employ can accept. If only electrons with high amounts of energy can be captured, which is often the case, then inevitably large amounts of energy are left behind. On the other hand, the lower the amount of energy that an electron acceptor can accept, the more energy can be extracted and captured from the original “food” molecules and the less energy is left behind. Molecular oxygen is unique in its ability to accept low energy electrons. For example, consider an organotroph that eats carbohydrates (molecules of the general composition [C6H10O5]n), a class of molecules that includes sugars, starches, and wood, a process known as glycolysis, from the Greek words meaning sweet (glyco) and splitting (lysis). In the absence of O2, that is under anaerobic conditions, the end product of the breakdown of a carbohydrate leaves ~94% of the theoretical amount of energy present in the original carbohydrate molecule remaining in molecules that cannot be broken down further, at least by most organisms. These are molecules such as ethanol (C2H6O). However, when O2 is present, carbohydrates can be broken down more completely into CO2 and H2O, a process known as respriration. In such O2 using (aerobic) organisms, the energy released by the formation of CO2 and H2O is stored in energetic electrons and used to generate a membrane-associated H+ based electrochemical gradient that in turn drives ATP synthesis, through a membrane-based ATP synthase. In an environment that contains molecular oxygen, organisms that can use O2 as an electron acceptor have a distinct advantage; instead of secreting energy rich molecules, like ethanol, they release the energy poor (stable) molecules CO2 and H2O.
No matter how cells (and organisms) capture energy, to maintain themselves and to grow, they must make a wide array of various complex molecules. Understanding how these molecules are synthesized lies within the purview of biochemistry. That said, in each case, thermodynamically unstable molecules (like lipids, proteins, and nucleic acids) are built through series of coupled reactions that rely on energy capture from light or the break down of food molecules.