Metabolism

d.w.rowlands [at] gmail.com

When I was in elementary school, the difference between plants and animals was explained to me as that "plants can make their own food from sunlight; animals have to eat other animals or plants for food." No one really bothered to define "food" very clearly, though, and I was always a bit confused by why plants needed fertilizer, or "plant food", if they made their own food.

Eventually, I figured out that the distinction was that plants could get energy from sunlight, although they still need lots of water and carbon (from the air), and animals can't. Plants also do need phosphorous and reduced nitrogen, which they usually absorb from the soil: this is why fertilizer is important. The distinction was slightly complicated by the existence of bacteria that can get their energy from chemicals at deep-sea vents and such, but I hadn't realized just how much variety there was until recently. It turns out that there are a number of different independent variables in terms of nutritional strategies that organisms can have:

• Energy source: can be sunlight (phototroph), or an exothermic chemical reaction (chemotroph). The latter category includes both animals, who get their energy from oxidizing organic matter, and bacteria who get their energy from inorganic chemical reactions.
• Carbon source: can be either inorganic carbon, generally carbon dioxide, (autotroph) or organic molecules produced by other organisms (heterotroph).
• Electron donor: can either be organic compounds found in the environment (organotroph) or inorganic compounds (lithotroph).

While plants are generally photoautolithotrophs (they get energy from sunlight, produce their own organic molecules from carbon dioxide, and get the electrons for their redox reactions from water) and animals and fungi are generally chemoheteroorganotrophs (they get energy from breaking down organic molecules they consume, and they get the electrons for their redox reactions and their organic carbon from those molecules), other strategies are seen in bacteria. Some, such as purple non-sulfur bacteria, are photoorganotrophic (they get energy from sunlight, but get electrons from organic molecules), some are photoheterolithotrophs (they get energy from sunlight and electrons from water, but consume organic molecules as their carbon sources), and some are chemoautolithotrophs (they get their energy and electrons from inorganic reactions and their carbon from carbon dioxide) or chemoheterolithotrophs (they get their energy and electrons from inorganic reactions but their carbon from organic reactions).

These aren't the only ways that organisms can differ in terms of nutritional strategies, though. A variety of both eubacteria and archaea are diazotrophs---they get their nitrogen by reducing atmospheric nitrogen gas---but most organisms need to get their nitrogen in a reduced form such as ammonia, nitrates, or nitrites that have been produced by diazotrophs. One of the main functions of fertilizers is to supply reduced nitrogen to the soil as plants other than legumes (which have symbiotic diazotrophic bacteria in their roots) can't reduce atmospheric nitrogen. Part of the reason nitrogen fixation is relatively rare, although it's evolved a number of times, seems to be that the chemical pathways for it are all very sensitive to oxygen, making it difficult for aerobic organisms to fix nitrogen. (Some do, though.)

Another, perhaps more familiar, difference is the electron acceptor used in cellular respiration to create ATP, which all cells use as their basic energy currency. Here, there are three options:

• Aerobic Respiration: In aerobic respiration, an electron transport chain is used to fully oxidize organic molecules with oxygen gas as the final electron acceptor.
• Anaerobic Respiration: Anaerobic respiration is similar to aerobic respiration, except that the final electron acceptor in the electron transport chain is a molecule other than oxygen gas. Anaerobic respiration is less efficient than aerobic respiration, since none of the final electron acceptors used in it are as oxidizing as oxygen gas.
• Fermentation: Fermentation is a process of metabolizing carbohydrates without an electron transport chain or an external final electron acceptor. It is significantly less efficient than aerobic or anaerobic respiration, since its waste products are still fairly reduced species.

Interestingly, although almost all eukaryotes are capable of aerobic respiration via their mitochondria, a recently discovered animal species turns out to be purely anaerobic, as its mitochondria have evolved to perform anaerobic respiration instead.