By Robert Hazen, George Mason University
Metabolism is the cell’s process of obtaining energy from its surroundings and converting that energy into molecules. Energy is the ability to do work; work is a force acting over a distance. If life is to survive, grow, reproduce, or escape its enemies, it needs energy.
Significance of Energy
Energy is essential for every aspect of living things. Nothing happens without energy. So, before looking at specific groups of organisms, it’s important to consider this characteristic that’s shared by all living organisms: the metabolic process that stores chemical energy.
One of the real challenges facing all cells, and every living thing, is that you have to transfer energy from one part of the physical structure of the cell to other parts of the cell. Any time you want to carry out a chemical reaction, you have to have energy available, and so you have to be able to move that chemical energy from one part of the cell to another.
What Does Adenosine Triphosphate (ATP) Do?
The way life transfers energy is remarkable. It uses several different kinds of small molecules, tiny molecules that basically act like batteries; energy-rich molecules that can move from one part of the cell and essentially plug in to larger molecules to facilitate these chemical reactions.
The most common of these molecular batteries is called adenosine triphosphate (ATP). The important part, at least as far as the energy is concerned, is the triphosphate, or the TP part. There are three groups of phosphorus and oxygen—called phosphates—that are linked end to end: one, two, three.
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Releasing Energy
Energy is released when a phosphorus-oxygen bond is broken. The reaction is very simple: It’s ATP—that is, triphosphate—goes to ADP—diphosphate, with 2 phosphates—plus an extra phosphate, an extra PO4 group, and it releases energy in the process.
This whole process is not at all unlike getting energy by breaking carbon-carbon bonds, when one burns wood, or coal, or natural gas, and so forth. The ATP molecules are formed in one part of the cell, called mitochondria; and then they’re shipped from the mitochondria to other parts, other places in the cell, where they power that cellular machinery. They really do act as batteries; one can plug them into molecules.
Fermentation: Metabolism without Oxygen
Fermentation is another form of gaining energy, another form of metabolism. It’s an incomplete reaction of pyruvic acid to alcohol, or some other small, carbon-based molecule, in the absence of oxygen. With no oxygen around to oxidize things, there is fermentation. One can tell the reaction is incomplete, because we know that alcohol—a product of fermentation—can be burnt. If we can burn it, we can get more energy out of it, so fermentation is incomplete.
Indeed, this inefficient process yields only two ATP molecules per starting glucose. In the most familiar form of fermentation, we have yeast cells. They convert sugar to pyruvic acid by glycolysis, and then on to ethyl alcohol. Ethyl alcohol is the alcohol of alcoholic beverages.
Fermentation in Human Cells
Fermentation of fruits produces wine, fermentation of grain produces beer; and if we allow the carbon dioxide to remain in the liquid—it must be remembered, CO2 is always a by-product—then we get a carbonated beverage, such as beer or champagne. The absolute maximum alcohol content by fermentation alone is about 15% alcohol. For getting stronger alcoholic beverages, we have to use distillation to get extra alcohol, and add it to the beverage.
Fermentation of a variety of other sorts occurs. For example, fermentation occurs in human muscle cells that are engaged in such strenuous activity as exercise. When the muscles cells are used over and over again, the supply of oxygen gets exhausted, leading to fermentation. The end product of fermentation is lactic acid. That’s a three-carbon molecule, and it causes that aching feeling and tired muscles when it concentrates in the muscles.
By-products of Fermentation
There are other organisms that produce a variety of commercially useful products by fermentation. For example, lactic acid—that’s the commonest by-product of fermentation—gives yogurt and rye bread, and some cheeses, their rather sour taste. The bacteria that makes Swiss cheese is another example. It forms the three-carbon propionic acid, which gives Swiss cheese its distinctive flavor; and it might be noted, the big holes in Swiss cheese are where the carbon dioxide comes out and forms large bubbles. Vinegar is another example. Vinegar comes from the fermentation of apples or grapes by an acetic acid-producing bacteria.
So we see how important fermentation is in a variety of ways. In the end, it needs to be remembered, all these products are just incidental; they’re by-products from the actual, vital process of making energy.
Common Questions about the Metabolic Process of Organisms
Fermentation is another form of gaining energy, another form of metabolism. It’s an incomplete reaction of pyruvic acid to alcohol, or some other small, carbon-based molecule, in the absence of oxygen. With no oxygen around to oxidize things, there is fermentation.
Fermentation occurs in human muscle cells that are engaged in such strenuous activity as exercise. When the muscles cells are used over and over again, the supply of oxygen gets exhausted, leading to fermentation. The end product of fermentation is lactic acid. It causes that aching feeling and tired muscles when it concentrates in the muscles.
Metabolism is the cell’s process of obtaining energy from its surroundings and converting that energy into molecules. It uses several different kinds of small molecules, tiny molecules, that basically act like batteries; energy-rich molecules that can move from one part of the cell and essentially plug in to larger molecules to facilitate these chemical reactions.
