By Robert Hazen, George Mason University
The process of transferring energy in organisms uses several different kinds of small 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, or ATP.
ATP as the Energy Battery
In an ATP, there are three groups of phosphorus and oxygen—called phosphates—that are linked end to end: one, two, three. Energy is released when a phosphorus-oxygen bond is broken. It’s ATP—that is, triphosphate—that goes to ADP—diphosphate, with two 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 we burn wood, or coal, or natural gas, and so forth. ATP molecules are formed in one part of the cell, called mitochondria, and then they’re shipped from the mitochondria to other places in the cell, where they power that cellular machinery.
Abundance of ATP
Each ATP molecule allows us to staple or put together two molecules, break apart two molecules, or do some other cellular chemical function. In the process, that ATP powers the molecule to do one operation at a time, so we need a lot of ATP in your cells.
In fact, a typical cell will have millions upon millions of ATP molecules available at any one time. This is true for virtually all of the cells in our body; the 100 trillion cells in our body all have to have ATP to carry out these functions—all but with a few exceptions.
Glycolysis for Synthesizing ATP
The ATP molecules are synthesized in several different ways. One important process, which occurs in all cells, is called glycolysis. In glycolysis, an energy-rich molecule, a sugar molecule with six carbons, is split into two smaller fragments, each with three carbons. That six-carbon sugar is called glucose, and we know that these are energy-rich solutions.
The actual process of glycolysis is quite complex; there are 10 different chemical steps that must be undertaken. But the ultimate reaction is that a glucose with six carbons breaks down to two pyruvic-acid molecules, each with three carbons, plus two ATPs, two of those little batteries.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Role of Phosphates in Glycolysis
The important role of phosphorus in glycolysis was discovered in 1905 by British biochemists Arthur Harden and W.J. Young, who were studying the breakdown of glucose by yeast cells. If we think about it–ATP is adenosine triphosphate. We need to have phosphorus atoms to do this reaction, but nobody knew that at the time, and their story illustrates something about the serendipitous nature of science.
What they found is that glucose breakdown slowed and stopped before all the glucose had been consumed. This implied that for glycolysis to occur we needed to have not only glucose but something else, some other raw material. What could it be? What limited this reaction? Using trial-and-error methods, they added one chemical after another.
They eventually discovered that the addition of phosphorus got things going again; as long as there was a little bit of phosphorus available, the yeast could keep going through glycolysis. This was the clue that phosphates play a key role in metabolism.
Respiration
There are several other metabolic pathways, including respiration and fermentation, and they both produce ATP, as well as other molecules that serve in the role of batteries. Recall that one of the products of glycolysis is that three-carbon molecule called pyruvic acid; pyruvic acid still holds lots of energy.
As one might expect, then, life has developed ways of exploiting that energy. The most efficient of these processes is called respiration, in which the glucose molecule is completely disassembled to make lots of ATP molecules.
Krebs Cycle
Respiration is always preceded by glycolysis, which splits the glucose molecule, and then we have pyruvic acid. The pyruvic acid then enters a metabolic cycle, variously called the citric acid cycle, or sometimes the TCA cycle, or sometimes that’s called the Krebs cycle, after the British biochemist Hans Krebs, who worked out the details in the 1930s.
Throughout the several steps of the Krebs cycle, carbon-based molecules are oxidized, and they therefore produce CO2, plus energy, which ultimately gets taken up as ATP and other energy-rich molecules.
A single glucose molecule can be converted to as many as 38 ATP molecules. In this process, this process of respiration, here’s the chemical reaction: we start with glucose, and make six CO2 molecules; we also make a bunch of H2O or water molecules, plus energy.
This looks like a very efficient reaction, but even so, the energy transfer is only about 40% of the chemical energy stored in glucose. In life, as in every other physical system, the second law of thermodynamics comes into play. One can never have a perfectly efficient system, for example, a system of respiration. Some of the energy is always lost in converting those chemical reactions.
Fermentation for Releasing Energy
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. If we don’t have oxygen around to oxidize things, there is fermentation.
One can tell the reaction is incomplete, because 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.
Common Questions about Synthesis of Adenosine Triphosphate (ATP)
The role of phosphorus in glycolysis was discovered in 1905 by British biochemists Arthur Harden and W.J. Young, who were studying the breakdown of glucose by yeast cells.
Glycolysis is a form of metabolism that makes energy available to the body. In glycolysis, an energy-rich molecule, a sugar molecule with six carbons, is split into two smaller fragments, each with three carbons.
A typical cell will have millions upon millions of ATP molecules available at any one time. Hence, the 100 trillion cells in our body all have to have ATP.
