11.5: Release of Energy in Metabolism

Earlier, in looking at how the body gets energy, we saw that it wasn’t a matter of direct burning. We don’t use our food fuel as a car does, by an explosive combining of the fuel with oxygen. Instead, although some of the energy is released as heat, the body takes energy from food in what is essentially a chemical form. It does this through ATP.

The making of ATP is the primary objective of our energy metabolism. It is to this end that our cells so laboriously break down the energy-supplying nutrients.

We saw in Chapter 2 that the fires of life—of each individual cell—burn continuously, or life stops. And the energy demands of life are far from constant. The fires burn brightly when we run, slowly when we sleep. Each individual cell has its own surges and resting phases. An intestinal cell has a lively bout of action after a meal. A white blood cell must suddenly do battle with a microbe at the site of a splinter.

Plainly, the mechanisms of getting energy from food must be closely controlled, not only for the body as a whole, but also for each individual cell. Thus, an extremely sophisticated system for energy use must exist. And it does. It makes the most elegant, electronic, automotive fuel-injection system look as simple as a kerosene lamp.

Only the specialist needs to understand all the intricate details of these chemical reactions. We only need to explore the outlines of how our cells are able to use and control the energy from our food to see that the cell’s use of nutrients is the essence of how foods sustain life.

Sugar Begins to Yield Energy by Glycolysis

It’s in the cytoplasm (“cell fluid”) of individual cells that glucose enters the first phase of metabolism. It’s here that glucose begins to yield energy which we can use.

Enzymes and coenzymes within the cytoplasm go to work. They begin by taking the glucose apart, generally reversing the process by which the plant’s enzymes and coenzymes put it together in the first place:

Plant Cells: Carbon Dioxide + Water + Solar Energy = Glucose + Oxygen

Animal Cells: Glucose + Oxygen = Carbon Dioxide + Water + ATP Energy

The first energy from glucose is extracted when glucose is broken in half. The process is known as glycolysis (meaning, logically enough, breaking sugar). From a single molecule with a skeleton of six carbon atoms, glucose is broken into two separate molecules, each with three of the carbons. These two smaller molecules are called pyruvate.

Although the breaking apart of glucose to pyruvate—along with a release of some of its energy —is essentially a simple idea, the process isn’t simple.

It isn’t done directly, say, as one would cleave two pork chops apart. Glycolysis involves ten distinct chemical reactions, precisely engineered by an array of enzymes and coenzymes, as well as some other special chemicals—“simply” to convert glucose to pyruvate.

A Vitamin Role in Glycolysis

At one point in the breaking down of glucose to pyruvate, a vitamin becomes essential. Without this vitamin, the process will stop after a few steps—indeed before any energy at all has been extracted from the glucose.

The vitamin is niacin. It’s the anti-pellagra factor which Goldberger sought. And it’s an indispensable part of a coenzyme needed in energy metabolism.

In fact, if you were to look closely at the chemistry of glycolysis, you would see that until the niacin-containing coenzyme enters the picture, the cell actually spends ATP energy. But once the niacin-containing coenzyme does its job, the energy balance changes, and glycolysis stops costing energy and begins to release energy. Thus, once we know that this essential coenzyme can’t be made without niacin, it’s easy to see why people who suffer from pellagra (a severe lack of niacin) feel weak.

They are weak. They are hampered in their ability to release energy from food. This isn’t the only reason why niacin deficiency produces pellagra symptoms, for the vitamin has other roles, too. But it shows dramatically why vitamins are defined as essential.

Energy Metabolism Without Air

The reactions of glycolysis are anaerobic (an[without] aerobic[air]), meaning that oxygen isn’t needed or used in these reactions. In other words, glycolysis enables ATP to be made in the absence of oxygen.

In weight lifting, for example, the strong muscle contractions squeeze the capillaries in the muscles, temporarily cutting off the muscles’ supply of oxygen through the blood. Through glycolysis, ATP is made despite the lack of oxygen, fueling the strong muscle action needed for the lift and hold.

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Muscle has a short-term back-up for ATP needed for bursts of intense anaerobic muscle action—a bit of phosphocreatine to replenish ATP. Our bodies make creatine, and also get it in meat and fish (animals have creatine in their muscles for the same reason we do). Some athletes take creatine supplements, which may help in “explosive” anaerobic events, e.g., sprinting, weight-lifting.

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But the anaerobic production of ATP energy can only fuel sudden energy needs for a very short time. For a sustained source of energy, the body must use an aerobic energy pathway.

A Turning Point—and Emergency Energy

The end product of glycolysis—pyruvate—can go in one of two directions (see Fig. 11-2). The normal pathway is to enter the aerobic (oxygen-requiring) phase of metabolism that takes place in the mitochondria of the cell (see Fig. 7-2). This aerobic metabolism is the means by which we ordinarily release some 90% of the energy in our food.

Figure 11-2: Glycolysis breaks glucose into pyruvate, releasing ATP energy in the process.

But there’s a possible detour. And virtually all of us have experienced what it feels like to use it. Understanding this detour for pyruvate helps us to know what really happens when a football halfback breaks through the line and sprints for a long touchdown. It helps to explain what we feel when we race desperately down a seemingly endless corridor for a departing airplane. We feel muscularly exhausted.

The burning of fuel—whether in a car, furnace, or in our body—takes place mainly through the (aerobic) oxidation of fuel. In the body, when all-out extremes of activity go on for more than a few seconds, the demands for oxygen quickly outpace the body’s ability to restore oxygen supplies by breathing and circulation.

Both the halfback and the late passenger, in going all out, quickly outpace the body’s oxygen-delivery system. So if the action is to continue, the detour for pyruvate must be taken—whether the activity is a 50-yard dash in a track meet or a terrified run from a burning building.

Run Now and Pay Later

When the need for energy becomes greater than the supply of oxygen to burn fuel, pyruvate is stopped from going onto the aerobic phase of energy metabolism. Instead, it gives up some of its energy by being converted into a compound called lactic acid.

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Detour for getting more ATP energy without oxygen: Pyruvate → Lactic acid

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When the action is either very vigorous or very prolonged, the lactic acid accumulates. Such accumulation can affect muscle performance.

Meanwhile, the failure of pyruvate to proceed along the normal energy pathway because of lack of oxygen sends a constant stream of messages to the heart and lungs—demands for a faster heart beat and more rapid deep breathing. This state, called oxygen debt, is tremendously stressful. Your heart hammers, your lungs ache, you gasp and feel faint.

The term oxygen debt is a very real one. For in effect, by using this detour, you’re merely deferring the need for oxygen. Eventually, oxygen must be used to deal with the lactic acid. Until your oxygen debt is paid, the pounding of your heart and your heavy breathing will continue, even after your vigorous physical activity has stopped.

Physical fitness extends the time one can exercise vigorously before becoming short of oxygen. Success in endurance events, for example, is very much dependent on how fast blood (which carries the needed oxygen) is delivered to the muscles.

Endurance training (aerobic exercise) increases the heart’s pumping capacity by increasing the amount of blood pumped with each beat. The heart can only beat so fast. To get more blood and oxygen to the muscles, you need to pump more blood with each beat.