Introduction to ATP Energy
Every living thing contains the chemical, adenosine triphosphate (ATP). ATP is used to provide energy for heat, nerve electricity, light (as in fireflies), and muscle movement. An ATP molecule is held together by strong electrical forces which are set free when the molecule is broken apart in a chemical reaction. Somehow these forces are converted into the kind of mechanical energy that will move our muscles. The part broken from the ATP molecule is a smaller molecule called phosphate. After the phosphate is broken off, living organisms can put it back and use the ATP molecule again. To do this, a rebuilding chemical called phosphagen, is used for a while until the powerful glycogen cycle takes over the production of fresh ATP energy. The glycogen cycle is a series of chemical steps in which sugar and other substances are used to make fresh ATP in large quantities. The whole process is so intricate that dozens of chemical reactions take place, literally, in the wink of an eye!
Fuel for the Muscle Motors
Muscular contraction is one of the most wonderful things in the operation of our body machinery. And though much about it remains a mystery, biochemists made many strides towards the understanding of this operation. In whatever form sugars are eaten, whether as starch, syrups, candy, or common table sugar, they are broken down by the juices of the alimentary canal (the digestive tract) to simple sugars. All these sugars are then changed to the simplest sugar, glucose, which is the only sugar found circulating in the blood.
The amount of glucose in the blood stays remarkably the same. It increases in the disease known as diabetes, otherwise the sugar content of the blood is about the same in all healthy persons.
The body's heat comes mostly from the burning of glucose. Cold-blooded animals such as the frog have less sugar in their blood than we do. Birds, which are warmer than humans, have more.
If we could not store so vital a substance as glucose in our bodies, we would need to eat constantly to maintain a steady supply of fuel. That would be a dangerous existence. If we were to fall asleep, we would never wake. Without food, our body's machinery would grind to a halt. On the other hand, large amounts of free glucose cannot be kept in the body either. It is too readily used up. Our bodies can, however, store the glucose in a more stable, less readily used form - glycogen. Glycogen is made up of many glucose molecules that are hooked together.
How Energy is Stored in the Body
Glycogen are "glucose" storage depots Glycogen is stored all over the body. There are large deposits of glycogen in the liver, in the muscles, and in the kidneys. As the body needs glucose, glycogen is broken down into glucose molecules to supply the body's immediate needs. If there is any excess glucose in the blood, as there is after a meal, it is shipped to the glycogen storage depots.
When energy is needed, glucose molecules are broken down to release energy in the form of heat. The release of energy takes place in an astounding series of reactions brought about within the body by chemical substances known as enzymes and co-enzymes.
The earliest glimpses of these reactions were obtained, oddly enough, from studies of yeast cells, not of animals. Yeast cells, up to a point, use sugars for their energy exactly as we do.
Before we can be qualified as engineers for the human body, this most marvelous of machines, we need to know how energy is released in chemical reactions.
All forms of energy are interchangeable. That is, heat can be changed into motion; motion can be changed into electricity, which, in turn, can give light or heat again. Such conversions or changes are often very wasteful. The average steam locomotive loses over 90 percent of the energy of the steam as it converts the remaining 10 percent to motion. The average light bulb uses 90 percent of its energy in creating heat, the rest of the energy is used to produce light
How Energy is Released
Energy cannot be destroyed. We can take a pound of coal and burn it in an ample supply of air while it changes to carbon dioxide. An amount of heat (n) will be given off. If we burn another pound of coal in a limited supply of air it will form carbon monoxide, but only about one-fourth (1/4) as much heat will be liberated as before. If we now burn all of this carbon monoxide to carbon dioxide, we get the rest of the original amount of heat (n). Whether we release the energy in one step or in several steps, the overall amount is the same.
What about the energy in glucose? Measurements of energy are always based on the chemist's unit weight, or molecular weight. Glucose weighs 180 grams. (One molecule of glucose weighs 180 times as much as one atom of hydrogen.) When a green plant makes glucose from carbon dioxide and water, it packs energy into it. Into about 180 grams (or six ounces) of glucose are packed 700 calories of energy. The calorie is a measure of heat energy. One hundred calories will heat one liter (about a quart) of ice-cold water to boiling. If we burn the 180 grams of glucose, 700 calories of heat will be released.
Where are these 700 calories hidden? They are used to form the bonds that hold together the six carbon atoms, the twelve hydrogen atoms, and the six oxygen atoms that make up the one glucose molecule. There is energy in chemical bonds. (1 glucose = C6 H12 O6)
Imagine a dozen large springs from a mattress squeezed into a hatbox. A good deal of energy had to be used to squeeze the springs together before the box lid could be safely locked. If the lid is opened, the jumping springs will release the same amount of energy that was used to put them into the box. This is a fair comparison of the energy used to lash the atoms of carbon, hydrogen, and oxygen together to form glucose. (This bond energy has nothing to do with the energy within the nucleus of the atom, known as atomic energy. And, of course, bond energy is not nearly so great as the monstrous energy of the nucleus.)
Glucose is Changed into Energy
The amount of energy in each chemical bond is not the same. Some bonds have more energy packed into them than others. The cell gets its warmth and its energy for work from the breaking of the bonds of glucose molecules. Cells have a complicated set of enzymes and co-enzymes for the step-by-step whittling of bonds of the glucose molecule and the step-by-step release of energy from these bonds. NADH is the coenzyme that plays the primary role in the release of energy.
Some of these steps are well known, and the overall picture is clear. The bonds in glucose are broken one at a time, and phosphates (salts containing phosphorus) play an important role in the process.
Phosphate molecules store much of the released energy in a form more useful to the cell. The howling wind has a lot of energy. The farmer's windmill catches some of that energy and at once puts it to work pumping water from a well. But some of that energy is also stored by the charging of batteries. The wind cannot light up the farmer's house, but the battery can. The heat from a crumbling glucose molecule does not make our legs move. It is the energy in the phosphate bond that supplies the energy that makes the muscles move. Phosphate bonds are our batteries. They are the stored energy for life's every need.
How does it work, this marvelous battery to which we owe our lives? The battery is a molecule - a molecule called adenosine triphosphate - abbreviated as ATP. Onto one molecule of ATP are lashed two, special phosphate groups. Ten calories of energy are packed into each of those bonds which hold these phosphates to the ATP. These phosphate cementing calories are the main form of energy the cell can use for its many tasks.
How Stored Phosphate-Bond Energy is Used in Our Body's Muscles
This stored phosphate-bond energy is used in a remarkable manner. Suppose a muscle cell needs ten calories of energy to set off a muscle contraction. A unit of ATP is alerted to act by an agent, called a coenzyme, which splits off one phosphate unit and then delivers the required ten calories. In the process, the ATP is reduced to ADP (adenosine diphosphate). If the energy set free by the splitting off of phosphate units is not used for muscle contraction, it can be transferred to the other compounds in the muscle such as creatine and glucose. In so doing, the glucose is taken from a glycogen storage depot and begins to be broken down. The energy flowing from the crumbling glucose is used to furnish heat, convert some of the broken-down glucose back to glucose, and attach the liberated phosphate units to ADP to reform ATP. When this cycle is completed, the ATP units are ready for any new emergency.
The ready supply of the stored ATP energy for movement is marvelously useful to all animals in emergencies. Sometimes an animal needs enormous amounts of energy when there is not enough time to break down glucose. After a vigorous sprint to the bus a person may pant for minutes before they can settle down to the calm reading of the morning paper. The panting is the body's way of taking in large amounts of oxygen needed for the burning of glucose to build back the ATP used up by the exertion of hurrying to the bus.
How Phosphate-Bond Energy Moves Our Legs
The bones in our legs are moved by the muscles attached to them. These muscles always come in matched pairs. As one muscle tightens or contracts, its opposite relaxes; then the other one contracts, and the first one stretches or relaxes. Our arms, legs, and fingers move by means of this alternate relaxing and contracting of muscles. The energy for this work is provided by the phosphate-bond energy of ATP. The energy for the replacement of this phosphate bond the sun's energycomes from glucose, and this energy in turn comes from the sun. So we are sun machines, fantastically complex sun machines, but sun machines nonetheless.
A Look Inside the Muscle Machinery
Let us lift the hood and take a look at the machinery. Muscle cells are made up of many long, tiny fibers composed chiefly of a protein called myosin. These fibers are chock full of ATP. In the early 1950's Albert von Szent Gyorgyi, a Hungarian born biochemist, (who had come to the United States,) brought about muscular contraction in a test tube. In 1937 he received the Nobel Prize in Medicine for his earlier work. He managed to cause the myosin threads of muscle to lose all of their ATP. He then added to these lengthened threads, some ATP. The long threads curled up instantly on contact with the source of energy. Contraction of the myosin threads is the mechanism for contraction of a muscle. This demonstration is a milestone in the history of science, for in Szent-Gyorgyi's words
"Motion is one of the most basic biological phenomena and has always been looked upon as the index of life. Now we could produce it in a test tube with constituents of the cell."
The contraction of muscle threads lacking ATP explains the hitherto puzzling stiffness (rigor mortis) which sets in soon after an animal cell dies. The ATP present in the muscle is slowly decomposed after death, and, since the enzymes that break down sugar are forever stalled, the ATP is never built up again. Without ATP, the muscle fibers shorten and cause the corpse to become rigid.
ATP has also been shown recently to be the source of energy for the light which some organisms are able to produce (like fireflies). It is also the source of electrical energy in nerve tissues and in the electric organs of animals which can accumulate and discharge electricity (like the electric eel). Thus, ATP can be tapped to supply all the forms of energy a living organism can generate: heat and light, and mechanical, electrical, and chemical energy.