The Energy of Life

In the past two topics, we’ve learned how matter is organized to make living things. We’ve seen how people discovered:

(1) the concept that offspring resemble their parents because they inherit genes which determine their physical characteristics

(2) that every individual is unique because they combine genes from both parents

(3) the idea that mutations can occur because the mechanisms of inheritance are not perfect

(4) the idea that the physical characteristics of a species will evolve over time if some variations make an individual more likely to survive and reproduce

(5) how the molecular properties of DNA account for all of these properties of biological inheritance

DNA can truly be described as the root cause of the biological organization of matter, because it:

(1) carries ‘information’ in the form of a sequence of nucleotides

(2) serves as a template to direct the synthesis of an identical DNA molecule, making it possible for living cells to replicate themselves.

This is indeed a remarkable story. But even if you understand how biological ‘information’ is transmitted from parent to offspring, you still have not completely explained how matter can be organized in the form of living things. Remember from our earlier discussion of thermodynamics that all systems tend towards an increase in entropy. The organization of matter in living things represents the opposite process—a systematic decrease in entropy. Yet no system, taken as a whole, can experience an overall decrease in entropy. So how is life possible?

The simple answer to this question is that when you consider the whole system of which living things are a part, there is not a decrease in entropy. The organizing processes that occur in living things require energy, and this energy comes from chemical reactions that release energy. So to fully understand how living things are able to exist, you need to understand where the energy comes from that supports their organization and how living things harness that energy to produce organization.

Considered in depth, this topic is quite complex and involves a fair amount of detailed biochemistry, but the general principles involved in energy transfers in living things are not too difficult to understand. To put it simply, plants (as well as bacteria and algae) capture light energy coming from the sun in a process called photosynthesis, and they use that energy to manufacture organic molecules like sugars, fats, proteins, etc. The plants make these molecules so they can ‘feed’ their roots, flowers, and other parts of the plant that cannot perform photosynthesis. But these same organic molecules can also be used as sources of fuel by other organisms. As a result, organisms like animals and fungi have evolved, which cannot perform photosynthesis but depend entirely on taking food molecules from other living things. They break down those high-energy food molecules, releasing enough energy to enable them to maintain their own organization, move around in search of food, grow, reproduce, and so on.

The story of how scientists initially thought about the growth of plants and the use of food by animals, and how they gradually arrived at our current understanding of these processes, is a fascinating example of the scientific method at work in the field of biology.

In this chapter, we will first consider what things people knew about the growth of plants and animals before anyone started looking at the question scientifically. We will then recount the evolution of scientific thinking about the use of food by animals and how the circulation of blood feeds the tissues in humans. These accounts will take us up to the threshold of the detailed biochemical experiments in the 20^th century that fully described cellular metabolism. Since the complete description of these processes involved a great many individual experiments, we will only summarize the main lines of these recent developments.

What was known about plants and animals?

Long before anyone thought about plants and animals scientifically, human beings had accumulated a fairly impressive store of knowledge about the way these creatures work. Not only are we ourselves animals, but humans have for the past ten thousand years raised both animals and plants as sources of food and clothing. Through careful tending of their herds and crops, humans developed a pretty detailed understanding of what animals and plants need to survive and what will harm them. What we will see is that the scientists who were investigating life didn’t necessarily learn all they could have from the farmers who were taking care of living things, but still it will be useful to review the main things humans already knew about life before scientists started looking into the problem.

By the 17^th century, people certainly knew that plants need water, since they dry up when it doesn’t rain. They must at least to some degree have realized that most or all plants need sun, since they grow better out in a field than under a tree. Also, Europeans describing the Caribbean islands in the 16^th century remarked on how quickly plants grow there since the sun is higher in the sky. The well-known fact that plants grow better in summer than winter may have been interpreted in terms of the shortening of days in the winter, or people may instead have supposed an effect of temperature on plant growth.

People also had strong reason to believe that plants derive something from the soil, since farmers had long since learned that a field can be ‘exhausted’ after only a few years of using it to grow crops and that it is restored if left unused for several years. Long before chemical fertilizers were introduced, farmers had been adding ‘manure’ to their fields to stimulate crop growth. The ‘manures’ of old included the feces of farm animals, seaweed, sea shells, and even fish. The value of this practice naturally led people to think that plants in some sense need food like animals do, and that these substances gave plants the nutritious material they need.

Finally, farmers were well aware that plants grow up from seeds produced by full-grown plants of the same type. They reserved a portion of their harvest each year to sow in the spring to produce the next year’s harvest. The idea of the seed as containing a generative principle from which life comes even had spiritual implications—finding its way, for example, into biblical parables.

Those basic points—the need of plants for water, sunlight, and something from the soil, as well as the generative potential of seeds—pretty well summarize what people knew about plant growth and nutrition. Farmers accumulated an extensive practical knowledge of which plants grew best at what times of year, which plants needed more water than others, which were more tolerant of shade, how to maximize seed production in particular plants, etc., but probably nothing else concerning plant growth and nutrition in general.

What did people in the 17^th century know about animals? As with plants, people certainly knew animals need water, since they would have experienced the horrors of thirst even in themselves. Also like plants, animals do grow up from little babies to full grown adults. The cause of babies was a mystery in many cultures until fairly recently—even the connection to sexual intercourse was not always recognized. In humans and other mammals, the development of the fetus takes place within the body, so it can’t be observed. Farmers did of course know that chicks hatch out of eggs, that eggs need to be kept warm, and that the gestation process takes a few weeks. Some people were also aware of the more complex developmental processes that occur in organisms like frogs and insects.

Animals differ from plants in that they have an obvious need for certain kinds of solid matter as food. It’s hard to say exactly how systematic they would have been in describing which solid matter made good food and which didn’t. They could hardly have failed to notice that food invariably comes from living things, plants or animals mostly. Of course, not all living material makes good food—people seldom eat wood or bones—but more important is the fact that non-living matter like dirt and sand is never the basis of a healthy diet. They were aware that certain substances tasted better to them, and that those substances are generally good sources of nutrition as well. Many grains, which were the basis for most people’s diet hundreds of years ago, don’t have much taste unless spiced, but at least they taste better than wood or sand.

It would also have been obvious that animals need air, and not just to farmers. Everyone can experience the horror of not being able to breathe. By observing that all other large animals breathe, people could infer that they must have the same desperate need to breathe as we do. Also, breathing is continuous throughout life, and before recently, the most obvious sign of death was when breathing stopped. Another sign of death (in mammals at least) is that the body grows cold. This led people to associate the heat that humans and many other animals give off with the condition of being alive.

This brief summary should give some idea of what material people had to work with in forming their opinions about the growth and nutrition of living things. The scientific investigations that began in the 17^th century grew out of this material. However, as we shall see, the working scientists were not always mindful of facts that would have been obvious to any farmer. That is of course excusable, since farmers have specialized knowledge that not everyone is aware of, and scientists are seldom also farmers. We may be more aware of some things that the scientists ignored, but we have the benefit of knowing how the processes of growth and nutrition actually work, so it’s also kind of unfair to compare their knowledge base with ours.

Early ideas about animal nutrition

In this field as in so many others, Aristotle established the conceptual standard that people worked from for centuries afterwards. Aristotle’s ideas about animal nutrition were based on the Greek idea of the four elements: earth, air, fire, and water. In Aristotle’s system, the material of living things consisted of a combination of these four elements. Likewise, the foods that animals eat are made up of part earth, part air, part fire, and part water. He believed that we eat in order to replace material that is lost from our tissues as a result of wear and tear. Each one of our tissues consists of a particular mixture of the four elements—for example, muscle contains more water than skin, and skin contains more water than bone. To replace lost material from a given tissue, the food material simply has to be processed so that it contains the same mixture of elements as the tissue it is being added to. This reorganization of matter in digestion is controlled by the nutritive soul.

Aristotle’s thinking about nutrition was revised and expanded by the eminent Roman physician Galen, who lived in the 2^nd century A.D. Galen’s biological and medical doctrines were built on Aristotle’s writings from hundreds of years before, but Galen also made observations and added concepts of his own. Galen left us an astoundingly large body of writings, and he was held in such high esteem by everyone who read his work for centuries afterwards that he actually stifled further creative thought in biology and medicine. As with Aristotle, the people of the Middle Ages could not imagine knowing better than Galen, so they tended to accept his views uncritically.

In Galen’s system, digestion consists of a series of ‘concoctions’ in which the gross material of food is refined and adapted to make it fit for consumption by the animal’s organs. Food absorbed by the intestines is first concocted in the liver. The refined product of this concoction flows to the heart through what we now refer to as the portal vein, a large blood vessel between the liver and the heart. The material is further concocted in the heart, producing blood. This blood flows out from the heart to the rest of the body (through both arteries and veins), where all of the various tissues take what they need from it to replenish their lost material. An additional concoction takes place in the brain, producing the more highly refined fluid in the cerebral ventricles and (as Galen thought) inside of the nerves communicating between brain and body.

Before the blood leaves the heart, there is also an exchange of material with the lungs. According to Galen, most blood flows from the right ventricle of the heart to the left ventricle via little holes in the inner wall of the heart. However, some waste material supposedly flows from the right ventricle of the heart to the lungs, where it is given off into the air when we exhale. The lungs also take in air and communicate it to the left ventricle of the heart. When blood is exposed to air in the left ventricle, a mysterious substance called pneuma is produced. This pneuma is, according to Galen, the vital spirit that makes life possible and produces the ‘vital heat’ that is characteristic of human and animal life. The production of pneuma in the left ventricle causes blood to become red.

Galen’s system was largely based on his anatomical observations, which he performed mostly with animals because human dissection was prohibited in Roman times. Some of the ‘anatomically correct’ components of Galen’s model included the communication between liver and heart, the flow of blood from the right ventricle to the lungs, the communication from the lungs to the left ventricle, the red color of blood leaving the left ventricle, and the existence of fluid in the ventricles in the brain. In these respects, Galen’s system (though mostly wrong) was consistent with the information available in his time. In other respects, however, Galen’s system is flatly contradicted by some simple facts about human anatomy that Galen wasn’t aware of because Roman law prohibited him from dissecting human cadavers.

One significant error that Galen made was in thinking that blood flows directly from the right ventricle in the heart to the left ventricle by means of holes in the inner wall of the heart. There are no such holes, and in fact all of the blood in the right ventricle is pumped to the lungs, where it gives off carbon dioxide and takes up oxygen before being returned to the heart. In the early years of the Scientific Revolution (in the 16^th century), Renaissance anatomists began to struggle with the discrepancies between Galen’s writings and what they were finding in their own dissections. The great Renaissance anatomist Andreas Vesalius, for example, candidly admitted that he could find no holes in the inner wall of the heart, but he reasoned that blood could still flow from the right ventricle to the left, since Galen testified that that is what happens. In the 1550’s, two other anatomists named Michael Servetus and Realdo Columbus more or less simultaneously published books in which they rejected Galen’s notion of holes in the inner wall of the heart. They correctly argued that all of the blood in the right ventricle flows first to the lungs and then back to the heart. It took real courage on their part to question Galen’s pronouncements, both because Galen was so overwhelmingly respected and because his thoughts on these subjects had become linked to Christian ideas about the interaction between soul and body.

In the 1620’s, William Harvey performed a series of experiments that added to the ideas of Servetus and Columbus. Harvey’s major conclusion, as we discussed in chapter 8, was that blood did not simply flow from the heart to the tissues in both arteries and veins, but that blood circulates through the body, leaving the heart in the arteries and returning to the heart in the veins. Harvey’s arguments in favor of this view represent an interesting combination of theoretical and experimental thinking.

First, Harvey examined the anatomy of the heart and observed the working of the heart as it beat in a still-living animal. Incidentally, the experimental surgeries he performed on living, un-anesthetized animals are an example of ‘vivisection’. This barbaric practice was critically important for the early advancement of the science of physiology. Eventually, however, scientists acknowledged how painful such procedures are for the experimental subject, and they refused to permit such cruel acts to be performed any more. As it happened, this change of views coincided with the discovery of the first anesthetics in the mid-19^th century. Whether or not this was a fortunate coincidence, scientists at least did not have to cease all physiological experimentation, since they were subsequently able to continue their work using properly anesthetized subjects.

Based on his observations of animal hearts, Harvey decided that the heart is in fact constructed of muscular tissue which generates force by contracting, just as skeletal muscles do. The ventricles contract together, which makes no sense if blood is supposed to flow from one ventricle to another, as Galen thought. Harvey also noted that the valves in the heart appear to be designed to pump blood out from the ventricles. He therefore concluded that the heart functions as a pump, driving blood out of the ventricle with each contraction.

Harvey was now able to calculate the amount of fluid pumped out of the heart with each contraction. Based on his measurements of the volume of the left ventricle, he calculated that each heart beat should pump about two ounces of blood. If blood simply flows from the heart to the tissues where it is absorbed, the pumping of the heart would provide ten pounds of blood to the tissues in one minute! Clearly this is way more than the amount of food that is consumed in the same period of time. This simple calculation persuaded Harvey that the blood must circulate from the heart to the tissues and back again.

If the blood circulates, then some blood vessels must communicate blood away from the heart, while others communicate it back to the heart. To see which way blood flows in the veins, Harvey used tourniquets to interrupt the flow of blood in veins of the arm. He found that blood always accumulated on the side of the tourniquet away from the heart, as it would if venous blood flows back to the heart. If, on the other hand, venous blood flows from the heart to the tissues, it should have accumulated on the side of the tourniquet closer to the heart.

Harvey’s experiments are from our point of view today simple and conclusive. Nevertheless, as we indicated in chapter 8, there was opposition to Harvey’s conclusions. Some such opposition is inevitable anytime a person challenges long-held views, no matter how strong their evidence. People simply grow accustomed to thinking in a particular way, and they do not readily change their minds.

Some of the opposition, however, was a result of Harvey’s inability to explain how the blood provides nutrition for tissues if it is not eaten up by the tissues but merely circulates from the heart to the tissues and back again. Also, Harvey had not been able to establish a connection between the arterial network and the venous network of blood vessels. The actual connection occurs in tiny vessels called capillaries within the tissues, but Harvey was not able to examine the tissues microscopically, or he would have seen the capillaries.

In spite of the opposition, Harvey’s new views gained support during the 17^th century. His experiments were so straightforward and easy to replicate, and his reasoning was so conclusive, that his views would inevitably prevail. The excitement generated by Harvey’s discoveries inspired further anatomical and physiological investigations of the circulatory system, including the discovery later in the 17^th century of the lymphatic system and the capillary networks by which blood flows from arteries to veins.

Nutrition as replacement of lost material

As described earlier, Aristotle and Galen viewed nutrition as a replacement of material lost from the tissues. Harvey’s discoveries challenged the Galenic model of blood flow, but they did not resolve the question of what blood is for. In the years after Harvey’s discoveries, people continued to view food primarily as a source of new material to replace what is lost from the tissues as a result of ‘wear and tear’, and to ‘lubricate’ the muscles and joints.

In the late 18^th and early 19^th century, this view was attached to the emerging idea that animal tissues consist of cells. Microscopic examinations of blood revealed the existence of ‘globules’ of material—what we now refer to as red and white blood cells. Since the imperfect microscopic examinations of animal tissues also suggested that globules were present there as well, scientists hypothesized that tissues are rebuilt by the assimilation of blood globules into tissues. Tissue growth could likewise be explained in terms of the assimilation of increasing numbers of globules into tissues, causing them to expand outwards. Around the same time, John Hunter hypothesized that incorporation of blood into growing tissues was related to the coagulation of blood that occurs in wounds.

The ‘globularist’ hypothesis was eventually rejected based on a number of observations, some reported by Johannes Müller and some by Theodore Schwann. First, the blood corpuscles did not appear to be the same size as the globules reported in tissues. Second, the part of the blood that clots was shown to be the liquid fraction called plasma, not the cellular fraction. Third, muscle tissue was shown to consist of long, fibrous cells rather than ‘globular’ cells. And finally, Schwann described the cellular membrane lining the walls of capillaries. Since there are no gaps in this membrane, the supposed globules go through it to get from blood to tissue, and therefore the material entering the tissues must be in the form of individual molecules.

On the basis of these findings, the globularist hypothesis had been more or less completely discredited by about 1835. Still, people viewed nutrition primarily in terms of the assimilation of molecules into tissues. A more complete understanding of nutrition didn’t come until the new science of thermodynamics enabled people to meld the idea of nutrition as assimilation with the idea of nutrition as combustion, which is discussed in the next section.

Nutrition as combustion

The idea that life is dependent on an ‘inner fire’ has been around for thousands of years. The warmth given off by human beings when they are alive, and the fact that they grow cold when they die, must have been an obvious and rather striking fact even to the earliest human beings. Aristotle believed in the existence of an ‘innate heat’ that animals could not live without. Galen spoke of ‘vital heat’ which was associated with the pneuma in the blood. And this idea persisted for hundreds of years, until Galen’s thinking began to be discredited in the early Renaissance.

The introduction of gunpowder to Europe in the Middle Ages influenced early Renaissance thinking about life as combustion. Just as Descartes explained living things by analogy to the mechanical devices of his day, and as people today try to explain the brain using the analogy of a computer, Renaissance scientists viewed all forms of combustion in relation to gunpowder.

The major ingredient in gunpowder is potassium nitrate, which was referred to as ‘nitre’ in the Renaissance period. Another key ingredient in gunpowder is sulfur. The idea in the 17^th century was that all forms of combustion, including volcanoes for example, occurred when a sulfurous material was incited to combust by nitre. In explaining respiration, therefore, people hypothesized that there must be some sort of ‘aerial nitre’ which is breathed in and produces heat in the body when it reacts with a sulfurous material in living tissue. This view was held by a number of prominent 17^th century scientists, including Robert Boyle, Robert Hooke, Johannes Mayow, and Thomas Willis.

These early chemical speculations were interrupted by the rise of the mechanistic point of view of Descartes and his follower. The ‘iatromechanists’ explained living things using mechanical rather than chemical analogies. For example, they viewed animal heat as arising from friction in the movement of the parts of the body, just as friction causes heat in machines. We have already discussed the limitations in the mechanistic theories of that time. Eventually, these limitations caused people to once again turn to chemical experimentation for insights.

The next big advance in the understanding of respiration as a kind of combustion was made by Lavoisier and his collaborator Pierre Laplace, in an article they published in 1780. Lavoisier and Laplace performed a series of experiments on combustion and respiration, by means of which they intended to show that respiration is a kind of slow combustion. In their key experiment, they produced carbon dioxide either by burning a measured quantity of goal or by letting a guinea pig breathe for a specific period of time. They also measured the heat produced in the combustion coal vs. guinea pig respiration. They found that roughly the same amount of heat was produced relative to the amount of carbon dioxide produced, regardless of whether the heat and carbon dioxide were produced in combustion or respiration.

From this experiment, Lavoisier and Laplace concluded that the production of heat and carbon dioxide were linked, and that the underlying process was the same in combustion as in respiration. As they put it:

Respiration is therefore a combustion, very slow it is true, but otherwise perfectly similar to that of charcoal; it occurs in the interior of the lungs, without producing perceptible light, because the liberated matter of fire is immediately absorbed by the humidity of these organs.

This conclusion also suggested an explanation for how animals use food, since other experiments had already shown that food materials can be combusted, releasing heat. Lavoisier and Laplace argued that food is necessary for animals in the same way as oil is needed to keep a lamp burning—without food, the ‘inner fire’ would go out.

As we see it now, Lavoisier and Laplace had this point backwards: they thought heat was necessary for life and food was just a way of keeping body heat up, while we view heat primarily as a by-product of metabolic activity in animals. But this perspective could not emerge until people had begun to think of heat as just one kind of energy.

The experiment of Lavoisier and Laplace are an interesting example of how much easier it is to appreciate a breakthrough with hindsight than it is at the time. We may disagree with their general interpretation of their findings, but given our current understanding of the chemistry and energetics of combustion and respiration we can still admire the fundamental principle revealed by their experiments. We now understand that heat is released as organic molecules are converted into carbon dioxide and water. We know that the amount of heat released is indeed related to the amount of carbon dioxide produced. We know that ultimately the chemical reactions involved in combustion and respiration are the same, only the reactions of respiration involve more intermediate steps. But at the time Lavoisier and Laplace published their results, their colleagues did not have these insights and were only able to judge the validity of their conclusions based on what was known at the time.

Lavoisier’s contemporaries were unconvinced by his idea of respiration as a kind of combustion, and not without good reason. The methods used to measure the amount of heat produced were fairly approximate, and some experimental variables could not be adequately controlled for, as is usually the case with any new experiment. Also, about 20% more heat was generated, relative to the amount of carbon dioxide produced, in respiration than in combustion. Lavoisier and Laplace were pleased that the numbers were so nearly equal. But more precise measurements of heat production by Dulong and Despretz, published independently in 1822 and 1824, confirmed that more heat was indeed generated by animal respiration than in the burning of charcoal, relative to the amount of carbon dioxide produced. Subsequent experiments also suggested that the amount of oxygen consumed by animals may be too great to account for the amount of carbon dioxide produced. These and other observations appeared to present difficulties for Lavoisier’s elegant equation of respiration with combustion. Eventually, it was all worked out, but at the time no one could have predicted confidently what the final conclusion would be.

Another topic of debate at the time concerned where the ‘combustion’ takes place. Lavoisier argued that oxygen is consumed and heat released in the lungs. Others asked why, if this were so, aren’t the lungs noticeably hotter than the rest of the body? With this in mind, other investigators demonstrated the release of heat by other tissues besides lungs. Lazzaro Spallanzani observed the production of carbon dioxide as well in a variety of tissues from many different animals, including worms, the skin of amphibians, and various parts of fishes studied in isolation from the rest of the body. Alexander von Humboldt demonstrated that detached muscle tissue survives longer when more oxygen is available to it, suggesting that muscle tissue uses oxygen. Remember also that the experiments of Ingen-Housz showed that plants consume oxygen and produce carbon dioxide in the dark. All of these findings argued that the lungs are not necessary for the production of carbon dioxide and the release of heat in respiration.

If respiration occurs throughout the body, then the oxygen taken in by the lungs must somehow be communicated to the rest of the body. The blood was immediately seen as the likely vehicle for oxygen transport. The difference in the color of arterial vs. venous blood had been known at least since Galen’s time, as we have already noted. In fact, Lavoisier suggested that the reddening of blood when exposed to oxygen may be related to the red color of the oxides of metal such as mercury and lead.

Although he never published this view himself, Joseph Louis Lagrange was reported in the 1790s as having hypothesized that:

In the course of the journey of the blood the oxygen little by little quitted the condition of dissolution in order to combine in part with the carbon and in part with the hydrogen of the blood, and so to form carbonic acid and water, which are set free from the venous blood as soon as this leaves the right side of the heart to enter the lungs.

Gustav Magnus was later able to confirm that blood transports gases by showing that oxygen and carbon dioxide are in fact dissolved in ordinary blood, and that arterial blood contains more oxygen than venous blood does. Following Lagrange, he hypothesized that combustion takes place in the blood of the capillaries, as it flows through the tissues. This was in the 1830s. In 1850 however, Georg Liebig demonstrated that isolated muscles still produce carbon dioxide when perfused with distilled water rather than blood. This finding provided evidence that respiration occurs within the cells of various tissues, rather than in the capillaries. As Moritz Traube stated this hypothesis in 1861:

The free oxygen enters in the dissolved state through the capillary walls and unites with the muscle fibre in a loose chemical combination, which is capable of giving up this oxygen again to other substances dissolved in the muscle fluid and endowed with higher affinity for oxygen; and then it can take up new oxygen.

That is close to the truth, though Traube was still unclear about many of the details. Subsequent experiments were needed to show that the red blood cells carry the oxygen, rather than the liquid portion of the blood (the plasma), and to establish the role of hemoglobin proteins as the oxygen carriers. Also, Traube could not adequately account for how the oxygen was used in the tissues. Understanding of that wasn’t to come until well into the 20^th century.

The final step in the understanding of respiration was the recognition that all respiration takes place in cells. This idea was actually expressed by Schwann in the late 1830s, in his formulation of the cell theory. But it was not generally accepted until Eduard Pfluger advocated that position in two papers in the 1870s. Pfluger did not actually present any decisive new experimental findings, but he did argue for the idea that respiration is a general property of all living things, including insects, animal embryos, and plants. The experimental basis for Pfluger’s ideas was already available in the experiments of Ingen-Housz on release of CO₂ by plants, almost 100 years earlier, and in the 19^th century experiments of Georg Liebig and others, which we have just described.

Animal nutrition as release of energy

In the past two sections, we have seen the development of two lines of thought about animal nutrition—the view that food is matter needed to replace material lost from tissues, and the idea that food is fuel for a kind of slow combustion that takes place in tissues. During the period we have reviewed—into the early part of the 19^th century, these two lines of thought developed more or less independently of each other and were not effectively reconciled. Lavoisier did speculate that food was necessary to provide fuel to keep the ‘slow combustion’ of respiration going, but he didn’t grasp why this respiration is necessary. In this section, we will discuss how the application to animal nutrition of the new science of thermodynamics in the mid-to-late 19^th century enabled people for the first time to appreciate why food is really so necessary for animal life.

But first, so you can appreciate where we’re going with this, here is how we understand nutrition now. We need food for two reasons: as a source of energy and as a source of certain organic molecules. Most of the food we take in is used as energy, and for this purpose any organic molecule is pretty much as good as any other. That is why people can get away with making one food source a major part of their diet—like potatoes, or pizza. But if we fail to get certain molecules, like some of the amino acids that we use to make proteins, a variety of different types of molecules that are collectively referred to as vitamins, particular minerals, etc., then our cells will not be able to perform certain functions and we will become sickly.

Food molecules are manufactured by other living things, which we eat. Food molecules can be used by us as sources of energy because they are made up of relatively weak, less stable chemical bonds. When these weak bonds are rearranged to produce stronger bonds (by means of chemical reactions), then energy is released. This energy can be used to generate force or it can be dissipated as heat. Combustion is the name of the process in which molecules with relatively weak bonds react with oxygen to form molecules with stronger bonds. Chemicals with especially weak, unstable bonds release a great deal of heat in combustion, which is why they can be burned. Since energy is released in the combustion of food molecules, we describe the weaker chemical bonds that make up such molecules as storing chemical potential energy. They can also be referred to as high-energy bonds.

To synthesize high-energy food molecules from more stable, lower-energy molecules, a source of energy is needed. On this earth, the source of energy that drives the vast majority of biosynthesis is the light energy coming from the sun. Plants, algae, and bacteria have evolved molecular mechanisms for harnessing the energy in photons of light and using that energy to synthesize organic molecules like sugars, fats, proteins, etc.

But none of this could be appreciated until people had developed a concept of energy and had learned how to determine the relative amounts of potential energy in different molecules. The scientists whose work we have described so far understood correctly that light is necessary for photosynthesis and that the carbon, hydrogen, and oxygen atoms in carbon dioxide and water molecules are used as raw materials to manufacture organic molecules. But these scientists did not appreciate that carbon dioxide and water are made up of especially strong chemical bonds and therefore have relatively low potential energy, while organic molecules are made up of weaker chemical bonds and therefore have higher potential energy. Not appreciating this, they could not understand why light is necessary for photosynthesis.

As you may recall from earlier in this semester, the science of thermodynamics emerged when people realized that energy can be converted from one form to another but never created or destroyed. For example, potential energy can be stored when objects are moved against a gravitational attraction, but it takes work to do so, and work is just the application of energy—in this case, kinetic energy. One especially important transformation of energy in the early development of thermodynamics was the transformation of kinetic energy into heat. We discussed how Thomson and Joule came to realize that heat is produced when movement causes friction. Heat gradually came to be understood as one kind of entropy—the random movement of the individual molecules that make up a material.

When we introduced thermodynamics, we spoke of chemical energy as the potential energy in weaker, less stable chemical bonds, but we did not describe how people came to appreciate this. As with the science of thermodynamics in general, the observations that led to the discovery of the thermodynamics of chemicals centered on the release of heat. Many chemicals can be burned to release heat, including organic chemicals. Wood has been used by hominids as a fuel source to produce heat for over one million years. Coal, which also consists of organic molecules, was used as fuel by Chinese people many hundreds of years ago and later played an important part in the Industrial Revolution, starting in the late 18^th century.

Once people realized that heat is just one form of energy, they also naturally realized that any substances which can be burned, releasing heat, must contain energy in their chemical makeup. They also devised instruments for measuring how much heat was released in particular chemical reactions, to determine how much potential energy was contained in the reactants relative to the products. The unit used to measure this chemical potential energy was (and is) the calorie: the amount of energy required to raise the temperature of one gram of water by one degree centigrade.

To find out how much energy was released in a particular chemical reaction, they merely measured how much the temperature of a known quantity of water was raised when that reaction was permitted to occur. Using this approach, chemists could determine precisely how much energy was released when particular food molecules were reacted with oxygen to produce carbon dioxide and water. In the 1870s, Jospeh Willard Gibbs described an equation that enabled chemists to more easily determine the relative amounts of chemical potential energy in the reactants and products in any chemical reaction.

Once people appreciated that food molecules are sources of energy, they could then look at photosynthesis and respiration in a whole new way. Since the combustion of food molecules to produce carbon dioxide and water releases energy, then the production of food molecules and oxygen must require energy. Once this was grasped, it did not require much imagination to realize why sunlight is necessary for photosynthesis. As we will see in the next section, the simple realization that sunlight is a source of energy still leaves many questions about photosynthesis unanswered—such as the question of how this energy is harnessed to synthesize organic molecules—but it is certainly a start.

The application of thermodynamic thinking to respiration enabled people to better appreciate what food is used for. Lavoisier had already shown that heat is released when food molecules are combusted, but later scientists realized that the generation of heat is not the main reason the energy in food molecules is released. Julius Mayer and Hermann Helmholtz hypothesized correctly that this energy is used in part to generate force in the muscles. We could not lift heavy objects if we did not have some source of energy that is converted into kinetic energy in our muscles. Max Rubner subsequently performed a great many experiments to ascertain just how much caloric energy was consumed by animals, what molecules were the most important sources of energy, and how much energy was converted into kinetic energy in muscles.

These concepts may seem obvious today, but up until this time, people actually do not appear to have wondered where the energy comes from that enables animals to generate the force needed to move their bodies. It is as though they took for granted that animals had the ability to move—like it was some sort of vital force. The idea that animals just transform the chemical potential energy of food into kinetic energy in the muscles was one more step towards a purely mechanistic explanation of living things. It enabled scientists to view animals as the biological equivalents of the steam engines that were used to convert heat energy into kinetic energy during the Industrial Revolution.

Gradually, people also came to realize that the generation of macroscopic forces, as in muscles, is only one of the uses of nutritional energy. As biologists began to describe more of the activities of living cells, they came to appreciate that energy is also needed to reduce entropy on the cellular level—to synthesize organic molecules, to transport molecules, to create structure out of chaos. The eventual fate of most of the energy released from food molecules is indeed heat (as Lavoisier thought), but it is the intermediate uses of that energy that make life possible.