While our focus is physiology, as we have already seen, understanding physiological functions depends on an appreciation of chemistry, biochemistry, physics, and related disciplines. This chapter introduces you to mechanisms that explain how cells satisfy energy needs and generate the building blocks required for growth and development. In many respects our discussions are rudimentary. However, this foundation will give you the tools and concepts to understand the roles and actions of each physiological system and to excel in advanced courses in nutrition, reproduction, lactation, or other biological and animal‐oriented courses.

Metabolism and Energetic Definitions

We have all heard the expressions, “there are no free rides” or “no free lunches.” This is certainly true when it comes to living our lives, but it is also a physiological truth. For example, acquiring the energy to maintain the ionic gradients we discussed in chapter two depends on the hydrolysis of adenosine triphosphate (ATP). The question becomes, where did the energy needed to produce ATP come from in the first place? Essentially, some molecules are catabolized so that others can be created. Simplistically, living systems are in a constant thermodynamic battle. As the second law of thermodynamics indicates, the natural tendency is the production of equilibrium with the dispersion of energy and increasing disorder. In other words, increased entropy. Living systems, cells, tissues, organs, systems, and organisms are just the opposite. The degree of complexity and organization in living systems is antithetical to this law. Unlike most of the nonliving universe, open biological systems can exchange matter and energy with their surroundings. This allows living systems to move away from the dispersion and energy equilibrium that the second law of thermodynamics dictates. The first law of thermodynamics is familiar as the maxim that energy can neither be created nor destroyed. More formally, it is expressed in this way: the total energy of a system plus its surroundings remains constant. Thus, the exquisite organization and complexity that characterize living systems are at the expense of free energy from the environment. The energy that can be harnessed to do work. Thus, living systems are analogous to an oasis in the desert. The oasis, often short‐lived in a geological sense, provides a place for relief for the weary traveler. Living systems represent transient conditions during which time nonequilibrium conditions related to energy–matter circumstances exist.

A fundamental postulate of theoretical biology is that life processes can be explained in terms of chemistry and physics—that is, in terms of matter and energy. Building on this idea, life processes are represented by the many chemical reactions (enzymatic and nonenzymatic) and physical reactions that occur within cells and tissues, or, simply stated, metabolism. The yin and yang of metabolism are anabolism and catabolism. Our discussion begins with catabolism.

The phrase intermediary metabolism appears often in nutrition and biochemistry texts. This refers to the many steps of reaction between the initiation of a biochemical process and its completion. For example, the complete oxidation of the critical nutrient glucose begins with the uptake of the glucose into the cell and the entry of the molecule into a sequence of reactions called glycolysis. This is an example of a biochemical pathway. The steps in the pathway detail the reactions necessary to convert this six‐carbon hexose sugar into two three‐carbon molecules of pyruvate. This process is also called anaerobic respiration. The various molecules that are produced in the 10 steps of glycolysis are called intermediates. Other biochemical pathways generate their group of intermediate molecules. Consequently, intermediary metabolism refers to the creation and existence of the hundreds of molecules that are fabricated as various molecules progress toward their final biochemical destination. Finally, although we typically describe important biochemical pathways singly, it needs to be emphasized that a host of biochemical reactions or pathways are occurring simultaneously. Intermediates from one pathway often supply materials used in other pathways. For example, one of the intermediate steps in glycolysis produces triose phosphate. This molecule can supply the carbon atoms to make pyruvate or, alternatively, be shuttled out of the glycolysis pathway to produce glycerol necessary in the anabolic pathway to make triglycerides. A key idea is that regulation and control of the rates of activity of these various, often competing biochemical pathways, is critical. Resources must be used effectively and efficiently.

Given the importance of energy to fuel the biochemical reactions in cells, some definitions are in order. What do we mean by energy? Are all forms of energy equally valuable from a physiological viewpoint? We often say that the diets we supply to our animals give them the energy they need for productive functions. For example, to allow draft horses to pull wagons, thoroughbreds to race, or cows to produce milk. Our animals do not consume energy directly, but rather it is the digestion of foodstuffs that liberates the nutrient molecules that can then be oxidized in a controlled, deliberate fashion to meet energy needs. In physics, energy is described as the capacity to do work. In biological systems, energy is expressed as heat units or calories. A calorie (cal) is the amount of energy needed to increase the temperature of 1 g of water 1 degree at a pressure of 1 atmosphere. Other related terms are the kilocalorie (1000 cal) or the megacalorie (Mcal = 1,000,000 cal), often used in descriptions of the energy content of feedstuffs. Other measures of energy also appear. For example, 1 joule (J) is the work done by a force of 1 N working over 1 m (m² × kg × second⁻²). Other measures of energy can be derived as well (1 cal = 4.187 J or 0.004 British Thermal Units (BTU)).

It is also clear that not all the bonds or chemical energy in food molecules can be captured for use by tissues or cells. If a known quantity of a food material or nutrient is completely combusted or oxidized in the presence of oxygen and the heat generated is measured, this estimates the gross or potential energy of the substance. This process is called calorimetry. This technique is valuable because it provides a measure of the potential energy available in each nutrient. However, as another old expression says, “it takes money to make money,” there are biochemical costs involved in acquiring the energy available in various nutrients. Just as there are overhead costs in running a business or a university for that matter, there are physiological costs that must be paid to capture the nutrient energy. For example, consider feeding your horse a carrot and the steps needed to capture the energy in the starch. For processing, there is mastication and swallowing, transport to the small intestine, secretion of gastric and intestinal enzymes, and transport of digested glucose molecules across the epithelial cells. All these events can be thought of as overhead or maintenance costs associated with the physiological processing of the carrot.

Thus, the amount of energy present in the diet does not equal the energy ultimately available to the animal. Animal science nutritionists often conduct feeding trials to evaluate the practical value of different feedstuffs. These feedstuffs are often complex mixtures. Consider the total mixed rations that are often fed to dairy cows with various combinations of forages and concentrates. In such studies, great care must be taken to account for measurement errors, for example, spilled feed and variation in the efficiency of digestion and ingestion. Particularly for complex rations, the gross energy content of the diet gives only a broad indication of how valuable the diet is to the animal. What happens if one of the components in the diet cannot be digested? What if there are interactions so that the breakdown component affects the microorganism population of the GI tract so that normally effective nutrients are lost? Even under the best conditions, especially for animals fed fibrous feeds, a part of the feed is not digested and contributes to the energy content of the feces. The difference between the gross energy content of the diet and the portion of the energy available to the animal is called digestible energy. Various diets can then be compared based on their digestibility energy, usually expressed as a fraction or percentage of the gross energy. From measurements made over the course of several days and sometimes weeks, average daily energy consumption minus the energy content of the feces and urine can determine the apparent digestible energy of the feed. It is called apparent because some, generally small, portion of fecal energy comes from sloughed intestinal epithelial cells, bacteria cells, and substances that have been excreted by way of the feces.

For nutrients that are ultimately absorbed, the energy content still does not match the energy ultimately available to the animal. The fraction represented by digested and absorbed nutrients is called metabolizable energy. This is the fraction of total or gross energy in the diet that is directly available to the tissues and cells to be processed. This energy can be used for maintenance, heat generation (sometimes considered a waste product), or for more recognizable practical productive functions, that is, muscle growth, egg production, or milk production. Metabolizable energy is less than digestible energy, typically about 80%, because of other losses in addition to the fecal losses. This fraction would be lower still if the heat that is generated is considered a waste product. The additional losses include molecules lost in urine and gaseous products from gut fermentation that are expelled. These products are especially plentiful in herbivores and particularly ruminants. The fraction of energy that is ultimately used for physiological activities is called net energy. Figure 3.1 illustrates the processing of dietary energy.

It is common to characterize energy that is needed for basal or resting life activities in animals as the energy of maintenance. These energy costs do not reflect energy needed for work in a production agriculture sense. For example, work done by a draft horse or racehorse or energy recovered from products such as meat, milk, or eggs. These are critical physiological functions: the action of the Na‐K ATPase pump, cardiac function, and so forth. As Figure 3.1 illustrates, there are additional energy costs associated with acquiring feed and assimilating nutrients themselves. Generally, maintenance activities are the energy costs of preserving an adult animal under resting or sedentary circumstances, in the absence of weight gain or loss, in thermoneutral conditions. In practical terms, this ideal status is rarely obtained, but it does give a framework to understand the meaning of maintenance energy costs. In humans, the term basal metabolic rate may be more familiar and is determined for subjects in a quiet, thermoneutral environment, about 12 hours after a meal.

Basal energy needs are determined indirectly by measuring oxygen consumption under these conditions. This is effective because the amounts of oxygen needed to completely oxidize fats, proteins, or carbohydrates are known. If CO₂ production is simultaneously measured, the respiratory quotient can be calculated (moles of CO₂ exhaled divided by the moles of O₂ consumed). These data can then be used to estimate the nature of the nutrients (protein, carbohydrate, or fat) being oxidized for energy production. The alternative to indirect calorimetry is to measure heat production along with the completed collection of gases and other wastes. This is clearly a difficult and expensive undertaking with large domestic animals. However, Table 3.1 shows an example of calorimetry data from a study by Tyrrell et al. (1988) designed to determine, in part, if the metabolic rate of dairy cows treated with bovine somatotropin (bST) was increased compared with controls. Briefly, nine cows received bST (51.5 IU/day) or a daily control injection in a single reversal experimental design, which utilized 14‐day treatment periods. With increased milk production after bST (22% increase), cows already in a negative tissue nitrogen balance (−21 g/day) tended to become more negative during the bST treatment period (−34 g/day). Energy and nitrogen balances were measured in open‐circuit respiration chambers. As predicted from increased milk production, there was a corresponding greater heat energy loss and increased milk energy secretion after bST treatment. The tissue energy balance was −1.1 Mcal/day during the control treatment period. Increased use of energy reserves with bST treatment decreased tissue energy balance to −9.8 Mcal/day. These researchers concluded that much of the effect of bST to increase milk production in dairy cows depends on increased use of tissue reserves and altered partitioning of nutrients rather than dramatic effects on the digestibility of nutrients or apparent changes in maintenance requirements of the animals. Simply, the increases observed would have been expected with the degree of increased milk production, regardless of the specific reason for increased production.

Production of ATP

For essentially all physiological activities, the most useful and available form of energy comes from ATP, or adenosine triphosphate. When the terminal phosphate group is cleaved to produce adenosine diphosphate (ADP), each mole of ATP yields 7400 calories. There are other similar high energy‐yielding molecules, but most are like ATP, for example, GTP or creatine phosphate. Indeed, the formation of creatine phosphate (which acts as a storage form of energy in muscle cells because of its capacity to regenerate ATP from ADP) gains this capacity only when ATP is produced more than immediate demand, as in the following reaction.

This means that the production of ATP is vital. The structure of ATP is shown in Figure 3.2. Mitochondria are critical because the final stages of oxidation of several coenzymes occur within these organelles. When the reduced coenzymes are oxidized, a portion of the energy produced is utilized to drive the synthesis of ATP from ADP as shown below.

As you might guess, since many different feedstuffs can be used to produce energy, a host of nutrient molecules can be modified to enter the biochemical pathway for ATP production. Although some ATP can be generated in the absence of oxygen through substrate‐level phosphorylation (essentially the regeneration of ATP from ADP as with creatine phosphate), production rates are minor compared with oxidative phosphorylation. In a nutshell, this explains the critical need for oxygen. Within the mitochondria, where reduced coenzymes are oxidized, oxygen serves as the final acceptor of electrons in this cascade of reactions. Without oxygen, the electron transport chain fails, and the energy normally available to drive phosphorylation of ADP to regenerate ATP also fails. Needs for continuous supplies of ATP are so acute that unless oxygen is quickly returned, death occurs in a matter of minutes. To understand the pathways for ATP generation, we will focus first on the catabolism of carbohydrates and specifically on the catabolism of glucose. Once we have this core of information, we will then discuss how other nutrients can be diverted to drive ATP generation.

Glycolysis

We begin our quest to understand energy production with glucose because it is the most common nutrient used for acute energy production. The initial processing of glucose molecules begins in the cytoplasm of the cell. Once inside the cell, six‐carbon glucose is converted in a series of reactions into two three‐carbon molecules of pyruvate. The 10 linked reactions responsible for this conversion are called glycolysis, the Embden–Meyerhof pathway, or anaerobic respiration. Carbohydrate catabolism usually begins with glucose, but you should remember that polymers of glucose (glycogen in animal cells) can be cleaved to provide the glucose monomers to enter the glycolysis pathway. Thus, the cell likely contains stores of glycogen that can be cleaved to supply glucose aside from the uptake of glucose across the plasma membrane. Table 3.2 provides a listing of some often confusing, similar‐sounding terms that have to do with carbohydrate metabolism. It may be useful for you to periodically refer to this table as you study this important topic. Figure 3.3 provides an outline of each of the steps in glycolysis. We have purposefully not given all the chemical structural details showing changes in carbons and functional groups as glucose is modified through glycolysis. If this is needed, it appears in any introductory biochemistry book. Our goal is for you to appreciate the highlights, the overall chemical events, and, most importantly, the physiological relevance. Although each of the reactions of glycolysis is enzyme mediated, we have focused on two glycolysis reactions and their associated enzymes along with one other reaction that is not strictly part of glycolysis. These are (1) the conversion of glucose to glucose‐6‐phosphate, step one of glycolysis, catalyzed by hexokinase; (2) step three, the conversion of fructose‐6‐phosphate into fructose 1,6‐diphosphate, catalyzed by phosphofructokinase (PFK); and (3) the conversion of pyruvate into lactate, catalyzed by lactate dehydrogenase (LDH). Some specific reactions and molecules associated with glycolysis are in Figure 3.4. At this point, it is worth remembering the significance of glycolysis. This pathway allows the conversion of the nutrient glucose into molecules that can then be shuttled into the mitochondria for use in the process of oxidative phosphorylation. However, as we indicated above, oxidative phosphorylation (simply the production of ATP linked to a series of oxidation–reduction reactions) requires oxygen. In addition to preparing molecules for entrance into the mitochondria, a small amount of ATP is produced during the glycolysis reactions. In contrast with mitochondrial activity, this occurs via substrate‐level phosphorylation. However, the amount of ATP made in this manner is very small compared with that which occurs with the complete catabolism of glucose (glycolysis reactions + mitochondrial activity), but it is nonetheless essential. This is because the production of ATP via glycolysis alone can occur in the absence of oxygen. For this reason, it is called anaerobic respiration.

Let’s outline the steps of glycolysis. Typically, glucose is captured by the action of membrane transporters and passed into the cytoplasm. At this juncture, a phosphate group is added to the sixth carbon of the glucose. This then produces glucose‐6‐phosphate. Somewhat ironically, even though glycolysis ultimately leads to energy production, in the first step of glycolysis, ATP is used. This is because the phosphate group added to the glucose is donated from ATP, as illustrated in Figure 3.4. In the next step (2), glucose‐6‐phosphate is converted into another hexose sugar by the action of the enzyme phosphohexose isomerase, that is, fructose‐6‐phosphate. Step 3 again utilizes another molecule of ATP as the enzyme PFK catalyzes the addition of another phosphate group to produce fructose 1,6‐diphosphate. Step 4 is a cleavage reaction catalyzed by the enzyme aldolase, which produces two three‐carbon molecules. These are dihydroxyacetone phosphate and glyceraldehyde 3‐phosphate. In step 5 the enzyme phosphotriose isomerase converts dihydroxyacetone into a second molecule of glyceraldehyde 3‐phosphate. There are now two identical molecules to continue through glycolysis so that products made from this point are doubled. Step 6 depends on the enzyme glyceraldehyde 3‐phosphate dehydrogenase. As we will discuss in more detail relative to mitochondrial activity, this enzyme requires the oxidized form of the coenzyme nicotinamide adenine dinucleotide, or NAD⁺. During this reaction, the NAD⁺ becomes reduced (NADH), and inorganic phosphate is added to the substrate to produce 1,3‐bisphosphate glycerate. The first direct production of ATP occurs in step 7 as a phosphate group is cleaved and the energy is utilized to simultaneously add a phosphate group to ADP. This is an example of substrate phosphorylation to produce a molecule of ATP, and the reaction is catalyzed by phosphoglycerate kinase. Step 8 depends on phosphoglycerate mutase to induce a rearrangement of the 3‐phosphoglycerate made previously to yield 2‐phosphoglycerate. Step 9 is the conversion of the 2‐phosphoglycerate into phosphoenolpyruvate catalyzed by enolase. Step 10 is another ATP‐making event as the enzyme pyruvate kinase acts to transfer a phosphate from phosphoenolpyruvate to ADP as pyruvate is also created. At this point, the pyruvate is at a crossroads. The redox state of the tissue determines which of the two alternative paths will be followed. If oxygen is available, the pyruvate is shuttled into the mitochondria as subsequently described. If oxygen is limited, that is, in anaerobic conditions, pyruvate is reduced by the action of LDH and the coenzyme NADH becomes oxidized again. This is a critical process under these conditions. You may recall that step 6 of glycolysis requires the oxidized form of this coenzyme (NAD⁺). Although only a small amount of ATP is derived directly from glycolysis, even this would be lost were it not for the action of LDH. As an aside, there are also several isozymes of LDH that are important clinically. For example, the unique structure of the LDH from heart muscle can be detected in blood serum in animals (or people) that have suffered cardiac injury (Figure 3.5). Table 3.3 summarizes ATP production that is associated with glycolysis.

The Cori Cycle

Most carbohydrates in the diet are readily converted into glucose, galactose, or fructose when digested. These molecules are absorbed into the portal vein that drains the intestinal tract for use by the liver and other tissues. There are various other compounds that are also considered glucogenic. These are molecules that can readily be converted into glucose to be processed via glycolysis for subsequent ATP production or for use in other biochemical pathways. For example, propionate which, is derived from the fermentation of dietary carbohydrates in ruminant animals, is an essential substrate to allow ruminants to synthesize the glucose they need. These glucogenic compounds can be divided into two groups: (1) those that are essentially direct conversions into glucose without a significant amount of recycling, such as propionate and certain amino acids, and (2) products of partial metabolism of glucose in selected tissues that are then transported in the liver or kidney for generation of glucose, such as lactate. In all animals, there are times when oxygen is locally limited so that anaerobic respiration is favored, and lactic acid accumulates. For example, lactic acid accumulates in active muscle tissue and produces the sensation of muscle fatigue. In addition, since erythrocytes lack mitochondria, they rely on glycolysis for all their ATP needs and consequently continually produce lactic acid regardless of oxygen availability. Most of the lactic acid from muscle or erythrocytes diffuses into the bloodstream. Fortunately, it is transported to the liver and, to a lesser extent, the kidney, where it can be converted into glucose. At this point, it can be stored as hepatic or renal cell glycogen or released back into the bloodstream for use by muscle or other tissues (see Fig. 3.10). The cycling of lactic acid from muscle (or other tissues) to the liver and the return of glucose is called the Cori cycle and is outlined in Figure 3.6.

Reaction	ATP Production Type	Number per Mole of Glucose
Glyceraldehyde 3‐P dehydrogenase	Creation of reduced NADH (2)	6
Phosphoglycerate kinase	Substrate‐level phosphorylation	2
Pyruvate kinase	Substrate‐level phosphorylation	2
Subtract ATP used by hexokinase and phosphofructokinase		−2
Summary		Net gain 8 (aerobic) Net gain 2 (anaerobic)

Krebs Cycle

Let’s now trace the fate of pyruvate that is produced at the end of glycolysis under aerobic conditions. Remember, our goal here is to understand how the catabolism of glucose and other carbohydrates is used to generate the ATP essential to cells and tissues. Once this foundation is established, we will then be able to understand how other nutrients can also be used for energy production. The next major biochemical pathway for this processing is called the citric acid or Krebs cycle. These reactions occur inside the mitochondria, so before pyruvate can be modified, it has to pass across the mitochondrial membrane.

This occurs via the action of a specific membrane transporter. The pyruvate is then quickly oxidatively decarboxylated (removal of CO₂) to produce acetyl‐CoA. This reaction is catalyzed by the action of the multienzyme complex pyruvate dehydrogenase. The product, acetyl‐coenzyme A (acetyl‐CoA), plays an especially central role in energy metabolism. This overall reaction, which involves two coenzymes, is outlined:

The oxidized‐reduced NAD⁺ and NADH are familiar from the action of glyceraldehyde 3‐phosphate dehydrogenase, or LDH, and reactions of glycolysis. Now a description of coenzyme A (CoA) is in order. CoA is a complex molecule derived from pantothenic acid (common in meats and grains), thioethanolamine, and ATP. The essential feature is that CoA acts as a carrier of acyl groups. Specifically, the thiol group of the thioethanolamine residue of the molecule functions in this manner in a variety of reactions involved in fatty acid oxidation, fatty acid synthesis, and acetylation reactions. The molecule is also important in oxidative decarboxylation reactions, as with pyruvate. A common convention is to abbreviate the structure of the reduced form of the molecule as CoA·SH, which designates the reactive SH group of the molecule. So, the acetyl group that is now part of acetyl‐CoA is derived from the catabolism of pyruvate (two carbons remaining after decarboxylation of pyruvate). Acetyl‐CoA is at the confluence of a variety of major metabolic pathways. Almost all carbohydrates and fats that are catabolized for energy production are utilized to generate acetyl‐CoA. In addition, several of the nonessential amino acids from degraded proteins are also cannibalized to generate acetyl‐CoA. As a special case in ruminants, one of the major products from the fermentation of dietary carbohydrates is acetate, which is readily converted to acetyl‐CoA for subsequent processing through the mitochondria.

Back to our story, at this point, the two carbons of the acetyl group of acetyl‐CoA and the four‐carbon molecule oxaloacetate condense to create the six‐carbon compound citrate. This is the first step of the Krebs cycle, as outlined in Figure 3.7. There are two critical physiological points to the Krebs cycle reactions. The first is that some ATP is produced directly via substrate‐level phosphorylation of ADP, like that which occurs in glycolysis. The second and most important is that with each turn of the cycle, reduced forms of the coenzymes NAD and flavin adenine dinucleotide (FAD) are produced. When enzymes of the electron transport chain, also located in the mitochondria, subsequently oxidize these molecules, this yields the energy for the synthesis of most of the ATP that can be created from the overall catabolism of glucose. We have provided only a skeleton outline showing the names of the intermediates of the Krebs cycle reactions and the locations of specific events. Remember that each molecule of glucose generates two molecules of pyruvate, so there are two turns of the cycle for each glucose molecule.

As was the case with glycolysis, the coupled oxidation–reduction reactions are a critical part of the processing. The combination of NAD+ and NADH appears again along with FAD and FADH. The enzymes that catalyze these oxidation reactions by the removal of hydrogen atoms are dehydrogenases, for example, LDH, whose action is illustrated in Figure 3.9. For these reactions to take place, the enzymes require the assistance of coenzymes that act to hold or carry these hydrogen atoms. It can be a source of confusion, but the transfer of the hydrogen atom, with its lack of a neutron but paired electron and proton, is effectively viewed as an electron transfer. Thus, oxidation–reduction reactions defined by either electron acceptance or electron donation are linked with the movement of the hydrogen atom. This explains the abbreviations related to FAD versus FADH₂ or NAD⁺ versus NADH + H⁺ in the Krebs cycle reactions that are outlined in Figure 3.7. Figure 3.8 gives an example of this type of reaction.

At this point, you are probably wondering just how much ATP gets generated from glucose catabolism and when this occurs. As we have seen, each turn of the Krebs cycle generates two molecules of ATP via substrate‐level phosphorylation. The key for the majority of ATP production depends on a cluster of interrelated membrane‐bound enzymes that make up the electron transport chain. The activity of these enzymes also accounts for essentially all our need for oxygen. As the electron chain functions, the hydrogen atoms (electrons) that are removed as various intermediates of glycolysis and Krebs cycle are oxidized and progressively passed along until they are combined with oxygen. Oxygen is the final electron acceptor in the chain, so water is formed. The reduced forms of both NADH and FADH₂ that were generated in the Krebs cycle become oxidized again as their hydrogens are donated to the electron chain enzymes. The energy that is produced as electrons pass ultimately to oxygen is indirectly used to power the attachment of inorganic phosphate groups to ADP to create ATP. The enzyme responsible for this final step is ATP synthase, whose activity is linked to the movement of hydrogen atoms down a concentration gradient across the membrane of the mitochondria. Some of the energy from the action of the electron transport chain acts to transport hydrogen ions out of the mitochondrial matrix space. The resulting electrochemical gradient drives hydrogen ions back across the membrane in conjunction with ATP synthase, leading to ATP generation. Because of the need for oxygen as the final electron acceptor in the electron transport chain, the making of ATP in this manner is called oxidative phosphorylation.

Interestingly, the position along the electron transport chain at which FADH₂ or NADH + H⁺ donate their electrons differs. Because of this, the amount of energy that is produced is greater for NADH compared with FADH₂. Each pair of hydrogen atoms from NADH + H⁺ supplies energy for the creation of three ATP, but the two hydrogen atoms from FADH₂ yield only two ATP. Most of the proteins of the electron transport chain are closely linked clusters within the inner mitochondrial membrane, along with more mobile proteins (coenzyme Q and cytochrome C) that act as carriers between complexes. Figure 3.9 illustrates the release of energy associated with the oxidation of NADH or FADH₂ in the electron transport chain, and Table 3.4 summarizes ATP production from the completed catabolism of glucose.

Under ideal conditions, the complete oxidation of 1 glucose molecule to CO₂ and water yields 36–38 ATP. Alternative figures come from uncertainty about the energy yield of reduced NAD⁺ that is produced from glycolysis. For these molecules to be utilized, they must be passed across the mitochondrial membrane by active transport. An estimate of this “expense” is that the net ATP gain from reduced NAD⁺ derived from the cytoplasm is only two ATP per molecule instead of the usual three ATP for those created inside the organelle. Since two of these molecules are produced in the cytoplasm during glycolysis, the total yield is reduced to 36 ATP per molecule of glucose. Regardless, when oxygen is available, energy captured from the biological oxidation of glucose is highly efficient. If a mole of glucose is completely combusted, as in a calorimeter, it yields 686 kcal. Energy obtained in the creation of high‐energy ATP bonds equals 262 kcal for an efficiency of 38% [262/686 × 100]. This is markedly more efficient than most machines. Energy not captured in the formation of ATP is liberated as heat.

Intermediary Metabolism: Processing and Pathways

Now that we have an appreciation for the processing of glucose to make ATP, we will explore some of the alternatives for storing glucose for situations when it is not immediately required for ATP generation, as well as pathways involved in mobilizing carbohydrate reserves. Similarly, we will also consider pathways that allow other important nutrients, that is, proteins and lipids, to be processed for ATP production. As we shall see, glycolysis, the Krebs cycle, and the electron transport chain are central to the capacity to catabolize many different nutrients.

Glycogenesis, Gluconeogenesis, and Glycolysis

While much of the available glucose is used to produce ATP, when energy demands are reduced, ATP production also declines. Cells have little capacity to “store” ATP; in fact, as ATP concentrations in the cytoplasmic rise, this produces allosteric inhibition of the regulatory enzyme PFK. So, what happens to excess glucose? Fortunately, this rise in ATP stimulates reaction pathways that act to convert excess glucose molecules into glycogen and into fat. Our animals have much more capacity to store fat than to store glycogen, but glycogen stores are nonetheless critical, especially for acute energy demands. We will consider fatty acid synthesis (lipogenesis) and catabolism (lipolysis) in a subsequent section.

When glycolysis is inhibited but glucose is available, this initiates glycogenesis (glyco = sugar + genesis for origin). Like the case with glycolysis, the first step depends on the uptake of glucose and conversion to glucose‐6‐phosphate by the ubiquitous enzyme hexokinase. However, instead of progressing through the glycolysis pathway, the glucose‐6‐phosphate is converted to its isomer glucose 1‐phosphate by the action of glucose‐6‐phosphomutase. Interestingly, the ability of this enzyme to bind glucose to its active site is substantially less than for hexokinase. In other words, its binding site has much less affinity for glucose. This means that when concentrations of glucose are low (likely also associated with a need for energy), then the hexokinase reaction pathway is favored because of the higher affinity binding site. Of course, high concentrations of ATP also allosterically inhibit PFK. As glucose concentration increases, the law of mass action promotes the activity of the mutase enzyme, favoring the path toward glycogen synthesis. The enzyme glycogen synthase catalyzes the attachment of glucose‐1‐phosphate molecules to growing glycogen chains (see Fig. 2.22).

As energy demands increase, stored glycogen molecules can be hydrolyzed to cleave glucose molecules for use by the cells. This process is called glycogenolysis and is catalyzed by the enzyme glycogen phosphorylase. This regenerates glucose‐1‐phosphate, which can then be converted to glucose‐6‐phosphate and processed for glycolysis. In most tissues (e.g., muscle cells), the glucose‐6‐phosphate is effectively trapped in the cells since it cannot interact with membrane carrier proteins. This means that for most cells, glycogenolysis can supply energy for specific cells with stored glycogen only. However, liver cells, along with some intestinal and kidney cells, express the enzyme glucose‐6‐phosphatase, which catalyzes the removal of the phosphate group. In these cells, when intracellular concentrations of glucose are increased, some of the glucose can leave the cells and enter the bloodstream. The capacity of the liver to utilize some of its glycogen stores to replenish blood glucose is critical for homeostasis. Pathways associated with glycogenesis and glycogenolysis are illustrated in Figure 3.10.

Describing gluconeogenesis completes our discussion of glucose metabolism. As we have seen, glucose and its intermediates from glycolysis and the Krebs cycle are essential. Maintenance of blood glucose concentrations within relatively narrow boundaries is vital to the homeostasis and health of our livestock and pets. However, in some situations (especially acute for ruminants), rations either do not supply sufficient carbohydrates or situations of high demand or depleted glucose reserves occur. Fortunately, there is a kind of metabolic backup system. Gluconeogenesis effectively protects the body, and especially the nervous system, which has an absolute requirement for glucose, from hypoglycemia. Luckily, many nonessential amino acids and some other intermediates can be converted into glucose. This conversion is acutely driven by increases in stress‐related hormones (epinephrine and glucocorticoids) and over longer periods by increased secretion of growth hormone and triiodothyronine. These topics will be covered in greater detail when we consider endocrinology (Box 3.1).