Medical Physiology, 3rd Edition

Energy Balance

Energy input to the body is the sum of energy output and storage

The first law of thermodynamics states that energy can be neither created nor destroyed; in a closed system, total energy is constant. This concept is illustrated in Figure 58-4. Humans acquire all of their energy from ingested food, store it in different forms, and expend it in different ways. In the steady state, energy intake must equal energy output.

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FIGURE 58-4 Energy balance.

The GI tract breaks down ingested carbohydrates (see pp. 914–915), proteins (see pp. 920–921), and lipids (see p. 925) into smaller components and then absorbs them into the bloodstream for transport to sites of metabolism. For example, the GI tract reduces ingested carbohydrates to simple sugars (e.g., glucose, fructose), which are then transported to liver and muscle cells and either oxidized to release energy or converted to glycogen and fat for storage. Oxidation of fuels generates not only free energy but also waste products and heat (thermal energy).

The body's energy inputs must balance the sum of its energy outputs and the energy stored. When the body takes in more energy than it expends, the person is in positive energy balance and gains weight. In the case of adults, this gain is mostly in the form of fat. In healthy children during growth periods, this gain is mostly in the form of muscle, organ, and bone growth. Conversely, when energy intake is less than expenditure, this negative energy balance leads to weight loss, mostly from fat and, to a lesser extent, from protein in muscle.

A person can gain or lose weight by manipulating energy intake or output. An optimal strategy to encourage weight loss involves both increasing energy output and reducing energy intake. In most people, a substantial decrease in energy intake alone leads to inadequate nutrient intake, which can compromise bodily function.

Nitrogen balance—the algebraic sum of whole-body protein degradation and protein synthesis—is an indication of the change in whole-body protein stores. It is estimated from dietary protein intake and urinary nitrogen (i.e., urea) excretion. Children eating a balanced diet are in positive nitrogen balance because they store amino acids as protein in the process of growth. Patients who have suffered burns or trauma are usually in negative nitrogen balance because of the loss of lean (mostly muscle) body mass.

The inefficiency of chemical reactions leads to loss of the energy available for metabolic processes

The second law of thermodynamics states that chemical transformations always result in a loss of the energy available to drive metabolic processes—the Gibbs free energy (G). The total internal energy (E) of the human body is the sum of the disposable or free energy (G) plus the unavailable or wasted energy, which ends up as heat (i.e., the product of absolute temperature, T, and entropy, S):

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(58-1)

For example when you ingest glucose, the total internal energy increases by a small amount (ΔE). Some of this energy will be stored as glycogen (ΔG), and some will be wasted as heat (T ⋅ ΔS). According to Equation 58-1, as long as the temperature is constant, the change in total internal energy will have two components:

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(58-2)

Thus, some of the increased total energy (ΔE) will be stored as glycogen (ΔG). However, because of the inefficiencies of the chemical reactions that convert glucose to glycogen, some of the ΔE is wasted as heat (T ⋅ΔS). Another way of stating the second law is that T ⋅ ΔS can never be zero or negative, and chemical reactions can never be 100% efficient.

If we add no energy to the body (i.e., ΔE is zero), the body's total free energy must decline (i.e., ΔG is negative). This decline in G matches the rise in T ⋅ S, reflecting inefficiencies inherent in chemical transformations. Consider, for example, what would happen if you took 1 mole of glucose (180 g), put it into a bomb calorimeter with O2, and completely burned the glucose to CO2 and H2O. This combustion would yield 686 kcal in the form of heat but would conserve no usable energy. Now consider what happens if your body burns this same 1 mole of glucose. In contrast to the bomb calorimeter, your mitochondria not only would oxidize glucose to CO2 and H2O but also would conserve part of the free energy in the form of ATP. Each of the many chemical conversion steps from glucose to CO2 and H2O makes available a small amount of the total energy contained in glucose. Converting 1 mole of ADP and inorganic phosphate (Pi) to 1 mole of ATP under the conditions prevailing in a cell consumes ~11.5 kcal/mole. Therefore, if a particular step in glucose oxidation releases at least 11.5 kcal/mole, it can be coupled to ATP synthesis. The conversion of the lower-energy ADP to the higher-energy ATP traps energy in the system, thus conserving it for later use. The cellular oxidation of 1 mole of glucose conserves ~400 kcal of the potential 686 kcal/mole; the remaining 286 kcal/mole is liberated as heat.

Free energy, conserved as high-energy bonds in ATP, provides the energy for cellular functions

ATP consists of a nitrogenous ring (adenine), a 5-carbon sugar (ribose), and three phosphate groups (Fig. 58-5). The last two phosphates are connected to the rest of the molecule by high-energy bonds. The same is true for a related nucleotide, GTP. If we compare the free energies of phosphate bonds of various molecules, we see that the high-energy phosphate bonds of ATP lie toward the middle of the free-energy scale. Thus, in the presence of Pi, ADP can accept energy from compounds that are higher on the free-energy scale (e.g., phosphocreatine), whereas ATP can release energy in the formation of compounds that are lower on the free-energy scale (e.g., G6P). ATP can therefore store energy derived from energy-releasing reactions and release energy needed to drive other chemical reactions.

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FIGURE 58-5 Hydrolysis of ATP to ADP, Pi, and H+.

Examples of the chemical reactions fueled by converting ATP to ADP and Pi include the formation of bridges between actin and myosin during muscle contraction and the pumping of Ca2+ against its electrochemical gradient during muscle relaxation. imageN58-4

N58-4

Hydrolysis of ATP

Contributed by Kitt Petersen, Gerald Shulman

Although the conversion of ATP to ADP and Pi is often referred to as a hydrolysis reaction, because the traditional representation is ATP + H2O → ADP + Pi + H+, the reaction actually occurs in two steps. The first typically involves transfer of a part of the phosphate group to an intermediate molecule, thus increasing the intermediate's free-energy content. The second step involves displacing the phosphate moiety, which releases Pi and energy. In contrast, true hydrolysis reactions merely release heat, which cannot be trapped to drive chemical processes.