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Chapter 5 Energy Metabolism

Section 1 Sources of Energy

1 ATP

3 Lipids and Proteins Can Also Be Used As Sources of Energy

2 Glucose Is the Major Energy Source for Most Cells

1 ATP

the energy present in the chemical bonds of our nutrients has to be transferred to a common energy “currency,” ATP ， in a process we name energy metabolism (figure 5.1).

figure 5.1

1 ATP

Many different processes in the cell need a supply of ATP to take place, and one of the most obvious of these processes is muscle contraction. Muscle contractile work is basically a question of transforming chemical energy into kinetic (mechanical) energy, much in the same way as the combustion engine in a car. Some of the energy used for muscle contraction is also converted to heat, which is taken up by the blood and carried to the skin, where it is dissipated to the environment. The CO2 resulting from muscle metablosim is transported to the lungs and expired, while the extra water may be removed by sweating or lost in the urine, depending primarily on the relative intensity of the muscular work and the ambient temperature and humidity.


• Glucose, the major energy source for most cells, is the only one that can be broken down anaerobically. Lipids and proteins, to the extent that the latter is used for energy purposes, can be broken down only areobiically (see figure 5.1).The first stepsin glucose catsabolism (brkeakdown) from glucose to pyruvic acid-collectively called glycolysis are always anaerobic;

• Although the glycolytic pathway enables the cell to function under anaerobic conditions, it remains an inefficient solution energetically. Only two molecules of ATP are the yield from the breakdown of each molecule of glucose, and two molecules of lactic acid accompany it. In comparison, the complete breakdown of one molecule of glucose yields 36 molecules of ATP in addition to water (H2O) and CO2 but, most important, no lactic acid.


• Lactic acid has, however, gained an undeservedly bad reputation, especially among laypeople.

• First of all, it should be called a metabolic intermediate rather than a waste product, simply because most of the energy originating from the glucose molecule is still intact. Secondly, since inteacellular glucose and all the metabolic intermediates of the glycolytic pathway are able to traverse the muscle cell membrane because they are phosphorylated, lactic acid becomes important as transferable energy. It can be released by muscle fibers working anaerobically and later taken up and metabolized by other fibers working aerobically (Chatham 2002).


• Although glucose is the most readily available source of energy for muscle fibers, both lipids and, to a lesser degree, proteins can be used as energy sources for cellular activity.

• Lipids emerge as an important source of energy both for muscle fibers and other cells, not least because, unlike carbohydrates, lipids can be stored in almost unlimited amounts in the body. In addition, lipids are a very efficient way of storing energy. Lipids storing the same amount of energy as carbohydrates would increase the weight of the energy stores by a factor of five. It must be considered a disadvantage, however, that the energy stored as lipid is not as readily accessible as energy stored as carbohydrates. The main lipid stores asubcutaneous fat tissue as triacylgycerol, which must be broken down to glycerol and free fatty acids (FFA) by the enzyme triavylglycerol lipase. In the blood, the FFAs ate transported bound


• to albumin, and even though the enzyme is located in an entirely different tissue, the triacylglycerol lipase reaction is the flux-generating reaction for fatty-acid oxidation in muscle. In spite of lipids’ abundance, they are unable to cover the energy demand during highintensity aeroblic activity. Therefore, when the carbohydrate stores are empty, lipid oxidation alone can provide only the energy necessary for activity at about 50% of VO2 max.

• Proteins, on the other hand, are not a preferred fuel for muscle cells, and when amino acids are used for energy purposes, the amino group must first be removed. This occurs in the liver. The resluting deaminated carbon skeleton can be converted to acetyl coenzyme A (acetyl CoA) and can enter the citric acid cycle or be converted to glucose in the process called gluconeogenesis.

Section 2 How Energy Metabolism Adapts to the Needs of the Body

• The energy requirement of the human body varies within wide limits, both among individuals and from one situation to another in the same individual. This is primarily due to variations in physical activity. Consequently, it is of utmost to the actual demand. This is achieved by means of a handful of basic biochemical mechanisms.

• Energy metabolism consists of a sequence of chemical transformations, coupled in series, that stepwise degrade a glucose molecule to H2O and CO2 (see figure 5.1). Some of the chemical steps are what commonly called equilibrium recations, that is, recations that can go in both directions, depending on the relative concentrations of substrate and reaction product. Since coupling in series means that the product of one chemical reaction is the substrate for the next, the flux through a series of equilibrium reactions is determined by the flux through the first reaction in the series.


• Other steps are nonequilibrium reactions, where the flux in one direction far exceeds the flux in the opposite direction. The nonequilibrium reactions make sure that the reactions proceed in the direction necessary to turn the glucose into H2O and CO2, at the same time covering the energy requirements of the cell.

• The long sequence of chemical transformations starting with glucose and ending with H2O and CO2 can be divided not only into an anaerobic and an aerobic part but also into shorter sequences of recations, each starting with a nonequilibrium reaction. Such sequences can be called metabolic patways (Newshlome and Leech 1984). The enzymes of the nonequeuilibrium reactions have the additional propertly that molecule can modulate their activity. Since this regulator site is different from the active site where the substrate binds, the regulator site is also called an allosteric site (allos=other). Such a regulatory molecule may be a downstream reaction product acting back on the first reaction in that particular sequence in a feedback mechanism, or it may be some other relevant molecule.


• The flux through the equilibrium reactions adjusts to the amount of reaction product coming out of the preceding nonequilibrium reaction. Consequently, the flux through the initial nonequilibrium reaction determines the flux through that particular part of the metabolic pathway. In this way allosteric regulation of nonequilibrium reactions makes it possible to tune the metabolic activity of the cell to its actual energy requirements.

• At rest the energy requirements of the muscle cell are modest, but when the cell faces an immediate danger, the energy demand rises sharply. Such acute changes are due to the action of adrenaline, released from the adrenal medualla as a result of sudden increased activity in the sympathetic nervous system. Through the action of cyclic adenosine monophosphate (cAMP) and a kinase cascade, adrenaline increases the release of glucose from intracellular stores of glycogen, thus providing an increased supply of substrate for the glycolytic pathway.