Fermentative and Aerobic Energy Metabolism

Skeletal muscle is composed of two different types of cells white and red muscle fibers. Some muscles are fairly homogeneous and contain mainly one type of fiber others contain mixtures of both types. White muscle fibers contain relatively few mitochondria, are mainly glycolytic, and produce lactic acid as an end-product of fermentation- Red muscle fibers are relatively rich in mitochondria and are well adapted to oxidising fatty adds and glucose to CO2- Their energy metabolism is dependent on an ample supply of oxygen. 

Lactate dehydrogenase is closely associated with the final step in fermentative metabolism conversion of pyruvate to lactate- Citrate synthase is closely associated with oxidative metabolism, because this enzyme catalyzes the introduction of acetyl groups into the Krebs cycle. Continued operation of the Krebs cycle is dependent on the continued transfer of electrons from reduced NAD and FAD to the respiratory chain, which catalyzes the reduction of Oi to HjO. Table 4.14 lists lactate dehydrogenasc/citrate synthase activity ratios for muscles that contain a large proportion of white or red muscle fibers. The data demonstrate that the ratio is relatively high in white cell muscles but low in red cell muscles. 

Would you expect carnitine, which is made in the liver, to be moie important to cardiac muscle or to vifhice skeletal muscle  

TABLE 4.14 Enzyme Activity Ratios for Muscles that Contain Large Proportions of White or Red Muscle Fibers 

Muscle RatLCi of activities of lactate dehydrofjenflse/ citrate TiynthaHe 

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Htucidation of muscle energy metabolism is complicated by several factors. Muscle can receive glucose and fatty acids from internal sources as well as from the bloodstream. The relative importance of internal versus external sources can be difficult to determine. Muscle is a heterogeneous tissue. Some muscle types carry out mainly aerobic forms of energy metabolism other types are fermentative and still others are composed of mixtures of cells, some aerobic and some fermentative. 

Alcoholic fermentation is less efficient in producing ATP than aerobic root respiration thus, net energy production by the anaerobic pathway is only a fraction of that produced by the aerobic respiration. Root energy metabolism under aerobic and anaerobic conditions is as follows ... 

Fermentation a form of metabolism producing incompletely oxidized end products. Per unit of substrate, F. yields far less energy than respiration, e.g. a yeast cell obtains 2 molecules ATP per molecule of glucose when it ferments glucose to ethanol, whereas complete respiration would yield 38 molecules of ATP (see Alcoholic fermentation). Strictly speaking, F. is an anaerobic process (Pasteur defined F. as life without air ) but the term is also widely and loosely applied to certain aerobic processes, such as acetic acid F., and to any industrial production process employing microorganisms in a fermentor (see Fermentation techniques). 

Aerotolerant anaerobes Microbes that grow under both aerobic and anaerobic conditions, but do not shift from one mode of metabolism to another as conditions change. They obtain energy exclusively by fermentation. 

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