Metabolism can be described as the sum of the chemical reactions in an organism that produce and maintain its molecular structure, and the chemical reactions that make energy available for those processes. Metabolism can therefore be divided into anabolism and catabolism.
Anabolism
This is the production of complex molecules from more simple precursors, e.g. the synthesis of amino acids from simpler carbon skeletons and a source of amino groups, the synthesis of proteins from amino acids. These are energy-requiring processes where energy is usually consumed in the form of adenosine triphosphate (ATP). The end products are often more reduced than the simple precursors, and so reducing power, in the form of NADPH, is often required (Figure 8.1).
Catabolism
This is the destruction or disassembly of complex molecules, often ultimately leading to the release of chemical energy, which is usually stored in the form of ATP (Figure 8.1). Examples of catabolism include the breakdown of complex lipids (fats) to glycerol and fatty acids, and the oxidation of the resulting fatty acids in mitochondria (β-oxidation) to acetyl coenzyme A, which in turn can be further oxidised to carbon dioxide and water (TCA cycle). Both processes lead to the reduction of NAD+ to NADH, which in turn can be used to drive the formation of ATP.
Energy requirements
It follows from the above that to maintain cells and, in addition, to support cell growth and division, energy is continually required. Note that many key molecules, especially proteins, once made do not last forever but rather are subject to turnover, that is continual synthesis and degradation back to amino acids. This turnover requires energy, therefore simply to maintain the normal complement of cellular proteins, outside of growth, requires energy input. The average daily energy requirement of a sedentary women is just under 2000 kcal/day, rising to 3080 kcal/day for a very active person. This energy is supplied by the oxidation of fuel molecules: carbohydrates, fats (lipids) and proteins, with carbohydrate being the major fuel for most humans. To sustain growth or support pregnancy, the calorific requirement will rise. If the calorific intake does not match the demands made on the organism then mechanisms will be invoked to try and match those demands by utilising stored fuels, such as fat reserves, and even by the breakdown of body proteins and oxidation of the resulting amino acids. It is recommended that human calorific intake should be 55% from carbohydrates, 30% from fats (lipids) and 15% from protein.
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