Generating energy from catabolism

The term metabolism comprises the entire physical and chemical processes involved in the maintenance and reproduction of life in which nutrients are broken down to generate energy and to give simpler molecules (catabolism) which by themselves may be used to form more complex molecules (anabolism). In case of heterotrophic organisms, the energy evolving from catabolic processes is made available for use by the organism (IUPAC). 

Bacterial cells obtain nutrients from the medium on which they grow. Many of their metabolic pathways for fuel oxidation are similar to those in eukaryotes and generate NADH and ATP however, individual steps in these pathways may use different coenzymes or very different enzymes for catalysis than do human cells. Like human cells, bacteria use intermediates of glycolysis and other basic degradative pathways to serve as precurors for biosynthetic pathways, and energy acquired from catabolic pathways is used iu anabolic pathways. Aerobic bacteria, such as Escherichia coli, contain enzymes of the tricarboxylic acid (TCA) cycle and the components of the electron transport chain, which are located in the cell membrane. However, many bacteria are anaerobes and can function in the absence of oxygen. 

The enzyme-catalysed reactions that living organisms use to generate the energy they need and to prepare the molecules they require, are collectively termed metabolism . In anabolism, enzyme-catalysed reactions are used to make large complex molecules, for example the synthesis of starch, cellulose and proteins. Photosynthesis is an example of an anabolic reaction. These processes require an input of energy from ATP. In catabolism, nutrient molecules are hydrolysed under enzyme control to provide smaller and simpler molecules for synthesis and also to supply energy (to convert ADP back to ATP). 

As is often the case, tissue-specific control mechanisms operate to optimise adaptation to particular conditions. For example, muscle contraction requires an increase in cytosolic calcium ion concentration (see Section 7.2.1, Figure 7.4). During exercise when energy generation needs to be increased, or from a more accurate metabolic point of view, when the ATP-to-ADP ratio falls rapidly, and the accompanying rise in [Ca2 + ] activate (i) glycogen phosphorylase which initates catabolism of... 

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