Anabolism versus catabolism

It is a common statement that life is always connected to the production of heat. That is true in most cases, perhaps with one (known) exception, i.e. the anaerobic production of CH and CO2 from acetate. In this process the entropy term outweighs the heat term and, at least in theory, life could be connected to net heat absorption [23]. However, if we ignore this exception and instead concentrate on what part of metabolism generates the heat. Is it due to catabolic or anabolic reactions The answer is in fact both catabolism and anabolism is involved. 

It is important to realise that heat generation is not a consequence or measurement of the rate of ATP turnover. In an aerobic process the heat production rate is directly proportional to the oxygen consumption rate. Whether there are 6 (P/O ratio = 3) or just 1 (P/O ratio = 0.5) ATP formed per 

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M. Sato, H.U. Bryant, and R.T. Turner. 2005. Molecular profile of catabolic versus anabolic treatment regimens of parathyroid hormone (PTH) in rat bone ... 

As everybody knows, all food in excess can be stored as fat. This is true for carbohydrates, proteins, and, of course, fats. In addition, these molecules can be interconverted, with the exception that fats cannot give a net yield of carbohydrates, as we saw in Section 19.6. Why can fats not yield carbohydrates The real answer lies in the fact that the only way a fat molecule would have to make glucose would be to enter the citric acid cycle as acetyl-CoA and then to be drawn off as oxaloacetate for gluconeogenesis. Unfortunately, the two carbons that enter are effectively lost by the decarboxylations. (We have already seen that, in one round of the citric acid cycle, it isn t really these same two carbons that are lost nevertheless, a two-carbon loss is a two-carbon loss, regardless of which two carbons they were.) This leads to an imbalance in the catabolic pathways versus the anabolic pathways. 

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