<P> Microbes simply secrete digestive enzymes into their surroundings, while animals only secrete these enzymes from specialized cells in their guts, including the stomach and pancreas, and salivary glands . The amino acids or sugars released by these extracellular enzymes are then pumped into cells by active transport proteins . </P> <P> Carbohydrate catabolism is the breakdown of carbohydrates into smaller units . Carbohydrates are usually taken into cells once they have been digested into monosaccharides . Once inside, the major route of breakdown is glycolysis, where sugars such as glucose and fructose are converted into pyruvate and some ATP is generated . Pyruvate is an intermediate in several metabolic pathways, but the majority is converted to acetyl - CoA through aerobic (with oxygen) glycolysis and fed into the citric acid cycle . Although some more ATP is generated in the citric acid cycle, the most important product is NADH, which is made from NAD as the acetyl - CoA is oxidized . This oxidation releases carbon dioxide as a waste product . In anaerobic conditions, glycolysis produces lactate, through the enzyme lactate dehydrogenase re-oxidizing NADH to NAD+ for re-use in glycolysis . An alternative route for glucose breakdown is the pentose phosphate pathway, which reduces the coenzyme NADPH and produces pentose sugars such as ribose, the sugar component of nucleic acids . </P> <P> Fats are catabolised by hydrolysis to free fatty acids and glycerol . The glycerol enters glycolysis and the fatty acids are broken down by beta oxidation to release acetyl - CoA, which then is fed into the citric acid cycle . Fatty acids release more energy upon oxidation than carbohydrates because carbohydrates contain more oxygen in their structures . Steroids are also broken down by some bacteria in a process similar to beta oxidation, and this breakdown process involves the release of significant amounts of acetyl - CoA, propionyl - CoA, and pyruvate, which can all be used by the cell for energy . M. tuberculosis can also grow on the lipid cholesterol as a sole source of carbon, and genes involved in the cholesterol use pathway (s) have been validated as important during various stages of the infection lifecycle of M. tuberculosis . </P> <P> Amino acids are either used to synthesize proteins and other biomolecules, or oxidized to urea and carbon dioxide as a source of energy . The oxidation pathway starts with the removal of the amino group by a transaminase . The amino group is fed into the urea cycle, leaving a deaminated carbon skeleton in the form of a keto acid . Several of these keto acids are intermediates in the citric acid cycle, for example the deamination of glutamate forms α - ketoglutarate . The glucogenic amino acids can also be converted into glucose, through gluconeogenesis (discussed below). </P>

The process of changing fatty acids to several molecules of acetyl coa is called