<Tr> <Td> Carbon source </Td> <Td> organic compound </Td> <Td_colspan="2"> </Td> <Td> hetero - </Td> </Tr> <Tr> <Td> inorganic compound </Td> <Td> auto - </Td> </Tr> <P> The most common set of catabolic reactions in animals can be separated into three main stages . In the first stage, large organic molecules, such as proteins, polysaccharides or lipids, are digested into their smaller components outside cells . Next, these smaller molecules are taken up by cells and converted to smaller molecules, usually acetyl coenzyme A (acetyl - CoA), which releases some energy . Finally, the acetyl group on the CoA is oxidised to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD) into NADH . </P> <P> Macromolecules such as starch, cellulose or proteins cannot be rapidly taken up by cells and must be broken into their smaller units before they can be used in cell metabolism . Several common classes of enzymes digest these polymers . These digestive enzymes include proteases that digest proteins into amino acids, as well as glycoside hydrolases that digest polysaccharides into simple sugars known as monosaccharides . </P>

Cells produce water as a by-product of energy metabolism