Digestion
taken from Wikipedia
Further information: Digestion and gastrointestinal tract
Macromolecules such as starch, cellulose or proteins cannot be rapidly taken up by cells and need to 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 monosaccharides.
Microbes simply secrete digestive enzymes into their surroundings,[27]HYPERLINK "http://en.wikipedia.org/wiki/Metabolism" \l "_note-19"[28] while animals only secrete these enzymes from specialized cells in their guts.[29] The amino acids or sugars released by these extracellular enzymes are then pumped into cells by specific active transport proteins.[30]HYPERLINK "http://en.wikipedia.org/wiki/Metabolism" \l "_note-22"[31]


A simplified outline of the catabolism of proteins, carbohydrates and fats.