Lecture 12. February 21, 2003 M. Martinez Digestion and Gut Structures

Digestion takes place inside cells in vacuoles (intracellular) and outside cells, typically in guts (extracellular). Extracellular digestion permits the handling of larger food particles, and is followed by intracellular digestion.

Common components of digestive systems are teeth/gizzard, crop, stomach, intestine, cecum/rumen, rectum. These components function variously in mechanical breakdown, storage, hydrolysis, absorption of digestive products, resorption of water, and fermentation by symbionts.

Digestion is the process of breaking down food into smaller components and making it soluble so that the body can absorb and utilize the food. Very specific enzymes break down food through hydrolysis. Hydrolysis is a spontaneous but very slow reaction, catalyzed by enzymes to yield digestive products on a time scale useful to organisms.

Proteins are cleaved in a step-by-step process with different enzymes working at each step. Fats are insoluble and thus require emulsification (such as by bile acids) before lipases can attack the molecules. Most animals, especially terrestrial ones, cannot digest wax, but some marine ones can. Marine fish and birds that consume copepods, which are up to 70% wax, appear to produce wax lipases that allow them to utilize the energy in these molecules. Disaccharides are broken down to monosaccharides (e.g. by sucrase and lactase) and then absorbed.

A small difference in structure of a large molecule makes starch a good source of energy, but cellulose indigestible by most animals. Most animals that survive on a diet that is high in cellulose do so with the assistance of microbial symbionts that product cellulase. Termites, ruminants and horses are well-known examples of animals with cellulase-producing symbionts.

Gut structure has evolved to match diet. Longer, more complicated guts with fermentation chambers indicate a poorer quality diet. Foregut fermenters can achieve efficient digestion of relatively high-quality feed. Hindgut fermenters cannot digest food as efficiently, but can survive on lower quality feed if a high quantity is available. Some hindgut fermenters reingest feces for a second round of digestion.