Physiology: Digestive Systems

Vertebrate digestive system - monogastric and digastric stomachs

The problem of digesting cellulose (b bonds)

The digestion of cellulose (organisms with their own cellulase enzymes (snails silverfish) and those in symbiotic relationship with cellulolytic microorganisms)

The vertebrate rumen; the production and utilisation of volatile fatty acids; the advantages and disadvantages of ruminant fermentative digestion Postgastric fermentation and coprophagy.

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The vertebrate digestive system

The vertebrate digestive tract is highly specialized in structure and function for the digestion of a wide variety of food items. The basic components of the gut tube include the buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, and anus/cloaca. The structure of the stomach can be highly modified, depending on the diet. A few vertebrates lack a stomach (e.g. lungfish) but in most it functions for storage and acidic/enzymatic digestion. It it highly modified for fermentation in ruminatnt mammals.

Most vertebrates have a monogastric stomach i.e. a single chamber which contracts so as to mix the contents with digestive juices. Multichambered digastric stomachs are found in ruminant animals such as cattle and sheep. These act as fermentation vats within which symbiotic bacteria and protozoa digest cellulose from the food - see later. Unlike the mongastric stomach, the digastric stomach both digests and absorbs foods.

The intestine may have been the primary or even the sole site of digestive enzyme production in primitive vertebrates, but this role has been supplemented by the stomach and pancreas (which is an outgrowth of the small intestine). Absorption is the primary function of the small intestine in higher vertebrates although some absorption also occurs in the stomach and large intestine. Higher vertebrtes have a much elongated small intestine to increase the absorptive surface area. The generalised tetrapod intestine is exemplified by that of amphibians, with a long, narrow, coiled small intestine and short, broad large intestine. A small pocket, the caecum, is usually present at the junction of the small and large intestines. The caecum is greatly enlarged as a fermentation chamber in certain post-gastric fermenting mammals (e.g. horses).

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The Problem of digesting cellulose

Many plant polysaccharides and all animal polysaccharides are monosaccharides joined by  bonds. These  bonds can be hydrolysed by common animal amylases, but these enzymes are ineffective at hydrolysing  bonds. Cellulose is difficult for most animals to digest because it is composed of monosaccharides joined by  bonds.

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The Digestion of Cellulose

A few animals (e.g. silverfish, snails) produce their own cellulase enzymes capable of hydrolysing  bonds, but many more have entered into symbiotic relationships with cellulytic protozoa and/or bacteria. The guts of these animals have frequently become highly modified to accommodate these symbiotic microorganisms. For instance, certain wood-feeding termites which are capable of living for long periods on pure cellulose, have rectal pouches that contain flagellate protozoans. The reliance of these termites on their symbiotic flagellates for cellulose digestion is easily demonstrated by exposing the termites to 3.5 atm pure oxygen, or 36C for 24 hours;…