Digestion of carbohydrates
Carbohydrate digestion in vertebrates and invertebrates is very similar. All the enzymes shown in Table are not required by all animals. The enzymes present are related to the food habits of the animal. However, amylase and maltase are of universal occurrence. Amylase is secreted in the saliva of man and in larger amounts by the pancreas. Enzyme production in some animals is also influenced by genetic characteristics and enzyme induction. For example, production of maltase and sucrase by the intestinal villi depends on the amount of ingested sugar. If a high maltose or sucrose diet is taken it induces the villi to produce more maltase and sucrase within 2-5 days. Lactase production declines in humans as gut develops after infamy. It ceases in some individuals so that they can no longer hydrolase this sugar. Now let us consider the digestion of cellulose, the most important structural material of plants and a major component of the diet of herbivores. Very few animals possess the enzyme cellulases. Then how do animals that feed on plants breakdown this carbohydrate? Cellulases enzymes are synthesised by many bacteria and protistans which live symbiotically in many herbivores and insects. Cellulose digestion is carried on by the help of these symbiotic microorganisms. The microorganisms live in the stomach of the ruminants (i.e. cow, sheep, etc.) and breakdown the cellulose. The breakdown products are then utilised by the host. In some invertebrates like silver fish (Ctenolepisma lineata) true cellulases have been reported but the insect cannot survive on an only cellulose diet. Some other invertebrates also have some cellulases that partly digest cellulose but none show conclusive evidence of a complete breakdown of cellulose into glucose without the help of symbionts.