Mechanism of digestion

a) Digestion of proteins

• Proteins are complex organic compounds of high molecular weight.
• They are composed of carbon, oxygen, hydrogen and Sulphur elements. For the digestion of protein following enzymes are required in the fishes as:

– Pepsin

– Trypsin

– Chymotrypsin

– Erypsin

• The fishes which possess stomach are generally carnivorous and secrete pepsin enzyme from gastric mucosa.
• The pepsin enzyme breaks down complex form of protein to simpler form or insoluble state.
• The optimum activity carried out at low pH 2 to4, so Hcl required for making low pH.
• At 100C the gastric secretion increases to three to four folds.
• The trypsin enzymes are present in the extract of pancreas of some elasmobranchs. The enterokinase enzyme is exclusively secreted by intestine of fishes.
• In the cyprinids, stomach less fish pepsin compensation is supplemented by some intestinal enzyme, erypsin.

b) Digestion of carbohydrate:

• The enzymes which break down the carbohydrate in the gut of fishes are as

follows:

– Amylase

– Lactase

– Sucrose

– Cellulose

– Maltase

• The most important enzymes is amylase which acts as starch and which breakdown to maltose and then to glucose by the process of digestion.
• The amylase is secreted from the pancreas in carnivorous fishes, but in herbivorous fishes, the presence of this enzyme is reported from the whole gastrointestinal tracts as well as pancreas.
• Starch ^Amylase → Maltose ^Maltase→ Glucose
• Sucrose ^Sucrase → Glucose + Fructose

c) Fat digestion:

• The main enzyme which acts on this lipid is lipase.
• The pancreas is also primary site of lipase production.