E.Digestion in Herbivores: Ruminants
These organisms must digest cellulose. However, mammals don’t produce the necessary enzymes. In order to digest the cellulose, mammals must have large flat teeth for grinding cellulose. Cellulose must be broken down to release plant nutrients. Once broken down the food goes through the esophagus to a four chambered stomach of such ruminant herbivores such as cattle, deer, giraffes, antelopes, and buffalo. The stomachs harbor protozoans and bacteria that break down cellulose.
F.Chemistry of Digestion
1. Carbohydrate digestion
Starch digestion begins in the mouth with amylase that is found in the salivea. The breakdown of carbohyrdrates is stalled in the stomach because the acidity and resumes in the small intestine where amylase from the pancreas converts all starch into maltose. Maltose is broken down by maltase into glucose which is absorbed. Sucrose, which is glucose and fructose, is broken down by sucrase. Lactose, milk sugar, is broken down in the gut by lactase. Lactase is absent in most blacks and Asians and in some whites preventing them from breaking down lactose.
2. Fat digestion
Fats reach the small intestine with little chemical change. Even though the stomach secretes a lipase. Bile separates fats into tiny droplets that are broken down further by lipases into fatty acids and glycerol. These cross the membrane and reform in the villi. Longer chans of fatty acids, more than 12 carbons, enter the lymphatic vessels of the villi. Fatty acids with fewer than 12 carbons go into the capillaries and are carried to the liver. The ones in the lymph system follow the lymphatic system to the thoracic duct near the heart and enter the blood stream. If there is too much fat in the blood after the meal, the blood will appear milky.
3. Protein digestion
Proteins are the most complex food molecule and their digestion is complex. Pepsinogen, secreted by the chief cells is changed by hydrochloric acid into pepsin. Pepsin is a nonspecific endopeptidase and hydrolyzes proteins into smaller peptides. Specific enzymes will break pepitides down further until individual amino acids are left. These amino acids are absorbed into the blood stream and sent to the liver. In the liver, the amino acids are used for eneg and to build proteins.
4. Nucleic Acids
Almost everything we eat contains some nucleic acids. Nucleases produced in the pancreas break nucleic acid bonds. They break down the nucleic acids into small units of either single bases or small chains.
G. Integration and Control of the Digestive Process
The realease of digestive enzymes must be precisely timed. They are controlled mechanically, neurally, and hormonally.
For example, the thought of chocolate stimulates saliva flow. The saliva flow can also be stimulated by chewing. Neural and mechanial stimulants can stimulate saliva flow. Gastric secretions can be stimulated by the presence of food. The neural message is sent along the vagus nerve from the brain to th stomach lining. Food in the stomach stimulates sensory neurons in the stomach and a hormone gastrin is released from stomach to the blood.