Digestion is the process whereby food substances are converted into soluble and diffusible forms. Digestion may be physical or chemical. Physical digestion is the mechanical breakdown of food into smaller particles. This increases the surface area of the food for enzyme action. Chemical digestion refers to the enzymatic hydrolysis of food substances.

What are enzymes?

Enzymes are biological or organic catalysts, i.e. substances that are protein in nature and can alter the rate of chemical reactions without themselves being chemical changed at the end of the reactions. Enzymes can/are:

• Alter or speed up the rates of chemical reactions without themselves being changed during the reactions.
• Bring about a large amount of chemical reactions with a small quantity of itself.
• Highly specific in their action. E.g. Amylase will only act on starch and break it down, but not on a protein.

Digestion

Digestion starts in our mouth. There we chew up our food and our tongue have special sense cells to help us select suitable foods. Saliva is secreted through the three salivary glands namely the, parotid gland, the sublingual gland, and the submandibular gland. The saliva excreted contains amylase, which breaks starch into maltose. The tongue then rolls the food into balls and pushes them down our oesophagus. Our epiglottis will then close up the trachea.

Oesophagus

• In the oesophagus, the food is transported down through a process known as peristalsis. Two sets of muscles work in such a way that when one contracts, the other one relaxes. The food is then pushed down and enters the next stage of digestion.

Stomach

• In the stomach, hydrochloric acid is excreted by the gastric glands. The hydrochloric acid serves the purpose of facilitating the enzymes in the stomach, namely rennin and pepsin as they work better in acidic natures. The rennin breaks soluble caseinogen into insoluble casein while the pepsin breaks proteins into peptones.

• Most of the digestion takes place in the small intestine. Many enzymes such as maltase, lactase, sucrase, erepsin and lipase are excreted to break down glucose, galactose, fructose, amino acids an, fatty acids and glycerol respectively.

• The pancreas secretes pancreatic juice, which passes through the pancreatic duct into the duodenum. The pancreatic juice contains pancreatic amylase that breaks down starch into maltose and trypsin that breaks proteins into peptones. Lipase in the juice also helps emulsify fats into fatty acids and glycerol.

• The gall bladder releases bile. Bile passes through the bile duct into the duodenum and helps to emulsify fats present.

• At this stage of digestion, many minerals and nutrients from the digested food is absorbed into our body through villi, present on the walls of the intestines. The villi are richly supplied with blood vessels and lymphatic vessels to carry away the absorbed food substances.

Liver

The liver plays a vital role in the functioning of the body:

• Production of bile                                                

The gall excreted is stored in the gall bladder before use.

• Regulation of blood glucose concentration

The liver keeps the amount of glucose in the blood constant. Blood normally contains about 70-90mg glucose. After a heavy meal, this glucose content rises. As the blood passes through the liver the excess glucose is converted into insoluble glycogen and stored. The blood leaving the liver contains a fairly constant amount of glucose. When the tissue cells of the body are in need of glucose, they obtain it from the blood. As a result the glucose level in the blood drops. This induces the liver to convert the glycogen in it back to glucose that enters the blood. In this way, the glucose level in the blood rises to normal. The deposition and mobilisation of glycogen is under the control of the hormones insulin and adrenaline.

• Deamination of amino acids

Excess amino acids are brought to the liver when their amino groups are removed and converted to urea. The remains of the deaminated amino acids are converted into glucose in the liver. Any excess glucose formed in this way is converted to glycogen.

• Detoxication

Harmful substances may be absorbed into the blood from the alimentary canal. Harmful substances such as benzoic acid, picric acid, chloroform are rendered harmless by the liver cells. The process of converting harmful substances into harmless ones is known as detoxication.

• Heat production

Heat is produced as a result of numerous chemical activities occurring in the liver and is distributed by the blood to other parts of the body, thus maintaining the body temperature.

Large Intestine