THE ALIMENTARY CANAL AND DIGESTION OF FOOD IN MAN

The alimentary canal of man includes the mouth, oesophagus, stomach, duodenum, small intestine or ileum, caecum, appendix, large intestine or colon, rectum and anus.

The description and importance of the parts are as follows:

Mouth: The alimentary canal of man starts from the mouth. The mouth contains the teeth, salivary gland and tongue.

 

The teeth

• The teeth are used to cut, grind or chew food into tiny particles.
• They expose large area of food for the action of enzymes.

 

The tongue

• The tongue rolls the food into bolus.
• It aids movement of food in the mouth.
• It allows mixing of food with saliva or ptyalin.
• It aids swallowing of food into the gullet or oesophagus.

 

The salivary gland

• The salivary gland secrets saliva which contains an enzyme called ptyalin. Ptyalin breaks down starch into maltose which is later swallowed into the gullet in form of bolus.
• It allows easy chewing or movement of food in the mouth for swallowing.
• It also serves as solvent for food.

The saliva is slightly alkaline.

 

Oesophagus or Gullet: The oesophagus connects the mouth to the stomach. The food swallowed is passed down through the oesophagus by a peristaltic movement into the stomach.

 

Stomach: In the stomach, the food is temporarily stored for few hours and it is released at regular intervals by the opening of the pyloric sphincter. In the stomach, the gastric gland secretes gastric juice which contains two enzymes – renin and pepsin.

The renin acts on milk (or it helps to curdle milk) while the pepsin breaks down proteins to peptones. The gastric gland also secretes hydrochloric acid (Hcl) which creates an acid medium for two enzymes to act. The Hcl also helps to kill some bacteria in the stomach. The food is churned by muscular contraction of the stomach wall (churning movement) which enables the mixing of food with digestive juice. The churning movement then converts the food into a semi-liquid state called chyme.

 

Duodenum: Digestion of food also takes place in this region of the alimentary canal. The duodenum contains pancreas which secretes pancreatic juice that contains three enzymes. These enzymes are:

• Amylase: This converts starch to maltose.
• Lipase: Lipase converts fats and oil to fatty acids and glycerol.
• Trypsin: It converts proteins and peptones to polypeptides.

The pancreatic juice is alkaline and provides that medium for enzymes.

The digestion of fats and oil is aided by a green alkaline liquid called bile which is secreted by the liver and stored in the gall bladder. The bile helps in the emulsification of fats, i.e. breaking down fats into tiny droplets. At the end of digestion in the duodenum the food now in liquid form called chyme passes to the ileum or small intestine.

 

Small intestine: the small intestine or ileum is found between the duodenum and the large intestine. Two major events take place in the small intestine. These events are (a) digestion and (b) absorption of the digested food.

Digestion: The digestion of food also takes place in the small intestine or ileum. The intestinal wall secretes intestinal juice which contains the following enzymes lipase, erepsin, maltase, sucrase and lactase. The lipase converts fats and oil to fatty acids and glycerol, erepsin converts polypeptides to amino acids, maltase converts maltose to two units of glucose, sucrase converts sucrose to glucose and fructose while lactase converts lactose to glucose and galactose.

In man, the final digestion of food ends in the small intestine. The end products in the digestion of protein are amino acids, fats and oil and fatty acids and glycerol, while that of starch is glucose.

 

Absorption of digestion food: The end products of digestion of food (amino acids, glucose, fatty acids and glycerol) are absorbed in the small intestine by tiny finger-like structures called villi (singular villus). The folding of the small intestine combined with the presence of numerous villi creates a large surface area for the absorption of digested food.

The inner surface layer or epithelium of each villus is thin. This allows the absorption of the end products by either diffusion or active transport through it. the glucose and amino acids are easily absorbed by blood capillaries through the epithelium while the fatty acids and glycerol enter the lacteal where they are carried through the lymph vessels which eventually empty their contents into the blood vessels near the heart. The blood then carries the fats and other food materials to various parts of the body where they are needed.

 

Caecum and Appendix: In man, the functions of caecum and appendix are not well known by the caecum usually contains some bacteria which aid minor digestion of cellulose. some vitamins such as K and B-complex are partially synthesized in this region.

 

Large intestine: The undigested food passes into the colon or large intestine. Here, water is absorbed. This absorption of water concentrates the waste products and turn them into faeces. The faeces is passed into the rectum and finally out of the body through the anus.

 

DIGESTIVE ENZYMES

An enzyme is an organic catalyst, usually proteinous in nature, which promotes or speeds up chemical changes in living cells but are not themselves used up in the process. Enzymes accelerate metabolic reactions without changing their composition in the process.

 

Types of Enzymes

There are two major types of enzymes. These are:

• Intracellular enzymes: These enzymes function inside the cells of living organisms. Common examples are the enzymes that catalyse cell respiration inside the mitochondria.
• Extracellular enzymes: These are enzymes which carry out their functions outside the cells. Examples of extracellular enzymes are the digestive enzymes.

 

Classification of Enzymes

The new method of classifying enzymes is based on the nature of chemical changes brought about on a substrate. Such changes may involve oxidation in which case the enzyme will be called an oxidase.

It is important to note that the earlier classification based on the substrate an enzyme works on, e.g. amylases for maltose, proteases for protein or lipases for lipids are no longer correct or biochemically acceptable.

From the above classification, it is clear that all digestive enzymes belong to the class called hydrolases. The class, hydrolases generally refer to enzymes that break up various bonds (i.e. breaking large molecules into smaller particles) in the presence of water.

 

Characteristics of Enzymes

1. Remain chemically unchanged at the end of a reaction.
2. Are specific in action/one or a group of enzymes will act on specific substrates.
3. Required in small quantities.
4. Act best over a specific range of temperature/between 35 – 40 degrees Celsius.
5. They are denature/destroyed at high temperature/inactivated at very low temperature.
6. Act best at specific Ph (either acidic or alkaline).
7. Their action is reversible.
8. Enzymes action is retarded by poison or inhibitors.
9. Some are inactive and require a coenzyme/agent to activate them.
10. They can function outside the body of the organism that produces them.
11. Enzymes are protein in nature.

 

Functions of Digestive Enzymes

Enzymes perform the following functions:

1. Enzymes help in breaking down proteins in foods into amino acids.
2. They also help in breaking down fats and oils into fatty acids and glycerol.
3. They help in breaking down of carbohydrates into glucose, fructose and galactose.
4. They aid the absorption of digested food through the addition of water to the food.