Digestion process
Monacia Williams, Contributor

Hello again, ready for another lesson? I certainly hope so! Did you try the experiments with the liver and the hydrogen peroxide, or did you use the irish potato? Fascinating wasn't it?

Learning does not always have to be centred around the classroom and the book. After all, biology is the study of living organisms and living organisms are everywhere so your learning can take place everywhere too.

As we begin our study of digestion, I hope it will encourage you to pay more attention to what you eat and also help you to develop a healthier diet for yourself.

You need to remember that some of the food that we eat do not need to be digested. These include simple sugars such as the monosaccharides, water, minerals and vitamins. This is because these are already small and can be absorbed as they are. Starches, proteins and lipids are large molecules and hence have to be digested. Starch is obtained from foods such as breads, cakes, biscuits, yam, potato, cassava and green bananas.

If food and nutrition is included among your subjects, you will recognise these as food from plants. Proteins are obtained from animals and plants, examples are, peas and beans (legumes) from plants, milk, eggs and meat from animals. Lipids also can be had from plant and animal sources examples are butter, oil and other fatty foods.

Digestion of these by enzymes produces the small molecules from which these compounds were made. Starch is digested by the enzyme amylase to produce simple sugars, namely glucose, protein is digested by a group of enzymes collectively known as the proteases to produce amino acids and lipids are digested by enzymes called lipases into fatty acids and glycerol.

The process of Digestion

Digestion begins in the buccal cavity. Note that most times, the word mouth is used, but this is technically incorrect since the mouth is just the opening to the buccal cavity.

The following are the steps that the food undergo here:

• The food is ground to a pulp by the teeth. Reminder, this is known as mechanical digestion.
• The food is mixed with saliva which is produced by the salivary glands.
• Saliva contains amylase; Reminder, amylase is the enzyme, which is responsible for digesting starch. This means that the digestion of starch begins here. Note that amylase works best at a pH of 7. Since this is the pH of the buccal cavity amylase will begin the digestion of starch.
• The ground food, which is mixed with saliva, is formed into a ball. This ball is known as a bolus.
• The tongue pushes the bolus to the back of the buccal cavity, that is, to the region of the throat.
• Note as well that the digestion of starch is not completed in the buccal cavity because the food spends only a short time there.

Oesophagus

This is the region of the alimentary canal that follows the buccal cavity.

When the bolus is swallowed the following occurs:

• The epiglottis closes the trachea to prevent food from going into it.
• The passage of the bolus is made possible by a series of alternate contraction and relaxation of the muscles of the oesophagus. This is known as peristalsis.

Stomach

• The sphincter muscle at the entrance to the stomach open and the food is released into the stomach.
• Peristalsis continues and the food is mixed with enzymes and mucus to form a mixture called chyme.
• The walls of the stomach contain goblet cells, which secrete mucus. They also contain special areas known as gastric pits.
• Gastric pits secrete:

Hydrochloric acid, which changes the pH of the chime from pH7 to pH 1.2 - 2. The stomach becomes very acidic. The acid conditions kill any bacteria that might be in the food.

Pepsin - This is a protease enzyme and it begins the digestion of proteins, breaking them down to polypeptides. This enzymes requires an acidic medium in which to work, hence the change in conditions provide optimum conditions for it to work.

Rennin - This enzyme is mostly produced in the stomach of young animals. It acts to change the soluble protein in milk into insoluble form. This soluble protein is casein and its insoluble form is caseinogen. Pepsin is now able to break down the protein caseinogen.

• The process takes between two to three hours.
• The sphincter muscles at the lower end of the stomach now open and food is released in small amounts into the next section of the alimentary tract, the duodenum. The duodenum is the first part of the small intestine.
• Let us think about what has happened so far:
• The enzyme amylase began the digestion of starch in the buccal cavity. Why could this happen? Because amylase was provided with its optimum pH, pH7.
• In the stomach, the pH changes. What are the implications for the digestion of starch? It has to stop because amylase can no longer work since it cannot work at pH 1.5 - 2.
• Hence there is no digestion of starch in the stomach.

Next week as we continue we will see what happens in the other regions of the alimentary canal. See you all next week. Have a productive week.