Digestion
“Never give up on a dream because of the time it will take to accomplish it. The time will pass anyway.”
H– Earl Nightingale ELLO AGAIN, students. How are you this week? I hope all your days have been good so far and that everything is going well with you. I hope that as you have been reviewing your lessons, you have been using your knowledge to answer past-paper questions. Many multiple-choice questions come from this section of the syllabus.
Last week, we looked at the structure and function of teeth in preparation for our study of digestion. This week, we will begin to look at the process of digestion. To do this properly, you will need to know the different parts of the alimentary canal. This is the section of the body in which digestion takes place. You need to know the sequencing of the different parts, as well as their names and their functions. Do not be surprised if you are asked to label these parts in an exam question. One advantage of getting this type of question is that it is quite easy to gain full marks, since this is one of the easiest sections of the syllabus to understand and remember. We begin by taking a general look at the process and then a more detailed look at what happens in each section of the canal.
WHAT HAPPENS TO THE FOOD WE EAT AND WHERE DOES IT HAPPEN?
The alimentary canal is a long, muscular tube running from the mouth to the anus. The walls of the tube contain muscles which contract and relax to send food through its lumen. This movement is known as PERISTALSIS. One misconception that students tend to have is that peristalsis takes place only in the oesophagus, but this is not so. It takes place through the entire length of the canal.
Along the tube, at specific places, there are special muscles which serve to block off the tube completely, causing A student of Ocho Rios High School effortlessly blazes around the track. the section that is blocked off to act independently of the others. These muscles are known as SPHINCTER MUSCLES. The alimentary canal also has special cells which secrete mucus. These cells are called goblet cells. Mucus helps the food to slide easily along the tube.
The parts of the canal are: the mouth, the buccal cavity, the oesophagus, the stomach, the duodenum, the small intestines, the large intestines, the colon followed by the anus.
The process by which food is taken into the buccal cavity through the mouth is known as ingestion. The food contains large molecules. Remember that these large molecules cannot enter or leave the cells by diffusion. This means that they must be broken down into smaller molecules so that they can be absorbed. This process is known as digestion. After the food is digested, it has to be sent from the alimentary canal to the blood stream. This process is known as absorption. Food which cannot be digested, and hence cannot be absorbed, is passed through the body in a process called egestion.
Two types of digestion take place in the alimentary canal. These are:
Mechanical – This is carried out by the teeth and the churning actions of the canal.
Chemical – This involves a chemical change from one molecule to another, e.g., large molecules converted to smaller ones using enzymes. Simple sugars, water, minerals and vitamins are small molecules and, hence, do not need to be digested. Starch, proteins and fats are large molecules and must be digested before they can be absorbed by the body cells.
Complex carbohydrates – Starchy foods, e.g., yam, potato, cassava and bread are broken down/digested by the enzyme amylase/ diastase to produce simple sugars.
Proteins – Example: milk, eggs and meat are broken down/digested by protein enzymes known as proteases to give amino acids.
Lipids/fats – Example: butter and oil are broken down/digested to produce fatty acids and glycerol.
A CLOSER LOOK AT THE DIGESTIVE PROCESS MOUTH
Digestion begins in the buccal cavity – some textbooks will state that it begins in the mouth, but the scientifically correct term is the buccal cavity, the mouth being the opening or entrance to the cavity.
Food is ground to a pulp by the molars and premolars (mechanical digestion or mastication).
Food is mixed with saliva produced by the salivary glands.
Saliva contains the enzyme amylase which begins the chemical digestion of the starch in the food.
The pH of the solution in the mouth is neutral, i.e., pH7. This pH is the one in which amylase works best.
Food is formed into a bolus or ball which the tongue pushes to the back of the mouth.
Digestion of the starchy material in the food is not completed in the mouth, because the food does not remain in the mouth for very long.
OESOPHAGUS
The oesophagus lies beside the larynx. The larynx is the tube leading to the lung. While the bolus is being swallowed, the epiglottis closes the trachea to prevent the bolus from going into it. The bolus passes through the oesophagus by alternate relaxation and contraction of it muscles in the walls. Do you remember what this movement is called? Of course, you do. You just read it! It is called peristalsis.
STOMACH
The sphincter muscles at the entrance to the stomach open and those at the other end close. The food is released into the stomach. The food remains in the stomach for two to three hours. Peristalsis continues and the food is mixed with enzymes and mucus to form a mixture called chyme. Mucus is secreted by the goblet cells. Gastric pits present in the stomach:
Protease enzymes – pepsin and rennin.
Hydrochloric acid (HCl) which: Changes the pH of the stomach, making it acid.
Kills any bacteria that might be in the food.
Pepsin – This begins the digestion of proteins, breaking them down into polypeptides. The pH required for pepsin to work is pH1-2 (very acidic). This means that the digestion of starch will stop because amylase cannot work in an acid medium.
Rennin – This is produced in the stomach of young animals. It changes the casein, the soluble protein in milk, to caseinogen, causing it to become insoluble and clot. The clotted milk can now be broken down by the enzyme pepsin.
The sphincter muscles at the lower end of the stomach now open and food is released into the duodenum, the first part of the small intestine. Next week, we will continue to trace the journey of food through the alimentary canal. Monacia Williams is an independent contributor. Send comments to kerryann.hepburn@gleanerjm.com.