O Level topics
Before we start this lesson we shall start with an activity to review what we know about Digestion
Forms of digestion
The Digestive system
Digestion in the mouth
In the mouth is found a set of teeth and the tongue.Both contribute to digestion as follows;
The teeth are categorised as incisors,canines and premolars and molars which help in cutting, tearing and chewing respectively.
Digestion in the stomach
In the stomach there is only digestion of proteins. This is so because the conditions in the stomach are favorable for protein digestion. Once in the stomach the food is acted upon by gastric juice secreted by gastric glands situated in the thick stomach wall. Gastric juice contains the following,
Pepsin breaks down proteins short polypeptides. Pepsin is secreted in an active form pepsinogen. This is activated by HCL. This is a safe guard because if it was stored in an active form it would destroy the stomach wall which is also a protein. Once secreted the active form of the enzymeis prevented from attacking the tissues by the mucus lining the stomach wall. In it’s absence the HCL attacks and destroys the gastric walls resulting in gastric ulcers. Rennin coagulates the soluble milk protein Caseinogen to an insoluble curd casein which is then acted upon by pepsin and broken down to polypeptides
HCL has the following functions
- Activates the in active form of pepsinogen to pepsin
- It provides an acidic medium for the action of pepsin.
- It kills bacteria and so stops the food from going bad.
- It stops the actions of ptyalin their by ensuring only protein digestion in the stomach.
Digestion of starch therefore does not occur in the stomach because.
- The enzyme necessary is not present.
- The low PH (acidic) stops the action of ptyalin.
The secretion of gastric juice is a reflex stimulated by,
While the enzymes are working, rhythmical contractions of the stomach pound the food into a semi fluid state called CHYME. The chyme is released in the small quantities by the pyloric sphincter, a ring of muscles.
Digestion in the duodenum
Duodenum is the main seat of digestion in the gut The agents of digestion come from three sources namely.
- The liver
- The pancreas
- Wall of the small intestine 'duodenum and ileum'
The liver produces bile which is stored in the gall bladder. It then flows down through the bile duct into the duodenum. It is a green liquid, a mixture of substances not all of which are involved in the digestion. The digestion components are inorganic components of sodium. These salts emulsify lipids by lowering their surface tension causing them to break up into numerous droplets there by increasing their surface area for enzyme action. The secretion of bile is controlled by a hormone CHOLECYSTOKININ, while its production in the liver is controlled by the hormone Secretin. Bile is also rich in sodium bicarbonate, which neutralizes the acid from the stomach. The PH of the small intestines is therefore alkaline, which favors the action of the various enzymes. The hormones secretin also stimulates the pancreas to release pancreatic juice into the duodenum via the pancreatic duct. Pancreatic juice contains three enzymes.
Trypsin is secreted in an in active form trypsinogen. This is converted into trypsin by the action of the enzyme enterokinase Secreted from the wall of duodenum. Trypsin breaks down proteins into polypeptides, pancreatic amylase breaks down starch into maltose. Pancreatic lipase breaks down lipids into fatty acids and glycerals.
Digestion in the ileum
Food moves from the duodenum to the ileum and along it by peristalsis Presence of food in the ileum stimulates the secretion of the intestinal juice also called succus entericus. The collective function of these enzymes is to complete the digestion of the various compounds. At the end of their activity, the food is a milky fluid called Chyle ready for absorption.
The table showing the action of intestinal enzymes.
- maltase breaks down Maltose into two Glusoe molecules
- Lipase breaks down Lipids into fatty acids and glycerols
- peptidase breaks down peptides and polypeptides into amino acids
- Sucrase breaks down sucrose into Glucose and fructose
The components of chyle are.
- Amino acids
- Fatty acids and glycerals.
The inner lining of the ileum secretes mucus which protects it from being digested by the protease enzymes
Absorption of digested food
This is the process where by the end products of digestion enter into the Blood stream. Water and soluble substances with relatively small molecules like simple sugars, common salt and alcohol are partly absorbed through the stomach wall. All the products of digestion are absorbed in the ileum. The ileum is therefore the main organ of absorption.
Adaptations of the ileum for absorbtion of food materials
- The great length of the small intestines provides a large surface area for absorption (it is approximately 6m long in adult man).
- The inner surface of the ileum is folded and this provides a large surface area for absorption.
- The inner surface is lined with tinny finger like structures called the villi which are in constant motion. The villi greatly increase the surface area over which absorption occurs and
- The lining of the ileum have a thin epithelium therefore digested food can easily diffuse through.
- The ileum has a rich supply of blood capillaries and lacteals for easy transport of
absorbed food substances.
Assimilation is the process by which food food materials after being absorbed are built up into complex constituents of the organism.
Metabolism refers to all the chemical processes/changes taking place within an organism. It involves breaking down of organic compounds from complex to simpler with liberation of energy –Catabolism.
It also involves building up of organic compounds from simple to complex using energy. –Anabolism. The body’s metabolic center is the liver.
The contents of the intestine gradually solidify and are excreted through the anus as feaces in the process of egestion. Feaces contains cellulose a good deal of mucus broken down cells and bacteria. Egestion of feaces is done by powerful muscle contraction of the wall of the rectum which ends in the opening of the anal sphincter