Digestion – Cont’d
“Live as if you were to die tomorrow: learn as if you were to live forever.”
– Mahatma Ghandi
HOW ARE you all this week? Have you been having a good week so far? I do hope so, and that you have learnt not to put off for tomorrow what you can do today.
We continue our study on what happens in the digestion of foods by humans. Do you remember where we stopped last week? We stopped at the sphincter muscles of the stomach opening at intervals to release small amounts of the chyme (acidic, partially digested food) into the duodenum, the first section of the small intestines. If the small intestine has a first section, then this means that it is made up of more than one section, doesn’t it? What are these sections?
The small intestine is a long, narrow, continuous tube which is about five metres long and is divided into three sections. These are:
Duodenum – which represents the first section. Jejunum – which represents the middle section. Ileum – which represents the last section.
DUODENUM
This section of the intestines receives, in addition to the chyme from the stomach, secretions from the pancreas. What is the pancreas? This is a cream-coloured gland found lying just below the stomach. The pancreatic duct connects the pancreas with the duodenum. The secretion that is released from the pancreas contains:
Amylase – Do you remember what this is? Of course, you do! This is an enzyme, and we discussed its use when we were looking at what happens to food in the buccal cavity. Remember, this is the enzyme which breaks down starch to simple sugars. What does this tell you about the pH of the duodenum? That’s right! It is different from the pH of the stomach; the pH of the duodenum is alkaline, while the pH of the stomach is acidic. Note that a change in conditions must occur for the required enzymes to break down the different substrates.
Trypsin – This is an enzyme which acts on protein (protease), breaking it down to amino acids.
Lipase – This is the enzyme which acts on lipids, breaking them down to fatty acids and glycerol.
Sodium hydrogen carbonate – This is alkaline and, hence, it can neutralise the acid of the chyme, making it neutral (changing the pH).
The duodenum also receives bile, which is produced by the liver and is stored in the gall bladder. Bile enters the duodenum through the bile duct. What is bile? Bile is:
Yellowish-green, watery liquid.
It does not contain enzymes.
It contains bile salts, which emulsify fats/lipids, breaking them up into smaller pieces and making it easier for the enzyme, lipase, in the pancreatic juice, to act on them, converting them to fatty acids and glycerol. This process involving the action of bile is known as emulsification.
JEJUNUM AND ILEUM
The small intestine also secretes enzymes, which complete the digestion of food. These also work in alkaline conditions, so a change of pH is not required. They are:
Maltase – This enzyme breaks down maltose to glucose.
Sucrase – This breaks down sucrose to glucose and fructose.
Lactase – This breaks down lactose to glucose and galactose.
Proteases – These break down polypeptides to amino acids.
Lipases – These break down fats to fatty acids and glycerol.
This concludes our discussion on digestion. We need to consider next what happens to the digested food? What do you think? That’s right! It is absorbed into the blood stream for transport. Let us see how this takes place.
ABSORPTION
The ileum of the small intestine is the site of absorption. This is logical, because this is where digestion is completed. Do you remember why digestion has to take place? Of course, you do. It has to take place so that the food is broken down into molecules that are small enough to diffuse across the cell membranes.
Absorption is described as the passage of digested foods, salts, vitamins and water through the walls of the small intestines into the transport system.
WHAT MAKES THE ILEUM SUITABLE FOR ABSORPTION?
It has the following adaptations:
It is very long, over six metres.
It has finger-like projections known as villi. These serve to increase the surface area of the ileum.
It has a network of capillaries which serve to transport the digested food from the ileum.
It has lacteals (part of the lymphatic system) to receive digested fats.
Our bodies are interesting, aren’t they? We will complete our study of digestion with a look at the functions of the liver, an organ which plays a supporting role in digestion. Let us see what this role is. The liver is the largest organ in the body. It weighs over 1kg and functions as a chemical factory, a food store and a central heating system.
FUNCTIONS OF THE LIVER
It:
1. Stores glucose as glycogen (plays a role in the maintenance of glucose balance in the body).
2. Stores copper, potassium and iron (for red blood cells).
3. Stores vitamins A, B, and D.
4. Deaminates amino acids – deamination removes the amino group from amino acids, converting these to urea, a waste material which is excreted by the kidneys.
5. Detoxifies the blood by removing toxins from germs, alcohol and blood.
6. Produces bile, which is stored in the gall bladder and used in digestion.
7. Produces fibrinogen, which is needed for the clotting of blood.
8. Produces heat because of its high level of activity – this heat serves to maintain our body temperature.
See you all next week. Protect your body, especially your liver – no alcohol, no drugs! Have a great week!