Digestion. I. What is digestion? II. Enzymes involved in digestion

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1 Digestion I. What is digestion? Digestion is the breakdown of large, insoluble food molecules into small, soluble food molecules so that they can be absorbed into the bloodstream. Large food molecules like proteins must be broken down into small amino acids. Large carbohydrate molecules like starch must be broken down into small sugar molecules. Large fat molecules must be broken down into small fatty acids. Large food molecules are insoluble. They cannot dissolve and they will not pass through the gut wall. Small food molecules are soluble. They will dissolve and can get through the gut wall. II. Enzymes involved in digestion Protease breaks down protein molecules into amino acids Carbohydrase breaks down carbohydrate (starch) molecules into sugar (glucose) Lipase breaks down fat molecules into fatty acids and glycerol 1

2 Diagram cited from: III. Your digestive system (the gut) your inner tube The gut is your digestive system and is approximately 9 meters long. It begins at your mouth and ends at your anus. It bends and twists a lot to enable itself to fit inside your body. It is made up of many different parts. IV. Your mouth The mouth is where it all begins. Here, you ingest food and drink into your body. Food enters in bitesized chunks and is chewed. Chewing food mixes it with saliva, which is produced in your salivary glands. Saliva contains a carbohydrase enzyme called amylase, which starts to digest starch into sugar. Before you swallow, your tongue rolls the food into a small soft ball and pushes it to the back of your mouth. The food pushes your soft palate upwards to stop it going up your nose. When you swallow, a flap of skin called the epiglottis covers your trachea to direct the food into the oesophagus. Swallowing is a reflex, you do it without even realising. 2

3 Diagram cited from: V. The oesophagus The oesophagus passes food down to your stomach. The oesophagus contains circular muscles in its wall, which contract and squeeze in behind the food to push it along. This motion of food travelling down your gut is called peristalsis. Diagram cited from: 3

4 VI. The stomach The stomach is a muscular bag that can hold up to two litres of food. The stomach makes digestive juices, these contain: Proteases these start the digestion (breaking down) of proteins to amino acids Hydrochloric acid this is because stomach acid works best in an acid ph Renin is produced by babies to make the milk solid so it stays in the stomach for longer The muscular stomach walls churn up the food, mixing it well with the digestive juices. After 23 hours, it is a runny liquid and a ring of muscle open and lets the food out into the small intestine, a small amount at a time. Diagram cited from: VII. The small intestine The small intestine first of all, is not very small! It is about 6 meters in length. Three important liquids are added here to the liquid food now in your intestine. 4

5 Pancreatic juice contains carbohydrases, proteases and lipases which carry on digesting the food Bile enters via the bile duct. Bile is made in the liver and stored in the gall bladder. It is alkaline and is there to neutralise the acid that was added to the food in the stomach. This provides the optimum ph for enzymes in the small intestine to work. Bile also emulsifies fats (breaks them down), which increases the surface area for lipase to work on them. Intestinal juice is made my glands in the wall of the small intestine and contains more lipases to digest fats, proteases to digest protein and carbohydrases to digest starch. VIII. Absorption The small intestine has another very important job besides from digestion, and that is to absorb the digested food through the wall of the small intestine, into the blood. The small intestine is well designed for this, it has: A thin lining A good blood supply A very large surface area (approx. 9 square meters) in the form of millions of tiny finger like prjections in the lining called villi 5

6 Diagram cited from: IX. The large intestine By the time your food reaches the large intestine, it is mainly just fibre, dead cells, bacteria and water. Here, some of the water passes back into the blood and the solid waste (faeces) is stored in the rectum. The solid waste is eventually egested out through the anus. Normally it takes between 24 and 48 hours for food to pass along the whole length of your digestive system. 6

7 QCM 1. Where does food pass through between the mouth and the stomach? a) The small intestine b) The large intestine c) The pancreas d) The oesophagus (gullet) 2. What happens when food reaches the stomach? a) Nothing, no digestion occurs here b) Digestive juices mix with the food and the stomach muscles contract to mix it c) It passes through quickly to the small intestine for digestion d) The food is completely digested and absorbed through the blood vessels into the blood stream 3. How does digested food finally reach the bloodstream? a) It passes from the small intestine, to the large intestine and eventually into the blood b) It is absorbed into the blood through the walls of the lungs c) It is absorbed into the blood through blood vessels in the small intestines d) It is absorbed into the blood in the appendix 4. Which of the following does not manufacture digestive juices? a) Kidneys b) Liver c) Stomach d) Pancreas 7

8 5. Which of the following does the liver produce? a) Bile b) Proteases c) Carbohydrases d) Digestive juices 6. Where is the product of the liver stored? a) Pancreas b) Appendix c) Gall bladder d) Rectum 7. Where in the digestive system is water reabsorbed? a) Liver b) Large intestine c) Small intestine d) Kidneys 8. Which is the acid produced by the stomach? a) Nitric acid b) Sulfuric acid c) Hydrochloric acid d) All of the above 9. What are the fingerlike projections of the small intestine called? a) Duodenum b) Illeum 8

9 c) Aveloa d) Villi 10. What is the motion called by which food moves through the digestive system? a) Peristalsis b) Gravity c) Waves of movement d) Muscular motion 9