Digestion
Visão Geral
A cheese sandwich is, chemically speaking, useless to you. The starch in the bread and the protein and fat in the cheese are all built from molecules far too large to slip through the wall of your gut, and most of them will not even dissolve in water. Left to themselves at body temperature they would sit there almost unchanged for years. Yet a few hours after lunch those same molecules are circulating in your blood as glucose, amino acids, fatty acids and glycerol. Something takes them apart, and it does it fast.
That something is a small set of protein molecules called enzymes, poured into the gut by glands that are not part of the gut wall at all. In this lesson you will find out what an enzyme is, why its shape decides everything it can and cannot do, why heat ruins it permanently while acid usually does not, and how amylase, protease and lipase divide the work of a meal between them. You will also see why the stomach floods itself with acid, why the liver sends a green fluid containing no enzymes at all into the small intestine, and why the same chemistry now sits inside a box of washing powder. This topic carries no required practical on this course, and yet both specimen papers build a long question on an enzyme experiment, so knowing how such an investigation is run matters as much as knowing the three enzymes.
Objetivos
- Starch (a carbohydrate), proteins and fats are insoluble. They are broken down into soluble substances so that they can be absorbed into the bloodstream in the wall of the small intestine. In the large intestine much of the water mixed with the food is absorbed into the bloodstream. The indigestible food which remains makes up the bulk of the faeces. Faeces leave the body via the anus. Students should be able to recognise the following on a diagram of the digestive system: salivary glands, oesophagus, stomach, liver, gall bladder, pancreas, duodenum, small intestine, large intestine, anus.
- Enzymes help the breakdown of food in the digestive system.; Enzymes are large proteins that act as biological catalysts. Catalysts increase the rate of chemical reactions and are utilized in the digestive process to speed up the breakdown of large molecules to small molecules for absorption into the bloodstream.; The shape of an enzyme is vital for the enzyme’s function. High temperatures denature the enzyme, changing the shape of the active site.; Different enzymes work best at different pH values.; Some enzymes work outside the body cells. The digestive enzymes are produced by specialised cells in glands and in the lining of the gut. The enzymes then pass out of the cells into the gut, where they come into contact with food molecules. They catalyse the breakdown of large molecules into smaller molecules.
- Digestive enzymes.; The enzyme amylase is produced in the salivary glands, the pancreas and the small intestine. Amylase catalyses the breakdown of starch into sugars in the mouth and small intestine.; Protease enzymes are produced by the stomach, the pancreas and the small intestine. These enzymes catalyse the breakdown of proteins into amino acids in the stomach and the small intestine.; Lipase enzymes are produced by the pancreas and small intestine. These enzymes catalyse the breakdown of lipids into fatty acids and glycerol in the small intestine.; The stomach also produces hydrochloric acid. The enzymes in the stomach work most effectively in acid conditions.; The liver produces bile, which is stored in the gall bladder before being released into the small intestine. Bile neutralises the acid that was added to food in the stomach. This provides alkaline conditions in which enzymes in the small intestine work most effectively.; Bile also emulsifies fats (breaks large drops of fats into smaller droplets). This increases the surface area of fats for lipase enzymes to act upon.
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- Which enzyme catalyses the breakdown of lipids? A. Amylase B. Protease C. Lipase D. Bile Answer: C
- Where is bile produced and where is it stored? A. Produced in the gall bladder, stored in the liver B. Produced in the liver, stored in the gall bladder C. Produced in the pancreas, stored in the gall bladder D. Produced in the liver, stored in the pancreas Answer: B
- What are the products of the complete digestion of a protein? A. Simple sugars B. Amino acids C. Fatty acids and glycerol D. Glycerol only Answer: B
- Why does an enzyme stop working when it is heated well above its optimum temperature? A. The enzyme is used up by the reaction B. The substrate molecules stop moving C. The shape of the active site is changed, so the substrate no longer fits D. The enzyme dissolves in the water Answer: C
- Bile emulsifies fat in the small intestine. What effect does this have? A. It breaks fat molecules down into fatty acids and glycerol B. It increases the surface area of fat for lipase to act on C. It denatures lipase so that digestion slows down D. It makes the contents of the small intestine acidic Answer: B
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