Organisation
Digestion You must be able to: • Explain how enzymes work • Describe the role of enzymes in digestion • Explain how bile can speed up the digestion of lipids.
Enzymes •
Enzymes are biological catalysts – they speed up chemical reactions in living organisms. Enzymes have a number of properties: – They are all large proteins. – There is a space within the protein molecule called the active site. – Each enzyme catalyses a specific reaction. – They work best at a specific temperature and pH called the optimum. The ‘lock and key theory’ is a model used to explain how enzymes work: the chemical that reacts is called the substrate (key) and it fits into the enzyme’s active site (lock). High temperature and extremes of pH make enzymes change shape. This is called denaturing. The enzyme cannot work once it has been denatured, because the substrate cannot fit into the active site – the lock and key no longer fit together.
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Substrate Enzyme
Combined substrate and enzyme. Reaction can take place.
Products
Enzymes in Digestion •
Digestive enzymes are produced by specialised cells in glands and in the lining of the gut: 1 The enzymes pass out of the cells into the digestive system. 2 They come into contact with food molecules. 3 They catalyse the breakdown of large insoluble food molecules into smaller soluble molecules. The digestive enzymes, protease, lipase and carbohydrase, digest proteins, lipids (fats and oils) and carbohydrates to produce smaller molecules that can be easily absorbed into the bloodstream.
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REQUIRED PRACTICAL Use qualitative reagents to test for a range of carbohydrates, lipids and proteins. Sample Method 1. To test for sugars, e.g. glucose, add Benedict’s reagent and heat in a water bath for two minutes. If sugar is present, it will turn red. 2. To test for starch add iodine solution. If starch is present, it will turn blue-black. 3. To test for protein add biuret reagent. If protein is present, it will turn purple.
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Considerations, Mistakes and Errors • Do not boil the mixture for a long time, because any starch present might break down into sugar and test positive. • Refer to ’iodine solution’ not ‘iodine’. • Sometimes the purple colour is difficult to see. Try holding the test tube in front of a sheet of white paper.
Substrate is broken down and enzyme can be reused.
Key Point The ‘Lock and Key Theory’ is an example of how models are used in science to try and explain observations.
Heat
Enzyme
Enzyme denatured by heat
GCSE Combined Science Revision Guide
227841 AQA Higher Tier Combined Science All-in-One_P016_031.indd 26
6/29/16 10:51 AM