a tissue is a group of cells with similar structure and function.
organs are collections of tissues performing specific functions.
organs are organised into organ systems, which work together to form organisms.
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B3.2: The Human Digestive System
organ systems of organs that perform specific functions in the body.
the digestive system in a mammal is an organ system where several organs work together to digest and absorb food.
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B3.3: The Chemistry of Food
carbohydrates are made up of units of sugar.
simple sugars are carbohydrates that only contain one or two sugar units. they turn blue Benedict's solution brick red on heating.
complex carbohydrates contain long chains of simple sugar molecules bonded together. starch turns yellow-red iodine blue black in its presence.
lipids consist of three molecules of fatty acid bonded to a molecule of glycerol. the ethanol test indicates the presence of a lipid in solutions.
protein molecules are made up of long chains of amino acids. Biuret reagent turn blue to purple in the presence of proteins
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B3.4: Catalysts and Enzymes
catalysts increase the rate of chemical reactions without chemically changing.
enzymes are biological catalysts and catalyse specific reactions in living organisms due to the shape of their active site. This is the lock and key theory of enzyme action.
enzymes are proteins. the amino acid chains are folded to form the active site and match the shape of a specific substrate molecule.
the substrate binds to the active site and the reaction catalysed by the enzyme.
metabolism is the sum of all the reactions in a cell or the body.
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B3.5: Factors Affecting Enzyme Action
enzyme activity is affected by temperature and pH.
high temperatures denature the enzyme, changing the shape of the active site.
pH can affect the shape of the active site of an enzyme and make it work very efficiently or stop it working.
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B3.6: How the Digestive System Works
digestion involves the breakdown of large insoluble molecules into soluble substances that can be absorbed into the blood across the wall of the small intestine.
digestive enzymes are produced by specialised cells in glands and in the lining of the digestive system.
carbohydrases such as amylase catalyse the breakdown of carbohydrates into simple sugars.
proteases catalyse the breakdown of proteins into amino acids.
lipases catalyse the breakdown of lipids into fatty acids and glycerol.
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B3.7: Making Digestion Efficient
the protease enzymes of the stomach work the best in acid conditions. the stomach produces hydrochloric acid which maintains a low pH.
the enzymes made in the pancreas and small intestine work best in alkaline conditions.
bile produced in the liver, stored in the gall bladder and released through the bile duct neutralises acid and emulsifies fat.
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