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South Tuen Mun Government Secondary School Biology Revision Note 2 Fluid mosaic model Characteristics Functions Phospholipid bilayer makes the cell membrane permeable to fat-soluble substances, simple and small substances; but impermeable to water, ions and water-soluble substance Protein (embedded half-way) acts as enzymes Channel protein makes the cell membrane permeable to certain ions, water and water-soluble (penetrate through the bilayer) substances Carrier protein (penetrate through the bilayer) carries out active transport Glycoprotein (embedded half-way) acts as receptors to substances in the blood e.g. hormones. acts as recognition substances e.g. antigens for recognize foreign cells Transport across the membrane Diffusion the net movement of substances from a region of higher concentration to that of lower concentration Osmosis the net movement of water from a region of lower solute concentration/higher water potential to that of higher solute concentration/lower water potential through a semi-permeable membrane Active transport the movement of substance through a membrane with the expense of energy, this method can move substance against a concentration gradient Phagocytosis large substance is engulfed, it is digested and useful substances are absorbed; energy is required e.g. white blood cells kill bacteria Osmosis and cells In hypertonic solution, water moves out of a plant cell by osmosis, the plant cell becomes flaccid, this is known as plasmolysis and the cell is plasmolysed. In hypertonic solution, water moves out of an animal cell by osmosis, the cell becomes crinkled. In isotonic solution (equal water potential) there is no net movement of water in or out of a cell. In hypotonic solution (higher water potential) – water moves in a plant cell by osmosis, the plant cell is turgid. In hypotonic solution – water moves in an animal cell by osmosis, the cell will burst, this process is known as haemolysis and the cell is said to be haemolysed. Enzyme acts as a catalyst to increase the speed of chemical reaction at 37oC A+ B C + D (A and B are substrates, C and D are products) speed of chemical reaction can be measured by the rate of disappearance of substrate and / or the rate of making products words ended with –ase, e.g. catalase : catalyse the break down of hydrogen peroxide to form oxygen enzyme is specific enzyme is not used by the chemical reaction, it remains unchanged after a reaction enzyme works best at 37oC, at low temperature, enzyme is inactive; at high temperature, enzyme (protein in nature) denatures and lose it function forever different enzymes work best at different optimum pH, enzyme, not at its optimum pH, denatures Lock and Key hypothesis Inhibitors Competitive inhibitor has a similar shape to substrate. It competes with the substrate for the active site. When it binds with the enzyme, at the active site, the substrate cannot bind with enzyme. Thus the rate of reaction is lower. Under high concentration of substrate, the rate of reaction can be increased. Non-competitive inhibitor does not has a similar shape to substrate. It binds with the enzyme, not at the active site. It changes the shape of the active site. The substrate cannot bind at the active site. Thus the rate of reaction is lower. Under high concentration of substrate, the rate of reaction cannot be increased. Application of enzymes in daily life Biological washing powder : protease and lipase to remove protein (blood) stain and grease Meat tenderizer : protease in fresh fruit (e.g. pineapple, papaya) is used to break down protein and makes meat soft