Turn over 9 4 Substances move into cells by several processes. (a) (i) Which of these processes requires energy from respiration? (1) A diffusion B endocytosis C facilitated diffusion D osmosis (ii) Which of these processes can occur against a concentration gradient? (1) A active transport B diffusion C facilitated diffusion D osmosis (iii) The graph shows the effect of increasing oxygen concentration on the rate of uptake of a substance other than oxygen. Rate of uptake Oxygen concentration Which of these processes is shown by the graph? (1) A active transport B diffusion C facilitated diffusion D osmosis 10 (iv) Which substance can enter a cell by diffusion? (1) A amino acid B carbon dioxide C glucose D protein (b) When plant cells absorb water, the water potential of the cells changes. The water potential of potato cells was investigated. The graph shows the percentage change in mean mass of potato samples after immersion in sucrose solutions of different concentrations. 30 20 10 0 −10 −20 1.2 1.0 0.8 0.6 0.4 0.2 0 X Sucrose concentration / mol dm−3 Percentage change in mean mass of potato samples (%) (i) Draw a suitable line on this graph. (1) (ii) Give the sucrose concentration that has the same water potential as the cells in these potato samples. (1) Answer .............................................................. mol dm−3 Turn over 11 (iii) The water potential (ψ) of plant cells can be calculated using the equation: ψ = P + π Explain the changes in the water potential of cell (ψ), turgor pressure (P) and osmotic potential (π), at point X on the graph. 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(iv) Explain why it is more difficult to determine the osmotic potential (π) than the water potential (ψ) of plant cells. 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(Total for Question 4 = 12 marks)