Movement of substances across the cell membrane

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1 Ch 4 Movement of substances across the cell membrane Think about (Ch 4, p.2) 1. The structure of the cell membrane can be explained by the fluid mosaic model. It describes that the cell membrane is mainly made up of phospholipids and proteins. The phospholipid molecules are arranged in a bilayer. The protein molecules are distributed among the phospholipid molecules. 2. The phospholipid bilayer allows lipid-soluble substances to pass through the membrane readily but is impermeable to water molecules, certain ions and water-soluble substances. Some membrane proteins serve as channels or carriers that allow these substances to pass through. Other membrane proteins serve as receptors that bind with specific chemicals and as markers for cell-to-cell recognition. 3. There are four main processes by which substances can move across the cell membrane: diffusion, osmosis, active transport and phagocytosis. Quick practice (Ch 4, p.8) B Quick practice (Ch 4, p.13) A Because diffusion depends on the spontaneous and random movement of particles, substances diffuse across membranes without using energy from the cell. (2) is incorrect. Particles move in all directions, but more particles move from the region of higher concentration to the region of lower concentration. (3) is incorrect. When the concentration gradient becomes zero, particles move randomly in all directions. Quick practice (Ch 4, p.24) B Quick practice (Ch 4, p.27) A The exchange of gases in the air sacs occurs by diffusion. Quick practice (Ch 4, p.28) C 1

2 Section exercise (Ch 4, p.32) 1. (a) fluid mosaic 2. (b) phospholipid (c) proteins 3. (d) laterally (e) interspersed 4. (f) water-soluble / polar (g) Carrier protein (h) Enzyme (i) (j) Receptor protein carbohydrate 5. A 6. D 7. T 8. T 9. F 10. F 11. F Cholesterol helps prevent the fatty-acid tails of the phospholipid bilayer from sticking together, which contributes to the fluidity of cell membranes. Cholesterol also increases flexibility and stability of cell membranes. Without it, membranes break up. 12. (a) Swell (b) turgid (c) Shrink (d) plasmolysed 13. (a) (i) pseudopodia (ii) vacuole (iii) digest (b) Phagocytosis is important for the nutrition of some single-celled organisms, e.g. Amoeba engulfs food particles by phagocytosis. It is also important for body defence against diseases, e.g. in humans, certain white blood cells engulf harmful microorganisms by phagocytosis. 2

3 Revision exercise (Ch 4, p.34) 1. A 2. B 3. C 4. D 5. C 6. B Structure I is glycoprotein. The hydrophobic interior of the phospholipid bilayer is permeable to lipid-soluble substances (e.g. alcohol). Simple, small molecules (e.g. oxygen, carbon dioxide) can also move across the phospholipid bilayer freely. Water molecules, certain ions and water-soluble substances (e.g. glucose) can move across the cell membrane through channel proteins or carrier proteins. It is the phospholipid bilayer that acts as a barrier to polar substances. Membrane proteins do not act as barriers. On the contrary, carrier proteins and channel proteins transport some substances across the membrane. All three factors affect the rate of diffusion. (1) At higher temperatures, particles have more kinetic energy and collide more frequently. This results in a higher diffusion rate. (2) The concentration gradient affects the tendency of particles to move from one region to another. The steeper the concentration gradient, the higher the diffusion rate. (3) Small particles diffuse faster than large particles. Distilled water has a higher water potential than 10% sucrose solution. Initially, there is a net movement of water molecules from the outside into the dialysis tubing by osmosis. As more water molecules move into the dialysis tubing, the water potential increases and finally becomes stable when the dialysis tubing is full. Although potato strips of the same size are used, the initial masses of the potato strips can be different due to different densities. To have a fair comparison, the percentage change in mass should be considered. 3

4 7. A 8. A 9. D 10. A 11. B 12. C 13. C 14. B In 0% and 5% sucrose solutions, the stored potato strips have a larger increase in mass than the fresh potato strips. This is because more water has entered the potato cells by osmosis. Whereas in 10% sucrose solution, the stored potato strip have a smaller decrease in mass. This is because less water is lost by osmosis. Therefore, the water potential of the stored potato is lower than that of the fresh potato. As starch is converted into sugars, the amount of solute increases. This lowers the water potential. (1) is incorrect. Replacing the 10% sucrose solution with 5% sucrose solution will reduce the volume of water moving in, hence leading to a lower final level of the sucrose solution. (3) is incorrect. While peeling off all the potato skin can increase the rate of osmosis, it does not contribute to final volume of water moving in. Different cells have slightly different water potential, because they have different concentrations of solutes. A and B are incorrect. The cells have been immersed for an hour. Equilibrium should have reached and all cells should be in good contact with the sugar solution. D is incorrect. The sizes of vacuoles may or may not affect water potential. It depends on the concentrations of solutes inside. (1) and (3) are passive processes that do not require energy from cellular respiration. Process (2) involves active transport which require energy from cellular respiration. These ions are moved from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient). 15. HKALE Biology 2010 Paper 1 Q7 16. (a) (i) ( ) % (1) = 12% (1) (ii) The water potential of distilled water is higher than that of sucrose solution. (1) There is a net movement of water from the beaker into the bag through the dialysis tubing by osmosis. (1) 4

5 (b) Line from 20 (g) (1) Line going upwards (1) (c) differential permeability (1) (Total: 7 marks) 17. (a) It would be bent downwards by the weight. (1) (b) osmosis (1) (c) Sucrose solution has a lower water potential than the cell sap. (1) Water moves out of the cells by osmosis. (1) Cells become flaccid / decrease in turgor / lose rigidity. (1) (d) The potato cylinder in tough B is shorter / lighter than that in tough A. (1) 18. HKALE Biology 2007 Paper 1 Q8 (Total: 6 marks) 19. (a) (i) glucose and galactose (1) (ii) Their absorption rates are reduced by cyanide. (1) Active transport needs energy. (1) Less / no energy available / released by respiration if cyanide is present. (1) (b) All of the sugars can be absorbed when the intestine is poisoned with cyanide or when no respiration occurs. (1) Diffusion does not need an energy supply. (1) (Total: 6 marks) 20. (a) (i) Diagram III should show two arms of membrane surrounding particle and almost meeting. (1) Diagram IV should show restored membrane surface and vacuole enclosed in cell. (1) Label vacuole and cell membrane. (1) (ii) phagocytosis (1) (b) Sea water has a lower water potential than the cytoplasm. (1) Water moves out of the cells by osmosis. (1) (Total: 6 marks) 21. (a) A: phospholipid head / hydrophilic head / phosphate group (1) B: hydrophobic tails / fatty acids (1) C: transmembrane protein / carrier protein / channel protein (1) 5

6 (b) (i) As the solubility in lipid increases, the rate of diffusion increases. (1) Lipid-soluble substances readily pass through / dissolve in the phospholipid bilayer of the cell membrane. (1) (ii) Smaller molecules diffuse faster. (1) They can pass between phospholipid molcules more easily. (1) (c) Any two of the following: (2) concentration / diffusion gradient / concentration difference (1) number of carrier proteins / channel proteins (1) temperature (1) (d) Vitamin B 1 is water-soluble and cannot pass through the phospholipid bilayer / hydrophobic region. (1) It passes into a cell through channel proteins / carrier proteins. (1) Vitamin K is lipid-soluble and can dissolve in the phospholipid bilayer / hydrophobic region. (1) It can pass directly through the phospholipid bilayer / hydrophobic region. (1) (Total: 13 marks) 22. (a) differentially permeable (1) (b) (i) Distilled water is hypotonic to red blood cells. (1) Water enter the cells by osmosis, the cells swell and finally burst. (1) (ii) 1.5% sodium chloride solution is hypertonic to red blood cells. (1) Water leaves the cells by osmosis. (1) (c) Phospholid in the cell membrane is dissolved / phospholipid bilayer is broken down. (1) Haemoglobin leaks out from the red blood cells. (1) (Total: 7 marks) 6

7 23. (a) (i) egg in water Egg in salt solution size of the egg larger smaller position in the liquid sunk to the bottom floating in the middle external appearance of the egg smooth / oval wrinkled / uneven 1 mark for correct answers in one row (1) 3 (ii) The membrane of the egg is differentially permeable. (1) Distilled water has a higher water potential / less solutes than the egg. (1) The egg in distilled water gains water by osmosis. (1) The egg gains in mass and sinks to the bottom. (1) The salt solution has a lower water potential / more solutes than the egg. (1) The egg in salt solution loses water by osmosis. (1) The egg becomes wrinkled / mass reduces and the egg floats in the solution. (1) (max. 5) (b) (i) Both axes scaled evenly (1) Plot points accurately (1) Points joined by best fit line (1) (ii) Check graph for answer but need correct units +/ 0.5 g dm 3 (1) (iii) Salt solution at this concentration has the same water potential as the egg / is isotonic to the egg. (1) There is no water potential gradient / no difference in water potential. (1) No net movement of water. (1) 24. HKCEE Biology 2009 Paper 1 Q4 (Total: 15 marks) 25. (a) (i) osmosis (1) (ii) Water molecules move across the cell membrane mainly via channel proteins. (1) Some water molecules may fit between phospholipid molecules. (1) 7

8 (b) Plant cells are surrounded by a cell wall (1) which prevents the cell membrane from over expanding / limits the uptake of water. (1) (c) (i) between and kpa (1) (ii) Plot water potential of sucrose solution on the x-axis and percentage of cell plasmolysed on the y-axis. (1) Read down from 50% plasmolysed to water potential. (1) OR Plot sucrose concentration on the x-axis and percentage of cell plasmolysed on the y-axis. (1) Read down from 50% plasmolysed to sucrose concentration and look up equivalent water potential. (1) (d) Make the results more reliable (max. 3) Observe more pieces of onion (1) Count more cells (1) Calculate a mean (1) Ignore anomalous results (1) Make the results more accurate, any one of the following: (1) Use more concentrations within existing range / smaller gap between concentrations (1) Use narrower range around 50% plasmolysis (1) Take photographs and mark cells by counting (1) (Total: 12 marks) 26. (a) (i) The water potential of the solution is higher than that of red blood cells. (1) Water molecules enter the red blood cells by osmosis. (1) The cells enlarge and finally burst. (1) (ii) 4 g dm -3 (1) (iii) Red blood cells have a range of water potential / solute concentration. (1) This would require external solutions with a range of water potential to burst the cells. (1) (b) (i) temperature (1) It would affect the rate of respiration and hence the supply of energy / ATP. (1) 8

9 (ii) active transport (1) It requires energy / ATP from respiration which uses oxygen. (1) As oxygen concentration increases, the rate of potassium ion uptake increases. (1) (c) All would be 7 arbitrary units. (1) Cyanide inhibits respiration / reduces energy supply. (1) Some potassium ions are taken up by the diffusion. (1) (Total: 14 marks) 27. Movement of substances through the cell membrane (max. 8) Simple, small molecules (e.g. oxygen) and non-polar molecules can diffuse directly through the phospholipid bilayer (1) from an area of high concentration to an area of low concentration / down the concentration gradient. (1) Certain ions and water-soluble substances (e.g. glucose, amino acids) can diffuse into cells through channel proteins or carrier proteins. (1) Water molecules move across the cell membrane from a region of higher water potential to a region of lower water potential (1) by osmosis. (1) Active transport requiring energy from respiration (1) moves substances against concentration gradients. (1) Large particles are brought into cells across membrane through phagocytosis. (1) Communication (max. 3) (Total: 11 marks) 28. The structure of the cell membrane can be explained by the fluid mosaic model proposed by Singer and Nicholson. (1) The cell membrane consists of phospholipids / a phospholipid bilayer. (1) It separates contents from outside / acts as a barrier. (1) The hydrophobic / water-repelling tails of phospholipid face each other. (1) The hydrophilic / water-loving heads face the aqueous environment inside and outside of the cell. (1) The presence of cholesterol affects fluidity of the cell membrane. (1) The inside of the phospholipid bilayer allows fat-soluble substances to pass through. (1) Some membrane proteins serve as carriers (1) for active transport. (1) 9

10 Some membrane proteins serve as hydrophilic channels (1) for polar molecules to diffuse through. (1) Some membrane proteins serve as enzymes. (1) Glycoproteins / glycolipids play an important role in cell recognition. (1) (max. 10) (Total: 10 marks) Article reading (Ch 4, p. 43) 1. Similarities Both involve the use of a differentially permeable membrane. (1) Both involve solutions of different concentration/water potential. (1) Differences Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential; while reverse osmosis is the net movement of water molecules from a region of lower water potential to a region of higher water potential. (1) Osmosis is a naturally occurring transport mechanism that does not require any external forces; while reverse osmosis requires the application of pressure. (1) 2. concentration gradient / temperature / surface area (1) 10

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