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S4 Biology Practice Paper: Respiration, Photosynthesis and Gas Exchange (12 Questions, 100 Marks, 75 Minutes, with Answers)
HKDSE Biology: Respiration, Photosynthesis and Gas Exchange
12 questions, 100 marks, 75 minutes: photosynthesis and respiration, leaf structure, stomata, bubble-rate data, the starch test, oxygen debt and ventilation, with marking points.
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Paper details Level: S4 | Subject: Biology | Time: 75 minutes | Marks: 100 | Questions: 12 | Answers included
Section A: Data and Leaf Structure (32 marks)
Answer all questions in the spaces provided.
| # | Question | Answer and marking notes |
|---|---|---|
| 1 | Source 1: The overall equation for photosynthesis is 6CO2 + 6H2O → C6H12O6 + 6O2. Source 2: Respiration breaks down glucose and releases the energy for cell activities. (a) State the two reactants in the equation in Source 1 and where each of them comes from. (3 marks) (b) Explain how photosynthesis and respiration depend on each other in gas exchange. (3 marks) | (a) Carbon dioxide diffuses from the atmosphere into the leaf through the stomata; water is absorbed by the roots and carried to the leaf in the xylem. (3 marks) (b) The oxygen released by photosynthesis is used in respiration, and the carbon dioxide released by respiration is used in photosynthesis; the two processes keep the oxygen and carbon dioxide content of the atmosphere stable. (3 marks) |
| 2 | The diagram shows a transverse section of a dicotyledonous leaf: upper epidermis, palisade mesophyll, spongy mesophyll, vascular bundle and lower epidermis. (a) Describe two structural features of the palisade mesophyll that favour photosynthesis. (4 marks) (b) Explain how the concentration of stomata on the lower epidermis helps a land plant to reduce water loss. (4 marks) | (a) The cells are column-shaped and closely packed, so they hold many chloroplasts; they lie in the uppermost layer, closest to the light; the small air spaces shorten the diffusion path of carbon dioxide. (4 marks, 2 each) (b) The lower epidermis receives less sunlight and is cooler, so transpiration is slower; the stomata avoid direct sunlight and water loss is reduced while gas exchange can still take place. (4 marks) |
| 3 | A student used pondweed to study the effect of light distance. The numbers of gas bubbles collected in 15 seconds were: 40 bubbles at 10 cm, 18 bubbles at 20 cm and 9 bubbles at 30 cm. (a) Calculate the rate of bubble production per minute at 20 cm. (4 marks) (b) Explain why the increase in rate when the lamp moves from 20 cm to 10 cm is not proportional to the increase in light intensity. (4 marks) | (a) 18 ÷ 15 × 60 = 72 bubbles per minute. (4 marks: working 2, answer with unit 2) (b) From 30 cm to 20 cm light intensity changes from limiting to more adequate, and the rate rises from 36 to 72, a doubling; from 20 cm to 10 cm the light intensity becomes four times greater, yet the rate only rises from 72 to 160, about 2.2 times; this is because carbon dioxide concentration or temperature has become the limiting factor and the light reaction is close to saturation. (4 marks) |
| 4 | A student kept a potted geranium in the dark for 48 hours, covered part of a leaf with opaque paper, and then placed the plant in sunlight for 6 hours. The leaf was decolourised in hot alcohol and then tested with iodine solution. (a) Explain the purpose of keeping the plant in the dark for 48 hours. (4 marks) (b) Describe the results expected. (3 marks) (c) Explain why hot alcohol, rather than boiling water, is used for decolourisation. (3 marks) | (a) Any starch already in the leaf is transported away or used up in respiration during darkness, so the leaf contains no starch at the start; any starch detected afterwards must have been made during the light period. (4 marks) (b) The uncovered part turns blue-black; the covered part stays yellowish-brown, the colour of iodine solution. (3 marks) (c) Chlorophyll dissolves in alcohol but not in water; hot alcohol dissolves the chlorophyll quickly so the leaf is decolourised and the iodine colour is easy to see. (3 marks) |
Section B: Experimental Design and Physiological Data (36 marks)
Show your working in calculations and state the units.
| # | Question | Answer and marking notes |
|---|---|---|
| 5 | (a) State two differences between respiration and burning in the way energy is released. (3 marks) (b) Explain why muscles become sore after vigorous exercise and why breathing stays rapid after the exercise stops. (6 marks) | (a) Respiration occurs at ordinary temperature, is catalysed by enzymes and releases energy in several steps, some of it stored in ATP; burning occurs at high temperature, releases energy in one step and most of it is lost as heat and light. (3 marks) (b) During vigorous exercise the oxygen supply is insufficient, so muscle cells respire anaerobically and produce lactic acid, which accumulates and causes soreness; breathing stays rapid afterwards to supply extra oxygen to oxidise the lactic acid or carry it to the liver and to restore the oxygen store of the muscles, that is, to repay the oxygen debt. (6 marks) |
| 6 | Data: at rest a person breathes 12 times per minute, taking in 0.5 L of air each breath; during exercise the rate is 30 times per minute with 2.0 L per breath. (a) Calculate the pulmonary ventilation per minute at rest and during exercise, and the ratio between them. (4 marks) (b) State two reasons why breathing becomes deeper and faster during exercise. (2 marks) (c) Explain how the increased ventilation helps muscles to keep contracting. (3 marks) | (a) At rest: 12 × 0.5 = 6 L per minute; during exercise: 30 × 2.0 = 60 L per minute; the ratio is 60 ÷ 6 = 10 times. (4 marks) (b) The carbon dioxide concentration of the blood rises; the pH of the blood falls and stimulates the breathing centre. (2 marks) (c) More oxygen enters the alveoli and diffuses into the blood for aerobic respiration to make ATP; carbon dioxide is removed faster, so less lactic acid accumulates and the muscles keep contracting. (3 marks) |
| 7 | Data: 20 g of germinating mung beans were placed in a vacuum flask and the temperature rose from 20.0 °C to 24.0 °C. A control with 20 g of boiled, cooled beans rose by only 0.4 °C. Assume the water in the flask has a mass of 100 g and a specific heat capacity of 4.2 J g⁻¹ °C⁻¹. (a) Calculate the heat energy released by the germinating beans and the energy released per gram. (4 marks) (b) Explain why a control of boiled, cooled beans is needed. (5 marks) | (a) Heat = mcΔθ = 100 × 4.2 × 4.0 = 1680 J; per gram of beans, 1680 ÷ 20 = 84 J. (4 marks) (b) Boiled, cooled beans are dead and cannot respire; they show that the rise in temperature comes from respiration of living seeds and not from bacterial decay or a change in room temperature; the control rose by only 0.4 °C, so the living seeds were the main heat source. (5 marks) |
| 8 | (a) Describe how the stomata change between day and night, and name the cells that control the opening. (4 marks) (b) Explain the advantages and disadvantages of stomata closing temporarily at noon on a hot, dry day. (5 marks) | (a) Stomata are open in the day and mostly closed at night; the opening is controlled by two guard cells, which open the pore when they take in water and swell and close it when they lose water. (4 marks) (b) Advantage: transpiration is reduced, so the plant avoids wilting and keeps its water and cell turgor; disadvantage: less carbon dioxide enters, so the rate of photosynthesis falls, oxygen cannot escape easily, and leaf temperature may rise if the stomata stay closed. (5 marks) |
Section C: Comparison and Application (32 marks)
Write full sentences for explanation questions.
| # | Question | Answer and marking notes |
|---|---|---|
| 9 | State four differences between photosynthesis and respiration in terms of: energy change, direction of gas exchange, organelle involved and time of occurrence. (8 marks) | Energy: photosynthesis stores light energy in organic matter while respiration releases the energy from organic matter; gas exchange: photosynthesis takes in carbon dioxide and gives out oxygen while respiration takes in oxygen and gives out carbon dioxide; organelle: photosynthesis occurs in chloroplasts while respiration occurs mainly in mitochondria; time: photosynthesis happens only in light while respiration goes on day and night. (8 marks, 2 each) |
| 10 | A farmer grows tomatoes in a greenhouse and wants a higher yield. (a) Explain why raising the carbon dioxide concentration in the greenhouse increases the yield when light is sufficient. (4 marks) (b) State and explain one other measure that could raise the yield. (4 marks) | (a) Carbon dioxide is a raw material and a limiting factor when light is sufficient; a higher concentration speeds up the light-independent reactions, so more glucose is made, the net rate of photosynthesis rises and more organic matter accumulates. (4 marks) (b) Keep the greenhouse near the optimum temperature of about 25 °C to raise enzyme activity; or extend the lighting period to supply more light energy; or supply water and minerals such as magnesium and nitrogen to maintain chlorophyll and protein synthesis. (4 marks) |
| 11 | A student measured 42, 8 and 45 bubbles per minute from pondweed under red, green and white light respectively. Explain the differences between the three results. (8 marks) | White light contains all colours and carries the most energy, so net photosynthesis is fastest and most bubbles are produced, 45 per minute; red light is strongly absorbed by chlorophyll, so the rate is close to that under white light, 42 per minute; most green light is reflected or transmitted by the leaf and little is absorbed, so photosynthesis is slowest, 8 per minute; the bubble count reflects the rate of oxygen production. (8 marks) |
| 12 | In a closed cabin on a space station, scientists plan to use algae and plants to maintain the oxygen supply for astronauts. Explain why photosynthetic organisms are needed and state one problem that still has to be handled. (8 marks) | Algae and plants take in the carbon dioxide breathed out by the astronauts and carry out photosynthesis, releasing oxygen for respiration and making organic matter as food while cleaning the cabin air; problems still to be handled include the energy for continuous lighting, the recycling of minerals such as nitrogen and phosphorus, plant pests and diseases, and control of oxygen and carbon dioxide fluctuations in a closed system. (8 marks) |
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