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DSE Biology Practice Paper: Ecosystems, Energy Flow and Human Physiology (12 questions, 100 marks, 75 minutes)
DSE Biology: Ecology and Human Physiology
DSE Biology practice paper on ecosystems, energy flow and human physiology: 12 questions, 100 marks, 75 minutes, with full answers and marking points.
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Paper details Level: S6 | Subject: Biology | Time: 75 minutes | Marks: 100 | Questions: 12
Section A: Energy flow and ecosystem data calculations (Questions 1-4, 24 marks)
Answer all questions. In calculations, write the formula, substitute the values and state the units.
| # | Question | Answer and marking notes |
|---|---|---|
| 1 | In a grassland ecosystem the producers fix 50 000 kJ m-2 yr-1 and the energy transfer efficiency between adjacent trophic levels is 10%. (a) Calculate the energy available to primary consumers. (b) Calculate the energy available to secondary and tertiary consumers. (c) Explain what happens to the other 90% of the energy. (6 marks) | Answer: (a) 50 000 x 10% = 5 000 kJ m-2 yr-1 is available to primary consumers. (b) Secondary consumers receive 5 000 x 10% = 500 and tertiary consumers 500 x 10% = 50 kJ m-2 yr-1. (c) The other 90% is lost as heat in respiration, retained in body parts that are not eaten and passed to decomposers as litter and corpses, or removed from the chain in faeces and undigested food. Marking: (a) 2; (b) 2; (c) 2 for the three routes. |
| 2 | A woodland has a gross primary productivity of 4 500 kJ m-2 yr-1 and loses 1 350 kJ m-2 yr-1 to respiration. (a) Calculate the net primary productivity. (b) Calculate respiration as a percentage of gross primary productivity. (c) Explain why net primary productivity matters to consumers. (6 marks) | Answer: (a) NPP = GPP minus respiration = 4 500 minus 1 350 = 3 150 kJ m-2 yr-1. (b) 1 350 divided by 4 500 x 100% = 30.0%. (c) Net primary productivity is the energy stored as plant biomass that herbivores can eat, so it sets how many consumers the ecosystem can support and how long its food chains can be. Marking: (a) formula and answer 3; (b) 2; (c) 1. |
| 3 | A person at rest has a heart rate of 72 beats per minute and a stroke volume of 70 mL; during exercise the heart rate is 150 beats per minute and the stroke volume is 110 mL. (a) Calculate the resting cardiac output in L min-1. (b) Calculate the cardiac output during exercise in L min-1. (c) Calculate the factor by which the cardiac output increases. (6 marks) | Answer: (a) Cardiac output = heart rate x stroke volume = 72 x 70 = 5 040 mL min-1 = 5.04 L min-1. (b) 150 x 110 = 16 500 mL min-1 = 16.5 L min-1. (c) 16.5 divided by 5.04 = 3.27 times. Marking: (a) formula 1, unit conversion 1, answer 1; (b) 2; (c) 1. |
| 4 | Snails in a meadow are estimated by mark-recapture: 80 snails are marked and released, then a later sample of 100 snails contains 16 marked individuals. The sampled area is 200 m2. (a) Estimate the population in the sampled area. (b) Calculate the density per m2. (c) State two reasons why the estimate could be too high. (6 marks) | Answer: (a) Population = marked in first catch x size of second catch divided by marked in second catch = 80 x 100 divided by 16 = 500 snails. (b) Density = 500 divided by 200 = 2.5 snails per m2. (c) The estimate is too high if marks rub off so fewer recaptured animals carry a mark, if marking makes animals easier to catch or easier to avoid traps, or if unmarked animals migrate into the area between samples. Marking: (a) formula and answer 3; (b) 1; (c) 2 each for any two. |
Section B: Energy flow, nutrient cycling and succession (Questions 5-8, 32 marks)
Write explanations in complete sentences and name the relevant ecological process or body system.
| # | Question | Answer and marking notes |
|---|---|---|
| 5 | Explain why food chains usually have only four or five trophic levels, and describe three routes by which energy is lost between trophic levels. (8 marks) | Answer: Each trophic level passes on only about one tenth of the energy it receives. Energy is lost in three ways: respiration oxidises organic matter and releases most energy as heat; parts such as bone, hair and roots are not eaten by the next level; faeces and corpses go to decomposers, so that energy never travels further along the chain. Because usable energy falls sharply at each step, by the fourth or fifth level too little is left to support a breeding population, so food chains stay short. Marking: mechanism of energy loss 2; three routes 3; link to chain length 3. |
| 6 | Describe how carbon moves between the atmosphere, producers, consumers and decomposers, and explain how burning fossil fuels upsets this balance. (8 marks) | Answer: Producers fix atmospheric carbon dioxide by photosynthesis into organic carbon; consumers pass that carbon along the food chain when they feed; producers, consumers and decomposers all oxidise organic carbon in respiration and return carbon dioxide to the air; decomposers break down corpses and faeces and complete the return. Some carbon is stored for long periods in peat, coal and oil. Burning fossil fuels releases carbon stored over millions of years within decades, so atmospheric carbon dioxide rises and the balance between photosynthesis and respiration is broken, strengthening the greenhouse effect. Marking: four pathways 6; effect of fossil fuels 2. |
| 7 | Describe how primary succession on bare rock develops into woodland, explain the role of pioneer species, and state how biodiversity and net primary productivity change. (8 marks) | Answer: Bare rock is first colonised by pioneers such as lichens and mosses. They tolerate drought, cling to rock and release organic acids that speed up weathering, and their dead remains form a thin soil in which other species can grow. Grasses then enter, their roots hold the soil and add organic matter; shrubs and trees replace them and woodland develops. During succession the soil deepens and gains nutrients, biodiversity rises and species composition keeps changing; net primary productivity rises at first and levels off in the mature stage while total biomass continues to increase. Marking: role of pioneers 2; stages 3; biodiversity and NPP changes 3. |
| 8 | Explain the role of decomposers in an ecosystem, including nitrogen fixation, nitrification and denitrification, and explain why removing leaf litter disturbs nutrient cycling. (8 marks) | Answer: Decomposers break organic matter in corpses and faeces into inorganic nutrients, release carbon as carbon dioxide and nitrogen as ammonium compounds so that producers can absorb them again. Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia; nitrifying bacteria oxidise ammonia to nitrite and then nitrate for plant uptake; denitrifying bacteria reduce nitrate back to nitrogen gas. Removing leaf litter takes away much of the organic matter and nitrogen, so decomposers lack substrate, nutrient return slows, soil fertility falls and primary productivity drops with the consumers that depend on it. Marking: role of decomposers 3; fixation, nitrification and denitrification 3; effect of litter removal 2. |
Section C: Human physiology and integrated essays (Questions 9-12, 44 marks)
Answer essay questions in points and support each argument with data, processes or examples.
| # | Question | Answer and marking notes |
|---|---|---|
| 9 | Describe mechanical and chemical digestion in the human alimentary canal, and explain how the structure of the small intestine is adapted for absorption. (11 marks) | Answer: In the mouth, teeth chew food as mechanical digestion while salivary amylase breaks starch into maltose; the stomach churns food while pepsin digests protein into peptides in acid conditions; in the small intestine pancreatic and intestinal juices supply amylase, protease and lipase, and bile salts emulsify fat, completing the breakdown to glucose, amino acids, fatty acids and glycerol. The small intestine is the main site of absorption because it is long and its lining has circular folds, villi and microvilli that give a very large surface area; the villus wall is one cell thick so the diffusion distance is short; each villus has a dense capillary network for glucose and amino acids and a lacteal for fat. Marking: mechanical digestion 2; chemical digestion and enzymes 5; four absorption features 4. |
| 10 | Explain gas exchange at the alveoli and at respiring tissues, and describe the mechanism of ventilation during inhalation and exhalation. (11 marks) | Answer: The alveolar wall and the capillary wall are each one cell thick, so the diffusion distance is short; there are very many alveoli, giving a large surface area; the alveolar lining is moist so gases dissolve first; the lungs have a dense capillary network and ventilation plus blood flow keep the concentration gradients steep. At the alveoli oxygen diffuses into the blood and binds to haemoglobin while carbon dioxide diffuses out; at respiring tissues oxygen diffuses into cells and carbon dioxide diffuses into the blood. During inhalation the intercostal muscles and diaphragm contract, the thorax enlarges and pressure falls so air enters; during exhalation the muscles relax, the thorax shrinks and pressure rises so air is pushed out. Marking: alveolar adaptations 4; exchange at both sites 3; ventilation mechanism 4. |
| 11 | Explain the significance of double circulation in mammals, and describe how the structure of the heart and its valves maintain one-way blood flow. (11 marks) | Answer: Double circulation means blood passes through the pulmonary circuit to the lungs for gas exchange and then through the systemic circuit to the body, both driven by the heart. This keeps systemic blood pressure high so blood reaches tissues quickly, and prevents oxygenated and deoxygenated blood from mixing freely, raising the efficiency of gas exchange. The right atrium receives deoxygenated blood returning from the body and the right ventricle pumps it to the lungs; the left atrium receives oxygenated blood from the pulmonary veins and the left ventricle pumps it to the body. Atrioventricular and semilunar valves open one way only and close during systole to stop backflow. The left ventricle must push blood round the whole body against greater resistance, so its muscular wall is thicker than that of the right ventricle. Marking: meaning and value of double circulation 4; blood pathway 3; valves 2; thicker left ventricle 2. |
| 12 | Explain how breathing, circulation and energy supply are coordinated during exercise, including aerobic respiration, anaerobic respiration and oxygen debt. Then compare energy flow in the human body with energy flow in an ecosystem. (11 marks) | Answer: During exercise the demand of skeletal muscle for oxygen and glucose rises and blood carbon dioxide increases, so the respiratory centre in the brain stem deepens and speeds up breathing; sympathetic stimulation raises heart rate and stroke volume, cardiac output rises, blood is redirected to skeletal muscle and heat loss increases. With enough oxygen, muscle carries out aerobic respiration, oxidising glucose completely to carbon dioxide and water and releasing a large amount of energy; when oxygen supply falls short, anaerobic respiration converts glucose to lactate and releases only a little energy. The lactate is oxidised later using extra oxygen, and this extra uptake is the oxygen debt. In an ecosystem energy fixed by producers flows one way along food chains and falls at each trophic level; in the body energy also flows one way and is lost as heat, but it is supplied continuously by the respiration of the same organism, is not passed between trophic levels and is not limited by a ten per cent transfer rule. Marking: coordination of breathing and circulation 4; aerobic and anaerobic respiration 3; oxygen debt 2; comparison with ecosystems 2. |
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