References only, not question text. Open Official paper ↗ where Cambridge publishes the paper,
or see the official Cambridge past-papers page; for other papers, ask your
school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Food chains & webs · Core · Paper 1 style · 1 mark
(a)In the food chain algae → water flea → small fish → heron, which organism is the secondary consumer?[1]
Show answer
(a) C small fish
The small fish eats the primary consumer (water flea), so it is the secondary consumer. The heron is the tertiary consumer, so D is wrong.
Q2Food chains & webs · Core · Paper 2 style · 4 marks
A food web in a wheat field in Punjab:wheat is eaten by grasshoppers, field mice and sparrowsgrasshoppers are eaten by sparrows and frogsfield mice are eaten by snakes and owlssparrows are eaten by owlsfrogs are eaten by snakes
(a)Name the producer in this food web.[1]
(b)Write out one food chain from the web that has four organisms.[1]
(c)Name one organism that is both a primary and a secondary consumer.[1]
(d)State the source of energy for the whole food web.[1]
B1 correct chain with arrows pointing to the eater
(c) sparrow
B1
(d) the Sun (light energy)
B1
Q3Food chains & webs · Core · Paper 2 style · 4 marks
(a)State what is meant by:(i) a producer(ii) a consumer[2]
(b)State the role of decomposers in an ecosystem.[2]
Show answer
(a) (i) organism that makes its own organic nutrients, usually by photosynthesis; (ii) organism that gets energy by feeding on other organisms
B1 each
(b) break down dead organisms and waste; release nutrients/minerals back into the soil (recycling)
B1 break down dead material; B1 recycle nutrients
Q4Food chains & webs · Standard · Paper 1 style · 1 mark
(a)What do the arrows in a food chain show?[1]
Show answer
(a) A the direction in which energy is transferred
Arrows point from the food to the feeder, showing energy transfer. They do not show size, so B is wrong.
Q5Food chains & webs · Standard · Paper 2 style · 5 marks
A food web in a wheat field in Punjab:wheat is eaten by grasshoppers, field mice and sparrowsgrasshoppers are eaten by sparrows and frogsfield mice are eaten by snakes and owlssparrows are eaten by owlsfrogs are eaten by snakes
(a)A farmer sprays insecticide that kills most grasshoppers. Explain two possible effects on other organisms in the web.[4]
(b)State why food webs give a more realistic picture of feeding than food chains.[1]
Show answer
(a) frogs decrease (less food), so snakes may eat more mice, reducing mice; sparrows eat more wheat (or fewer sparrows), affecting owls; wheat may increase (less eaten by grasshoppers)
2 marks each for two effects with explanation (B1 effect, B1 reason): frogs fall due to less food; sparrows rely more on wheat/fall; snakes eat more mice; wheat increases
(b) most animals eat more than one type of food / are eaten by more than one predator
B1
Q6Food chains & webs · Standard · Paper 2 style · 5 marks
In the Arabian Sea off the coast of Karachi: phytoplankton are eaten by zooplankton; zooplankton are eaten by sardines and by jellyfish; sardines are eaten by tuna; jellyfish are eaten by turtles.
(a)Construct a food web using all five organisms.[3]
(b)State the trophic level of the tuna.[1]
(c)Plastic bags in the sea are mistaken by turtles for jellyfish. Suggest the effect on the jellyfish population if many turtles die.[1]
B1 producer at start; B1 all arrows from food to eater; B1 all five links correct
(b) tertiary consumer (fourth trophic level)
B1
(c) jellyfish increase (fewer predators)
B1
Q7Food chains & webs · Challenge · Paper 2 style · 6 marks
In a village pond in Sindh, small fish eat mosquito larvae and dragonfly nymphs. Dragonfly nymphs also eat mosquito larvae. All the fish are removed by netting.
(a)Explain why the number of mosquitoes might increase after the fish are removed.[2]
(b)Explain why the dragonfly nymphs might first increase and then decrease.[3]
(c)Suggest one reason why using fish to control mosquitoes may be better than spraying insecticide.[1]
Show answer
(a) fish eat mosquito larvae; fewer predators so more larvae survive to become adults
B1 fish are predators of larvae; B1 more survive
(b) at first more food (more mosquito larvae) and no competition from fish, so they increase; then the larger nymph population eats most of the larvae / food becomes scarce, so their numbers fall
B1 more food (larvae) / less competition; B1 increase; B1 food runs out / competition rises so decrease
(c) does not kill other (useful) insects / no chemical pollution or bioaccumulation / mosquitoes do not become resistant
B1
Q8Food chains & webs · Challenge · Paper 2 style · 5 marks
(a)Explain why food chains rarely have more than five trophic levels.[3]
(b)Explain why eating plant food such as lentils feeds more people from the same area of land than eating meat.[2]
Show answer
(a) energy is lost at each level (respiration, heat, movement, egestion, excretion, parts not eaten); only about 10 % passes on; too little energy remains to support another level
B1 energy lost at each transfer; B1 named loss; B1 not enough energy for further level
(b) shorter food chain; less energy lost, as energy is lost when crops are fed to animals
B1 humans as primary consumers / shorter chain; B1 less energy lost between levels
Q9Energy flow & pyramids · Core · Paper 2 style · 5 marks
Energy in a grassland food chain (kJ per m² per year):grass: 25 000grasshoppers: 2 500lizards: 200hawks: 15
(a)Calculate the percentage of energy transferred from the producers to the primary consumers.[2]
(b)Calculate the percentage transferred from the primary to the secondary consumers.[1]
(c)State two ways energy is lost between trophic levels.[2]
Show answer
(a) 2 500 ÷ 25 000 × 100 = 10 %
M1; A1 10 %
(b) 200 ÷ 2 500 × 100 = 8 %
B1 8 %
(c) respiration (heat); movement; egestion (faeces); excretion; parts not eaten (bones, roots)
1 mark each for two
Q10Energy flow & pyramids · Standard · Paper 1 style · 1 mark
(a)One large mango tree is eaten by 2000 caterpillars, which are eaten by 50 birds. Which describes the pyramids of numbers and biomass for this food chain?[1]
Show answer
(a) B pyramid of numbers has a narrow base (not pyramid-shaped), pyramid of biomass pyramid-shaped
One tree supports many caterpillars, so the bottom bar of the number pyramid is narrowest, but the tree has the greatest biomass, so the biomass pyramid is pyramid-shaped. C has them the wrong way round.
Q11Energy flow & pyramids · Standard · Paper 2 style · 4 marks
A field of fodder crop receives 1 500 000 kJ of light energy per m² per year and fixes 15 000 kJ per m² per year in photosynthesis.
(a)Calculate the percentage of the light energy that is fixed by the crop.[1]
(b)Goats eat all the energy fixed by the crop and convert 10 % of the energy they receive into new body tissue. Calculate the energy in the new goat tissue, in kJ per m² per year.[1]
(c)Suggest two reasons why most of the light energy is not fixed by the crop.[2]
Show answer
(a) 15 000 ÷ 1 500 000 × 100 = 1 %
B1
(b) 15 000 × 0.1 = 1 500 kJ per m² per year
B1 1 500
(c) light misses leaves / reflected; wrong wavelength; transmitted through leaves; used to evaporate water; photosynthesis limited by CO₂/temperature
1 mark each for two
Q12Energy flow & pyramids · Standard · Paper 2 style · 3 marks
(a)Explain why a pyramid of biomass is usually a better way to show a food chain than a pyramid of numbers.[2]
(b)State one disadvantage of measuring biomass to construct a pyramid of biomass.[1]
Show answer
(a) pyramid of numbers ignores size, e.g. one tree with many insects gives an odd shape; biomass takes the mass of organisms into account so is nearly always pyramid-shaped
B1 numbers ignores size of organisms; B1 biomass accounts for mass/always upright
(b) organisms must be killed and dried to find their dry mass (or biomass changes with the season / hard to sample all organisms)
B1 any one: organisms killed/destructive; dry mass takes time; varies with season/time of year; difficult to collect all organisms (e.g. roots, soil organisms)
Q13Energy flow & pyramids · Challenge · Paper 2 style · 4 marks
In a lake, producers fix 36 000 kJ per m² per year. Transfer efficiency is 8 % from producers to primary consumers, and 10 % at each later transfer.
(a)Calculate the energy reaching the tertiary consumers, in kJ per m² per year.[2]
(b)Suggest why the transfer from producers to herbivores (8 %) is less efficient than the transfers between animals (10 %).[2]
(b) plant material contains much cellulose/lignin that herbivores cannot digest; so a larger proportion of the food is lost in faeces (egested); animal food is more digestible
B1 cellulose/plant material hard to digest; B1 more energy lost in faeces/egestion
Q14Energy flow & pyramids · Challenge · Paper 2 style · 4 marks
(a)Explain why a pyramid of energy is always upright.[2]
(b)Fish farms in Thailand feed farmed fish with pellets made from wild-caught smaller fish. Use ideas about energy flow to explain why this may not be sustainable.[2]
Show answer
(a) energy is lost at every transfer between trophic levels (e.g. as heat from respiration); so each level always has less energy than the one below
B1 energy lost at each level; B1 so less available at each higher level
(b) much energy is lost when small fish are fed to farmed fish; more mass of wild fish is caught than farmed fish produced; wild stocks may be overfished
B1 energy losses mean more wild fish needed than produced; B1 overfishing of wild stocks
Q15Populations & sampling · Core · Paper 1 style · 1 mark
(a)Which factor would NOT normally limit the growth of a population of rabbits?[1]
Show answer
(a) D a high birth rate
A high birth rate increases the population; food shortage, predators and disease limit it. A (food) is a limiting factor.
Q16Populations & sampling · Core · Paper 4 style · 4 marks
Students counted clover plants in ten 0.5 m × 0.5 m (0.25 m²) quadrats placed randomly in a school field 50 m × 20 m.Counts: 3, 5, 0, 7, 4, 6, 2, 5, 8, 4
(a)Calculate the mean number of plants per quadrat.[1]
(b)Calculate the mean number of plants per m².[1]
(c)Estimate the total number of plants in the field.[1]
(d)State how the students made sure the quadrats were placed randomly.[1]
Show answer
(a) 44 ÷ 10 = 4.4
B1 4.4
(b) 4.4 ÷ 0.25 = 17.6
B1 17.6
(c) 17.6 × (50 × 20) = 17 600
B1 17 600
(d) used random number tables/generator to choose coordinates on a grid (not chosen by eye)
B1
Q17Populations & sampling · Standard · Paper 2 style · 5 marks
(a)Describe the phases of the sigmoid growth curve of a population of yeast in a flask of sugar solution.[3]
(b)Suggest two reasons why the yeast population eventually falls.[2]
Show answer
(a) lag phase: slow growth as yeast adapt; exponential (log) phase: rapid growth, plenty of food, no limits; stationary phase: birth rate = death rate as food runs out / waste builds up; (death phase: numbers fall)
B1 lag; B1 exponential with reason; B1 stationary with reason
(b) sugar used up; ethanol (toxic waste) builds up; lack of oxygen
1 mark each for two
Q18Populations & sampling · Standard · Paper 4 style · 4 marks
Snail counts in eight 1 m² quadrats placed randomly in a garden:12, 15, 9, 0, 18, 11, 6, 13
(a)Calculate the mean number of snails per quadrat.[1]
(b)Estimate the total population of snails in the 400 m² garden if each quadrat is 1 m².[1]
(c)Suggest why the result of quadrat 4 (0 snails) should not be removed.[1]
(d)Suggest how the estimate could be made more reliable.[1]
Show answer
(a) 84 ÷ 8 = 10.5
B1
(b) 10.5 × 400 = 4 200
B1
(c) it is a real result from random sampling / snails may be absent from some areas
B1
(d) use more quadrats / sample on several days
B1
Q19Populations & sampling · Challenge · Paper 2 style · 5 marks
(a)Describe and explain the shape of the human population curve over the last 200 years.[3]
(b)Suggest two factors that could limit the future growth of a human population in a country.[2]
Show answer
(a) rapid (exponential) increase; death rate fell due to better medicine, vaccination, sanitation, food supply; birth rate stayed higher than death rate
B1 rapid/exponential increase; B1 lower death rate with named reason; B1 birth rate > death rate
(b) food/water shortage; disease; family planning/contraception; space/housing; war
1 mark each for two
Q20Populations & sampling · Challenge · Paper 2 style · 5 marks
In a greenhouse of tomato plants in Murree, aphid and ladybird numbers were counted weekly.week | aphids | ladybirds0 | 20 | 22 | 60 | 34 | 400 | 56 | 1200 | 158 | 500 | 4010 | 150 | 25
(a)Describe and explain the change in aphid numbers from week 2 to week 6.[2]
(b)Explain the changes in aphid and ladybird numbers after week 6.[2]
(c)Suggest why the ladybird peak comes after the aphid peak.[1]
Show answer
(a) aphids increased rapidly because food (plants) was plentiful and there were few predators; exponential growth
B1 rapid increase; B1 reason
(b) ladybirds increased (more food) and ate more aphids, so aphids decreased; also limited by food
Q23Carbon cycle · Standard · Paper 2 style · 4 marks
(a)Explain the role of decomposers in the carbon cycle.[2]
(b)Describe how fossil fuels were formed.[2]
Show answer
(a) they feed on dead organisms and waste; respire, releasing CO₂ into the air
B1 break down dead material; B1 respiration releases CO₂
(b) dead organisms did not decay fully (no oxygen/waterlogged); compressed over millions of years into coal/oil/gas
B1 incomplete decay; B1 pressure/millions of years
Q24Carbon cycle · Challenge · Paper 2 style · 6 marks
Mean CO₂ concentration at a monitoring station (parts per million):2000: 3702010: 3902020: 414Within each year the concentration is about 6 ppm higher in late spring (May) than in early autumn (September).
(a)Explain why the CO₂ concentration rises through autumn and winter and falls through summer each year.[2]
(b)Calculate the mean yearly increase in CO₂ from 2000 to 2020.[2]
(c)Give two human activities that cause the long-term increase.[2]
Show answer
(a) in summer more photosynthesis (more leaves, more light), removing more CO₂ than released; in winter less photosynthesis but respiration/combustion continues
B1 photosynthesis greater in summer; B1 respiration/combustion continue in winter
Q25Carbon cycle · Challenge · Paper 2 style · 4 marks
(a)Explain how cutting down and burning forest affects the amount of CO₂ in the atmosphere.[3]
(b)A company plants trees to 'offset' its emissions. Suggest one limitation of this.[1]
Show answer
(a) burning (combustion) releases CO₂; fewer trees so less photosynthesis removes CO₂; decomposition of remaining wood releases more CO₂
1 mark each for three
(b) trees take many years to grow / may be cut down or burned later / cannot absorb as much as is emitted
B1
Q26Nitrogen cycle · Core · Paper 2 style · 3 marks
(a)State why plants need nitrate ions.[1]
(b)Name the structures on the roots of pea and bean plants that contain nitrogen-fixing bacteria.[1]
(c)State one way, other than by bacteria, that atmospheric nitrogen is fixed.[1]
Show answer
(a) to make amino acids/proteins
B1
(b) root nodules
B1
(c) lightning (or the Haber process/making fertilisers)
B1
Q27Nitrogen cycle · Standard · Paper 1 style · 1 mark
(a)Which type of bacteria converts ammonium ions into nitrate ions?[1]
Show answer
(a) C nitrifying bacteria
Nitrifying bacteria oxidise ammonium to nitrite and then nitrate. Denitrifying bacteria turn nitrate back into nitrogen gas, so B is wrong.
Q28Nitrogen cycle · Challenge · Paper 2 style · 4 marks
(a)Explain why farmers in waterlogged rice paddies may lose nitrate from the soil.[2]
(b)Explain why ploughing to aerate soil helps to keep nitrate levels high.[2]
Show answer
(a) waterlogged soil lacks oxygen; denitrifying bacteria (anaerobic) convert nitrate to nitrogen gas
B1 anaerobic/no oxygen; B1 denitrifying bacteria convert nitrate to nitrogen
(b) nitrifying bacteria need oxygen to convert ammonium to nitrate; denitrifying bacteria less active in aerobic soil
B1 nitrifying bacteria aerobic/need oxygen; B1 less denitrification
Q29Nitrogen cycle (option) · Standard · Paper 2 style · 5 marks
(a)Describe how the nitrogen in protein of a dead buffalo can become part of a protein in a grass plant.[5]
Show answer
(a) decomposers (bacteria, fungi) break down protein in dead body; releasing ammonia/ammonium ions; nitrifying bacteria convert ammonium to nitrite; then to nitrate; nitrate absorbed by root hairs (active transport); used to make amino acids, then proteins
1 mark each for up to five of: decomposers; ammonium/ammonia; nitrifying bacteria; nitrite → nitrate; absorption by roots/active transport; amino acids → protein in plant
Q30Nitrogen cycle (option) · Challenge · Paper 2 style · 5 marks
(a)Explain why growing a crop of beans before a crop of wheat increases the wheat yield.[3]
(b)Suggest one advantage and one disadvantage of crop rotation compared with using nitrate fertiliser.[2]
Show answer
(a) bean roots have nodules with nitrogen-fixing bacteria; these convert nitrogen gas into ammonium compounds/amino acids; when bean remains decay, nitrate is added to the soil, used by wheat for protein and growth
B1 nodules with nitrogen-fixing bacteria; B1 nitrogen gas → nitrogen compounds; B1 more nitrate in soil after decay → more growth
(b) advantage: cheaper / less nitrate runoff and eutrophication; disadvantage: slower/less nitrate added, land used for a less valuable crop
B1 each
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
