Cambridge O Level Biology 5090: Relationships of organisms with one another and with the environment (5090) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover Topic 19, Relationships of organisms with one another and with the environment, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): every learning outcome in sections 19.1 to 19.5. The syllabus is not tiered, so every question applies to every candidate.

Revise first with the study guide and the revision notes. The course hub and the printable checklist list the other topics; the free 10-minute diagnostics show what to revise next.

Questions

1. Describe what is meant by each term.

(a) population [1] (b) community [1] (c) ecosystem [2]

2. In a pond, water fleas eat algae. Snails eat algae and pondweed. Dragonfly larvae eat water fleas. Small fish eat water fleas, snails and dragonfly larvae. Herons eat small fish.

(a) Name the producers in this food web. [1] (b) Write out a food chain from this web that contains five organisms. [1] (c) Name one herbivore in this web. [1] (d) Most dragonfly larvae die of a disease. Suggest and explain the effect on the water fleas and the small fish. [3] (e) Explain why herons depend on photosynthesis. [2]

3. The energy in each trophic level of a food chain was measured in kJ per m² per year: producers 30,000; primary consumers 3,600; secondary consumers 270; tertiary consumers 27.

(a) Calculate the efficiency of energy transfer from producers to primary consumers. [2] (b) Calculate the percentage of the producers’ energy that reaches the tertiary consumers. [2] (c) Explain why the transfer of energy from one trophic level to the next is inefficient. [3] (d) Use the data to explain why a fifth trophic level is unlikely. [2]

4. One apple tree (dry mass 800 kg) supports 40,000 aphids, each of dry mass 0.5 g. The aphids are eaten by 600 ladybirds, each of dry mass 0.04 g.

(a) Calculate the total dry biomass, in kg, of the aphids and of the ladybirds. [2] (b) Describe the shape of the pyramid of numbers for this food chain and explain why the pyramid of biomass has a different shape. [3]

5. A field of maize contains 6,000,000 kJ of energy per year. Assume that 10% of the energy is passed on at each transfer.

(a) Calculate the energy available to humans who eat the maize. [1] (b) Calculate the energy available to humans who eat chickens that were fed on the maize. [1] (c) Explain why eating crop plants is more energy efficient than eating livestock fed on them. [2]

6. This question is about the carbon cycle.

(a) Describe how the carbon in a fallen tree is returned to the atmosphere. [2] (b) Describe how carbon in organisms that lived millions of years ago is returned to the atmosphere today. [3]

7. This question is about the nitrogen cycle.

(a) Name the process in which ammonium ions are converted to nitrate ions. [1] (b) State two ways in which nitrogen gas is fixed. [2] (c) Describe how nitrogen in the protein of a dead plant becomes available to a living plant. [3] (d) Name the process that returns nitrogen gas to the air. [1]

8. Dissolved oxygen was measured along a river near a pipe releasing untreated sewage.

Distance from pipe / m (− = upstream)	−50	100	200	400	800
Dissolved oxygen / mg per dm³	9.0	6.3	2.7	4.5	8.1

(a) Calculate the percentage decrease in dissolved oxygen from −50 m to 200 m. [2] (b) Explain why the dissolved oxygen is low at 200 m. [5] (c) Suggest the effect on fish at 200 m. [1]

9. Large areas of a tropical forest are being cleared.

(a) Describe the consequences of this deforestation. [5] (b) Explain how quotas and replanting can help to conserve forests. [2] (c) Give two reasons for conserving species that live only in this forest. [2]

10. This question is about pollution.

(a) Name two greenhouse gases. [2] (b) Describe two likely effects of global warming. [2] (c) Explain how a non-biodegradable plastic bag in the sea can harm a turtle. [2]

11. Goats were introduced to an island where they had no predators.

(a) Suggest why the number of native plant species on the island fell. [3] (b) A farmer on the island sprays an insecticide on a crop. Explain how this could reduce the yield of a nearby fruit orchard. [2]

12. A fish population has a mass of 50,000 tonnes. Reproduction and growth add 8,000 tonnes each year.

(a) Calculate the mass of the population after one year if 11,000 tonnes are caught. [1] (b) Explain whether a catch of 11,000 tonnes a year is sustainable. [2] (c) Explain how three methods could help to conserve this fish stock. [3] (d) Other than fishing, state two factors that affect the rate of growth of the fish population. [2] (e) Suggest why the growth of the human population makes over-fishing more likely. [1]

Answers

1. (a) A group of organisms of one species, living in the same area, at the same time [1] (b) All of the populations of different species in an ecosystem [1] (c) A unit containing the community of organisms and their environment [1], interacting together [1] [4] Examiner insight: A population needs one species, same area and same time; “animals living together” is too vague.

2. (a) Algae and pondweed [1] (b) algae → water fleas → dragonfly larvae → small fish → herons [1] (c) Water fleas or snails [1] (d) Water fleas increase [1] because fewer are eaten [1]. Small fish may decrease (less food), or stay the same by eating more water fleas and snails [1]. (e) The fish they eat fed on consumers of algae or pondweed [1]; that energy entered the chain by photosynthesis in the producers [1]. [8] Examiner insight: In (d) each predicted change needs its own reason.

3. (a) (3,600 ÷ 30,000) × 100 [1] = 12% [1] (b) (27 ÷ 30,000) × 100 [1] = 0.09% [1] (c) Any three [3]: not all of the organism is eaten; some is not digested (faeces); energy released in respiration is lost as heat; energy is used for movement; energy is lost in excretory products. (d) Only 27 kJ per m² per year reaches the tertiary consumers [1]; after another transfer too little would remain to support a population [1]. [9] Examiner insight: The question asks for a percentage, so give 12%, not the decimal 0.12.

4. (a) Aphids: 40,000 × 0.5 g = 20,000 g = 20 kg [1]; ladybirds: 600 × 0.04 g = 24 g = 0.024 kg [1] (b) The pyramid of numbers has a narrow base (one tree), a very wide middle (aphids) and a narrower top (ladybirds) [1]. The pyramid of biomass is a true pyramid: 800 kg, 20 kg, 0.024 kg [1], because biomass allows for size: one tree is heavier than all the aphids [1]. [5] Examiner insight: The question asks for kg; an answer in grams loses the mark.

5. (a) 6,000,000 × 0.1 = 600,000 kJ [1] (b) 6,000,000 × 0.1 × 0.1 = 60,000 kJ [1] (c) Eating livestock adds an extra trophic level [1], and energy is lost at every transfer, so less energy reaches humans [1]. [4] Examiner insight: The explanation must be about energy lost between trophic levels; points about cost or health gain nothing.

6. (a) Decomposers (fungi and bacteria) feed on the dead tree [1] and respire, releasing carbon dioxide [1]. (b) The dead organisms did not decay and were compressed over millions of years into fossil fuels [1]. These are burned (combustion) [1], releasing carbon dioxide [1]. [5] Examiner insight: In (a) name respiration by decomposers; “the tree rots” does not explain how carbon dioxide is released.

7. (a) Nitrification [1] (b) Lightning [1]; nitrogen-fixing bacteria [1] (c) Decomposers break down the protein to ammonium ions [1]; nitrifying bacteria convert ammonium ions to nitrate ions [1]; the living plant absorbs the nitrate ions through its roots [1]. (d) Denitrification [1] [7] Examiner insight: Spell process names exactly; “nitrogenation” or a mix of “nitrification” and “fixation” is not credited.

8. (a) (9.0 − 2.7) ÷ 9.0 × 100 [1] = 70% [1] (b) The sewage increases the availability of nitrate and other ions [1]. Producers such as algae grow more [1]. When they die, there is more decomposition [1] and more aerobic respiration by decomposers (bacteria) [1]. This reduces the dissolved oxygen [1]. (c) Fish die (or move away) because they need dissolved oxygen [1]. [8] Examiner insight: Each step in the sequence is a separate point; leaving out the decomposers’ aerobic respiration loses that mark.

9. (a) Any five [5]: habitats lost, so biodiversity falls; species found only there become extinct; no roots to hold soil, so soil is lost; less water taken up, so flooding; less photosynthesis; burning trees releases carbon dioxide, raising its atmospheric concentration. (b) Quotas limit how many trees are felled [1]; replanting replaces trees that are felled [1]. (c) Any two [2]: to maintain biodiversity; to reduce extinction; to protect a vulnerable environment. [9] Examiner insight: In (a), “causes global warming” is too vague; link it to carbon dioxide concentration.

10. (a) Carbon dioxide [1]; methane [1] (b) Any two [2]: melting ice and rising sea levels; more extreme weather; changes to where species can live; extinction of species unable to adapt. (c) The bag is not broken down by decomposers, so it persists [1]; the turtle swallows it and starves, or becomes entangled and drowns [1]. [6] Examiner insight: In (a) give carbon dioxide and methane, the two gases the syllabus names.

11. (a) With no predators the goat population grew quickly [1]. Goats ate native plants faster than they could regrow [1]. Some plant species died out, so biodiversity fell [1]. (b) Insecticide kills non-target insects such as pollinators [1], so fewer fruit flowers are pollinated and fewer fruits form [1]. [5] Examiner insight: Apply the answer to the orchard; “harms the environment” does not explain the lower yield.

12. (a) 50,000 + 8,000 − 11,000 = 47,000 tonnes [1] (b) Not sustainable: fish are removed faster than they are produced (11,000 > 8,000 tonnes) [1], so the stock falls and will run out [1]. (c) Any three, each with how it works [3]: closed seasons let fish breed; larger mesh lets young fish escape; quotas limit the catch; protected areas ban fishing; monitoring shows stock sizes so quotas can be set; education about sustainable fishing. (d) Any two [2]: food supply; competition; predation; disease. (e) More people means a greater demand for food, so more fish are caught [1]. [9] Examiner insight: In (b) compare the two figures using the definition of a sustainable resource.

Where marks are usually lost

• Food chain arrows pointing from predator to prey.
• Efficiency calculated the wrong way up (30,000 ÷ 3,600), or left as a decimal.
• Energy described as “used up” or “recycled” rather than lost as heat.
• Confusing nitrification with nitrogen fixation.
• Leaving decomposers out of eutrophication and the carbon cycle.
• Biodiversity defined as the number of organisms, not species.

Next steps

• Revision notes for this topic
• Study guide for this topic
• Cambridge O Level Biology course hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 19: Relationships of organisms with one another and with the environment, published by Cambridge Assessment International Education.