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

These revision notes cover Topic 19, Relationships of organisms with one another and with the environment, for Cambridge O Level Biology 5090. They follow 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 all of it applies to every candidate.

For full explanations and worked examples, use the study guide. When you are ready, try the practice questions. The course hub and the printable checklist list every topic, and the free 10-minute diagnostics show where to focus.

19.1 Energy flow

Must-know facts

• The Sun is the principal source of energy input to most biological systems.
• Most life depends completely on photosynthesis: it brings light energy into food chains as chemical energy, and every food chain starts with a producer.
• Energy flows one way: light → producers → consumers → decomposers, and is eventually transferred to the environment as heat. It is not recycled.

Definitions

Term	Definition to learn
Producer	Organism that makes its own organic nutrients, usually by photosynthesis
Consumer	Organism that gets energy by feeding on other organisms
Herbivore	Animal that gets energy by eating plants
Carnivore	Animal that gets energy by eating other animals
Decomposer	Organism that gets energy from dead or waste organic material
Food web	A network of interconnected food chains
Trophic level	The position of an organism in a food chain

Method: reading a food web

1. Find the producer(s): no arrows point into them.
2. Arrows point from the eaten to the eater.
3. To predict the effect of removing a species, list what it eats (those may increase) and what eats it (those may decrease or switch prey).
4. Give a reason for each change: “fewer eaten” or “less food”.

Why energy transfer is inefficient

Energy is lost between trophic levels because:

• parts are not eaten (bones, roots, wood);
• parts are eaten but not digested (lost in faeces);
• energy is released in respiration and lost as heat;
• energy is used for movement;
• some is lost in excretory products.

Fewer than five trophic levels: so much energy is lost at each transfer that too little remains to support another population.

Crops versus livestock: eating crops removes one trophic level, so less energy is lost and more people can be fed from the same land.

Method: efficiency calculation

efficiency (%) = (energy in higher level ÷ energy in lower level) × 100

Worked reminder: wheat 1,500 kJ → mice 120 kJ. (120 ÷ 1,500) × 100 = 8%.

Pyramids: must-know distinctions

	Numbers	Biomass	Energy
Each bar shows	Number of organisms	Dry mass	Energy transferred over a set time
Can be inverted or odd-shaped?	Yes (one tree, many insects; many parasites on one host)	Rarely	Never
Main weakness	Ignores size of organisms	Snapshot at one time	Hardest data to collect

Producers always go at the bottom. Label each bar with the trophic level or organism.

19.2 Nutrient cycles

Carbon cycle: in steps

1. Photosynthesis removes carbon dioxide from the air; carbon becomes carbohydrate in plants.
2. Feeding passes carbon compounds to animals.
3. Respiration by plants, animals and decomposers returns carbon dioxide to the air.
4. Decomposition: decomposers respire dead remains and waste, releasing carbon dioxide.
5. Fossil fuel formation: dead organisms that do not decay are compressed over millions of years into coal, oil and gas.
6. Combustion of fossil fuels and wood releases carbon dioxide.

Nitrogen cycle: the seven syllabus stages

Stage	What happens
(a) Decomposition	Protein in dead plants and animals is broken down to ammonium ions
(b) Nitrification	Ammonium ions are converted to nitrate ions by nitrifying bacteria
(c) Nitrogen fixation	Lightning or nitrogen-fixing bacteria convert nitrogen gas to compounds plants can use
(d) Absorption	Plants absorb nitrate ions through their roots
(e) Amino acids and protein	Plants use the nitrogen to make amino acids, then proteins
(f) Feeding and digestion	Animals eat plants, digest protein to amino acids and make their own proteins
(g) Denitrification	Denitrifying bacteria convert nitrate ions to nitrogen gas

Names of individual bacteria are not required.

Decomposers

Fungi and bacteria secrete enzymes onto dead material, absorb the products and respire them. They return carbon dioxide to the air and ammonium ions to the soil.

19.3 Ecosystems and biodiversity

Term	Syllabus description
Population	A group of organisms of one species, living in the same area, at the same time
Community	All of the populations of different species in an ecosystem
Ecosystem	A unit containing the community of organisms and their environment, interacting together
Biodiversity	The number of different species that live in an area

Factors affecting the rate of population growth: food supply, competition, predation, disease. The growing human population is increasing the demand for global resources.

Must-know distinctions

• Energy flows; elements cycle. Carbon and nitrogen are reused; energy leaves as heat.
• Nitrogen fixation (nitrogen gas → compounds) vs nitrification (ammonium ions → nitrate ions) vs denitrification (nitrate ions → nitrogen gas).
• Population (one species) vs community (all species) vs ecosystem (community plus its environment).
• Biodiversity counts species, not individual organisms: a field of 10,000 wheat plants has very low biodiversity.

19.4 Effects of humans on ecosystems

Deforestation: five consequences

Loss of biodiversity; extinction of species; loss of soil (no roots to hold it); flooding (less water taken up, faster run-off); higher carbon dioxide concentration (less photosynthesis; burning releases carbon dioxide).

Over-harvesting and non-native species

• Over-harvesting: species removed faster than they reproduce; populations fall, may become extinct; food webs disrupted.
• Non-native species: no natural predators or diseases; spreads fast; out-competes or eats native species.

Eutrophication: the six-step chain

1. More nitrate and other ions (from untreated sewage or fertiliser).
2. More growth of producers.
3. More decomposition after producers die.
4. More aerobic respiration by decomposers.
5. Less dissolved oxygen.
6. Death of organisms that need dissolved oxygen.

Other pollution

• Greenhouse gases: carbon dioxide and methane trap heat, contributing to global warming; likely effects include rising sea levels, more extreme weather and changes in species distribution.
• Insecticides: kill non-target insects such as pollinators; harm animals that eat poisoned insects.
• Herbicides: kill non-target plants, removing food and habitats.
• Non-biodegradable plastics: not decomposed, so they persist; animals in water and on land are entangled or swallow them.

19.5 Conservation

• Why conserve species: maintain biodiversity; reduce extinction; protect vulnerable environments.
• Forests: education, protected areas, quotas, replanting.
• Fish stocks: education, closed seasons, protected areas, controlled net types and mesh size, quotas, monitoring.
• Sustainable resource: one produced as rapidly as it is removed from the environment, so that it does not run out.

Quick self-test

1. State the principal source of energy input to most biological systems.
2. In a food chain, which way do the arrows point?
3. Grass contains 9,000 kJ and the cattle eating it 900 kJ. Calculate the efficiency of transfer.
4. Give two reasons why energy transfer between trophic levels is inefficient.
5. Why can a pyramid of energy never be inverted?
6. Name the process that converts ammonium ions to nitrate ions.
7. Name two ways nitrogen gas is fixed.
8. Define a community.
9. State the four factors affecting the rate of population growth named in the syllabus.
10. In eutrophication, which organisms use up the dissolved oxygen?
11. Name two greenhouse gases.
12. Why does a mesh size limit help conserve fish stocks?

Answers

1. The Sun.
2. From the organism eaten to the organism that eats it, in the direction of energy flow.
3. (900 ÷ 9,000) × 100 = 10%.
4. Any two: not all eaten; not all digested (faeces); respiration releases energy lost as heat; energy used for movement; excretion.
5. Energy is lost at every transfer, so each level always has less energy than the one below.
6. Nitrification.
7. Lightning; nitrogen-fixing bacteria.
8. All of the populations of different species in an ecosystem.
9. Food supply, competition, predation, disease.
10. Decomposers (bacteria), by aerobic respiration.
11. Carbon dioxide and methane.
12. Young fish escape through the larger holes, survive and breed, so the stock is replaced.

Where marks are usually lost

• Arrows drawn from predator to prey, or a food chain that does not start with a producer.
• “Energy is used up” or “energy is recycled” instead of “lost to the environment as heat”.
• Efficiency calculations divided the wrong way round, or the “× 100” left out.
• Drawing a pyramid of numbers as always pyramid-shaped.
• Confusing nitrification, nitrogen fixation and denitrification.
• Eutrophication answers that skip the decomposers, or say algae “use up the oxygen”.
• Defining biodiversity as the number of organisms rather than of species.
• Deforestation answers that say “global warming” without linking to carbon dioxide concentration.
• Conservation methods listed without saying how each one works.

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.