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

This study guide teaches Topic 19, Relationships of organisms with one another and with the environment, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 19.1 Energy flow, 19.2 Nutrient cycles, 19.3 Ecosystems and biodiversity, 19.4 Effects of humans on ecosystems and 19.5 Conservation. The 5090 syllabus is not tiered, so every candidate studies all of it.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic, and the printable checklist lets you tick off each outcome. This topic builds on Plant nutrition and Respiration. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
19.1.1–19.1.3	Sun as main energy source; dependence on photosynthesis; energy flow through chains and webs
19.1.4–19.1.6	Construct and interpret food chains and webs; producer, consumer, herbivore, carnivore, decomposer
19.1.7–19.1.9	Why energy transfer is inefficient; why chains are short; why eating crops is more efficient
19.1.10	Construct and interpret pyramids of numbers, biomass and energy
19.2.1–19.2.3	Carbon cycle; nitrogen cycle; fungi and bacteria in decomposition
19.3.1–19.3.6	Population, community, ecosystem, biodiversity; factors in population growth; human demand for resources
19.4.1–19.4.3	Deforestation; over-harvesting; non-native species; eutrophication; greenhouse gases; insecticides and herbicides; plastics
19.5.1–19.5.4	Reasons for conservation; conserving forests and fish stocks; sustainable resources

19.1 Energy flow

Where the energy comes from (19.1.1–19.1.3)

The Sun is the principal source of energy input to most biological systems. Plants trap light energy in photosynthesis and store it as chemical energy in organic compounds.

Most forms of life depend completely on photosynthesis because:

• it is how light energy enters food chains; animals and fungi cannot use light and must eat organic food, so chains start with a photosynthesising producer;
• photosynthesis releases the oxygen most organisms use in aerobic respiration.

Energy flows in one direction: light → chemical energy in producers → chemical energy in consumers and decomposers. At every stage some energy is released by respiration and lost to the environment as heat. Energy is not recycled. Chemical elements, such as carbon and nitrogen, are.

Key terms (19.1.5)

Term	Meaning
Producer	Makes its own organic nutrients, usually by photosynthesis
Consumer	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 chains and food webs (19.1.4, 19.1.6)

A food chain shows the transfer of energy from one organism to the next. Each arrow points in the direction energy flows, from the organism eaten to the organism that eats it:

grass → grasshopper → frog → snake

Each feeding level is a trophic level: grass is the producer, the grasshopper the primary consumer (herbivore), the frog and snake secondary and tertiary consumers (carnivores).

A food web is a network of interconnected food chains. Most animals eat more than one kind of food, so a web is more realistic than a single chain. To interpret a web, follow every arrow into and out of the organism named. If frogs died out, grasshoppers might increase (fewer eaten) and snakes might decrease, or eat more of another prey species.

Why energy transfer is inefficient (19.1.7–19.1.8)

Only a small fraction of the energy in one trophic level becomes part of the next. Energy is lost because:

• not all of the organism is eaten (roots, bones, fur);
• not all that is eaten is digested and absorbed; some leaves as faeces;
• energy is released by respiration and lost as heat;
• energy goes to movement and other life processes, not to growth;
• some energy stays in excretory products, such as urea.

So food chains usually have fewer than five trophic levels: by the fifth level there is too little energy left to support a population.

Worked example 1. A grassland receives 20,000 kJ per m² per year in grass, 2,000 kJ in rabbits and 180 kJ in foxes. Calculate the efficiency of each transfer.

efficiency = (energy in next level ÷ energy in level below) × 100
grass → rabbits: (2,000 ÷ 20,000) × 100 = 10%
rabbits → foxes: (180 ÷ 2,000) × 100 = 9%
energy not passed to rabbits = 20,000 − 2,000 = 18,000 kJ per m² per year

Eating crops versus livestock (19.1.9)

People eating crops are primary consumers. People eating livestock fed on crops are secondary consumers, so there is one extra transfer and more energy is lost.

Worked example 2. A crop contains 100,000 kJ. Assume 10% is passed on at each transfer.

humans eat the crop:            100,000 × 0.10 = 10,000 kJ
cattle eat crop, humans eat cattle: 100,000 × 0.10 × 0.10 = 1,000 kJ

Eating the crop gives humans ten times more energy, so the same land feeds more people.

Pyramids of numbers, biomass and energy (19.1.10)

Draw horizontal bars, producer at the bottom, widths in proportion to the values.

Pyramid	Each bar shows	Can it be an odd shape?
Numbers	Number of organisms at each level	Yes: one large tree supports thousands of insects
Biomass	Dry mass of organisms at each level	Usually pyramid-shaped
Energy	Energy at each level over a set time	Never inverted: energy is lost at every transfer

Worked example 3. One oak tree (2,000 kg) supports 5,000 caterpillars (0.004 kg each), which feed 30 birds (0.02 kg each).

numbers: 1 → 5,000 → 30        (narrow base, wide middle: not a pyramid)
biomass: 2,000 kg → 5,000 × 0.004 = 20 kg → 30 × 0.02 = 0.6 kg   (a true pyramid)

The pyramid of numbers ignores size; the pyramid of biomass allows for it.

19.2 Nutrient cycles

The carbon cycle (19.2.1)

Process	Effect on atmospheric carbon dioxide
Photosynthesis	Removes it; carbon becomes carbohydrates in plants
Feeding	Passes carbon compounds along food chains
Respiration (plants, animals, decomposers)	Returns it to the air
Decomposition	Decomposers respire dead organic matter, releasing it
Formation of fossil fuels	Dead organisms that did not decay are compressed over millions of years into coal, oil and gas, locking carbon away
Combustion	Burning fossil fuels or wood releases it

The nitrogen cycle (19.2.2)

Plants and animals need nitrogen to make amino acids and proteins, but cannot use nitrogen gas directly. The stages:

1. Decomposition: bacteria and fungi break down protein in dead plants, animals and waste to ammonium ions.
2. Nitrification: nitrifying bacteria convert ammonium ions to nitrate ions.
3. Nitrogen fixation: lightning, or nitrogen-fixing bacteria (in soil and in root nodules), convert nitrogen gas into compounds plants can use.
4. Absorption: plants take up nitrate ions through root hair cells by active transport.
5. Production of amino acids and protein: plants combine nitrogen with carbohydrates to make amino acids, then proteins.
6. Feeding and digestion: animals eat plants, digest the protein to amino acids, and build their own proteins.
7. Denitrification: denitrifying bacteria convert nitrate ions back to nitrogen gas.

You do not need the names of individual bacteria.

Decomposition (19.2.3)

Fungi and bacteria are decomposers. They secrete enzymes onto dead organic matter, absorb the digested products and respire them. This returns carbon dioxide to the air and ammonium ions to the soil, recycling elements for producers.

19.3 Ecosystems and biodiversity

• 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.

The rate of population growth depends on food supply, competition, predation and disease. More food speeds growth; more competitors, predators or disease slow it, or make the population fall.

The growth of the human population is increasing the demand for global resources: food, water, land, fuel, timber and minerals. This pressure drives many of the impacts in 19.4.

19.4 Effects of humans on ecosystems

Deforestation (19.4.1)

Forests are cleared for farmland, timber and building. Consequences:

• Biodiversity: habitats and food sources are destroyed, so fewer species live there.
• Extinction: species found only in that forest may die out.
• Loss of soil: without roots to hold it, rain washes soil away.
• Flooding: fewer trees take up and hold back rain, so water runs off quickly, and eroded soil silts up rivers.
• Carbon dioxide: fewer trees photosynthesise, and burning or decomposing trees release carbon dioxide, so its atmospheric concentration rises.

Over-harvesting and non-native species (19.4.2)

Over-harvesting removes plants or animals faster than they can reproduce, so populations fall and may become extinct. Predators that relied on them lose food. Introducing a non-native species can harm an ecosystem when it has no natural predators or diseases there: it spreads quickly and out-competes or eats native species, reducing biodiversity.

Eutrophication (19.4.3a)

Untreated sewage and nitrogen-containing fertilisers washed into water cause this sequence:

1. Increased availability of nitrate and other ions.
2. Increased growth of producers (algae and plants).
3. Increased decomposition after the producers die.
4. Increased aerobic respiration by decomposers.
5. Reduction in dissolved oxygen.
6. Death of organisms requiring dissolved oxygen, such as fish.

Greenhouse gases, pesticides and plastics (19.4.3b–d)

Carbon dioxide (from combustion and deforestation) and methane (from cattle, rice fields and landfill sites) are greenhouse gases. They trap heat in the atmosphere and contribute to global warming. Likely effects include melting ice and rising sea levels, more extreme weather, changes to where species can live, and extinction of species that cannot adapt.

Insecticides can kill useful insects, such as pollinators, and harm animals that eat poisoned insects. Herbicides kill non-target plants, removing food and habitats. Both can wash into rivers.

Non-biodegradable plastics are not broken down by decomposers, so they persist. In water and on land, animals become entangled in plastic, or swallow it and starve or choke.

19.5 Conservation

Reasons to conserve species (19.5.1): to maintain biodiversity, reduce extinction and protect vulnerable environments. Each species sits in food webs, so losing one affects others.

Forests (19.5.2)	Fish stocks (19.5.3)
Education about the value of forests	Education about sustainable fishing
Protected areas where felling is banned	Closed seasons during breeding
Quotas on how much timber is felled	Protected areas where fishing is banned
Replanting felled trees	Controlled net types and mesh size so young fish escape
	Quotas on the catch
	Monitoring of stock sizes

A sustainable resource (19.5.4) is one which is produced as rapidly as it is removed from the environment, so that it does not run out.

Common errors

• Drawing food chain arrows from predator to prey. The arrow means “is eaten by”.
• Saying energy is “recycled” or that it is “used up”. Energy is transferred and lost as heat.
• Mixing up nitrification (ammonium to nitrate) and nitrogen fixation (nitrogen gas to compounds).
• Writing that algae use up the oxygen in eutrophication. It is the decomposers’ aerobic respiration.
• Defining biodiversity as “the number of organisms” instead of the number of species.

Next steps

Test yourself with the revision notes, then the practice questions. Fungi and bacteria as kingdoms are in Classification.

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.