Cambridge IGCSE Biology 0610: Organisms and their environment – Revision Notes

These notes condense topic 19, Organisms and their environment, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (sections 19.1 to 19.4) for your final weeks of revision. Supplement content is marked Extended only; Core candidates (Papers 1 and 3) can skip it, while Extended candidates (Papers 2 and 4) need all of it. For full explanations and worked examples, use the Organisms and their environment study guide.

Also useful: the IGCSE Biology hub, the 0610 checklist, the practice questions for this topic, and the free Core and Extended diagnostics.

19.1 Energy flow (Core)

• The Sun is the principal source of energy input to biological systems.
• Route: light energy → (photosynthesis in producers) → chemical energy in organic compounds → passed along by feeding → eventually transferred to the environment as heat (mainly from respiration).
• Energy flows through an ecosystem in one direction. It is not recycled.

19.2 Food chains and food webs

Definitions (Core) – learn the wording

Term	Definition
Food chain	Shows the transfer of energy from one organism to the next, beginning with a producer
Food web	A network of interconnected food chains
Producer	Makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
Consumer	Gets its energy by feeding on other organisms
Herbivore	An animal that gets its energy by eating plants
Carnivore	An animal that gets its energy by eating other animals
Decomposer	Gets its energy from dead or waste organic material
Trophic level	The position of an organism in a food chain, food web or ecological pyramid

Trophic levels in order: producers → primary consumers → secondary consumers → tertiary consumers → quaternary consumers.

Method: answering a food-web question

1. Find the organism that changes. Note what it eats and what eats it.
2. Its prey usually increases (less eaten). Its predators usually decrease (less food).
3. Check for knock-on effects: a predator that loses one food source may eat more of another.
4. For every change you state, give the reason.

Human impact (Core)

• Overharvesting a food species: its predators lose food and may decline; its prey may increase.
• Introduced foreign species: may have no natural predators, may prey on natives, or may outcompete natives for food. Native populations can fall.

Ecological pyramids

Pyramid	Shows	Always pyramid-shaped?	Tier
Numbers	Number of organisms at each level	No – can be inverted (one tree, many insects)	Core
Biomass	Dry mass of living material at each level	Almost always	Core
Energy	Energy transferred to each level over time (e.g. kJ m⁻² yr⁻¹)	Yes	Extended only

• Biomass better than numbers (Core): accounts for the size of organisms; gives a truer picture of the amount of living material each level holds.
• Energy better than both (Extended only): shows energy transferred over a period of time, not at one moment; never inverted; lets you compare transfer efficiencies.

Energy transfer (Extended only)

Formula	Use
efficiency (%) = (energy at higher level ÷ energy at lower level) × 100	Efficiency of one transfer
energy lost = energy at lower level − energy at higher level	Energy not passed on

Why transfer is inefficient: heat from respiration; parts not eaten; parts not digested (faeces); excretion (urea); energy used in movement, eventually lost as heat.

Why fewer than five trophic levels: energy is lost at every transfer, so too little is left at the top to support a viable population.

Crops versus livestock: eating crops means one transfer; eating livestock fed on crops means two, with energy lost by the animals as heat, in movement and in faeces. More people can be fed from the same area if crops are eaten directly.

Small worked reminders

Food-web change (Core). Web: phytoplankton → krill → penguins; krill → whales; penguins → seals. If whales are overharvested, krill are eaten less and increase; penguins then have more food and may increase; seals may increase because there are more penguins to eat. State each change with its reason.

Inverted pyramid (Core). One rose bush supports 600 aphids, which support 15 ladybirds. The pyramid of numbers has a narrow base, a wide middle and a narrow top. The pyramid of biomass is a normal pyramid, because the bush is far heavier than all the aphids together.

Efficiency (Extended only). Always divide the higher trophic level by the lower one. If your answer is over 100 %, you have divided the wrong way round.

19.3 Nutrient cycles

Carbon cycle (Core) – six processes only

Process	Carbon moves from → to
Photosynthesis	CO₂ in air → carbohydrate in producers
Feeding	Producers → consumers → further consumers
Respiration	All organisms → CO₂ in air
Decomposition	Dead organisms and waste → decomposers → CO₂ in air (by respiration)
Formation of fossil fuels	Dead organisms not fully decomposed → coal, oil, gas (over millions of years)
Combustion	Fossil fuels and wood → CO₂ in air

Nitrogen cycle (Extended only)

Stage	Change	Who or what does it
Decomposition	Protein in dead organisms and waste → ammonium ions	Decomposers (bacteria and fungi)
Nitrification	Ammonium ions → nitrate ions	Nitrifying bacteria
Nitrogen fixation	Nitrogen gas → nitrogen compounds	Lightning; nitrogen-fixing bacteria (in soil and root nodules)
Absorption	Nitrate ions from soil → plant	Plant roots
Amino acids and proteins	Nitrate → amino acids → proteins	Plants
Feeding and digestion	Plant protein → amino acids → animal protein	Animals
Deamination	Excess amino acids → urea	Liver
Denitrification	Nitrate ions → nitrogen gas	Denitrifying bacteria

Microorganism roles you must state: decomposition, nitrification, nitrogen fixation, denitrification. Names of individual bacteria are not required.

19.4 Populations

Definitions (Core)

• 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.

Factors affecting growth rate (Core)

Limited to: food supply, competition, predation, disease.

Sigmoid curve

Phase	Shape	Why (Extended only)
Lag	Slow rise	Few individuals breeding; adjusting to conditions
Exponential (log)	Steep rise	Resources plentiful; births far exceed deaths
Stationary	Level	Limiting factors (e.g. food, space) act; births = deaths
Death	Fall	Food exhausted or toxic waste builds up; deaths exceed births

Method: calculating a growth rate from a graph

1. Read two values from the curve, with their times.
2. Rate = (later number − earlier number) ÷ (later time − earlier time).
3. Give units: individuals per day, per hour, etc.

Must-know distinctions

• Energy flows and is lost as heat; carbon and nitrogen are recycled.
• Nitrification (ammonium → nitrate) versus nitrogen fixation (N₂ → compounds) versus denitrification (nitrate → N₂).
• Community (living organisms only) versus ecosystem (community plus environment).
• Pyramid of numbers can be inverted; pyramid of energy never is.
• Consumer is a feeding role; carnivore and herbivore describe diet.

Quick self-test

1. State the principal source of energy input to biological systems.
2. In the chain seaweed → limpet → crab → gull, name the secondary consumer.
3. Define a decomposer.
4. Why can a pyramid of numbers be inverted?
5. Name the carbon-cycle process that releases carbon dioxide when coal or wood is burned.
6. Extended: rabbits receive 4 500 kJ m⁻² yr⁻¹ and stoats gain 360 kJ m⁻² yr⁻¹. Calculate the efficiency of transfer.
7. Extended: name the process that converts ammonium ions to nitrate ions.
8. Extended: state two ways nitrogen gas can be fixed.
9. Define a community.
10. A population rises from 40 to 400 in 6 weeks. Calculate the mean growth rate.
11. Name the phase of the sigmoid curve in which births equal deaths.
12. Extended: give two reasons why energy transfer between trophic levels is inefficient.

Answers

1. The Sun.
2. Crab.
3. An organism that gets its energy from dead or waste organic material.
4. One large organism (such as a tree) can support many small ones, and the pyramid shows numbers, not size.
5. Combustion.
6. (360 ÷ 4 500) × 100 = 8.0 %.
7. Nitrification (by nitrifying bacteria).
8. Lightning; nitrogen-fixing bacteria.
9. All of the populations of different species in an ecosystem.
10. (400 − 40) ÷ 6 = 60 per week.
11. Stationary phase.
12. Any two: heat lost in respiration; parts not eaten; parts not digested (faeces); excretion; energy used in movement.

Where marks are usually lost

• Arrows in food chains drawn the wrong way. They point from the food to the feeder.
• Writing “energy is used up” or “energy is recycled” instead of “transferred to the environment as heat”.
• Listing processes in the carbon cycle without saying whether carbon dioxide is added to or removed from the air.
• Calling nitrogen fixation “turning nitrogen into nitrate” without mentioning nitrogen gas, or mixing it up with nitrification.
• Naming “bacteria” in the nitrogen cycle without saying which type: nitrifying, nitrogen-fixing, denitrifying or decomposers.
• Describing the stationary phase as “no growth” without stating births equal deaths or naming a limiting factor (Extended).
• Giving a percentage efficiency with the numbers inverted (lower level ÷ higher level), producing a value above 100 %.
• Leaving units off a population growth rate.
• For crops versus livestock, saying “cows waste energy” instead of naming the extra trophic level and where the energy goes.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge University Press & Assessment (Cambridge International Education). Topic 19, sections 19.1 to 19.4.