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Cambridge IGCSE Biology · 0610

Chapter 19: Organisms and their environment (Part 1)

Energy flow

Source
The Sun is the principal source of energy input to biological systems.
Flow
Energy is introduced as light energy, which plants absorb and transfer into chemical energy. This energy then passes to other organisms through feeding and is eventually transferred to the environment.

Exam Traps

  • Do not say energy is recycled like nutrients — energy flows through and is lost as heat.

Food chains and webs

Food chain
A diagram showing the transfer of energy from one organism to the next, beginning with a producer.
Food web
A network of interconnected food chains illustrating how different chains interact.
Producer
An organism that makes its own organic nutrients, usually using sunlight through photosynthesis.
Consumer
An organism that gets its energy by feeding on other plants or animals.
Herbivore
An animal that gets energy by eating plants.
Carnivore
An animal that gets energy by eating other animals.
Decomposer
An organism that gets energy from dead or waste organic material.
Trophic levels
Refers to the position of an organism in a food chain, web, or pyramid. These are classed as producers, primary consumers, secondary consumers, tertiary consumers, and quaternary consumers.
A simple food chain from grass to grasshopper, frog, and owl with arrows showing energy transfer
Diagram 1: A simple food chain. A producer (e.g., grass) on the left, with arrows pointing to a primary consumer (e.g., grasshopper), then to a secondary consumer (e.g., frog), and a tertiary consumer (e.g., owl) on the right.

Exam Traps

  • Do not draw arrows pointing from predator to prey — arrows show energy flow to the eater.

Efficiency of energy transfer

Inefficiency
Energy transfer between trophic levels is often inefficient because:
  • Not all material is digestible (e.g., fur and bones).
  • Energy is lost through excretion and decay.
  • Energy is used for movement, respiration, and keeping warm (released as heat).
Trophic level limits
Due to this energy loss, food chains usually have fewer than five trophic levels.
Human diet
It is more energy efficient for humans to eat crop plants directly than to eat livestock that have been fed on those plants, as there are fewer stages for energy loss.

Exam Traps

  • Do not say 100% of energy passes to the next trophic level — typically about 10% is transferred.
  • Avoid claiming decomposers return energy to producers unchanged.

Pyramids of numbers, biomass, and energy

Pyramid of numbers
Shows the number of organisms at each trophic level. Numbers typically decline as you move up due to energy loss.
Pyramid of biomass
Measures the total mass of living material (biomass) at each level. These typically have a true pyramid shape as biomass is lost at each level.
Pyramid of energy
Shows the amount of energy within the biomass at each level. These provide the most accurate representation of energy flow and efficiency.
Ecological pyramids of numbers, biomass, and energy with trophic levels labelled from producers upward
Diagram 2: Ecological pyramids. Three pyramids (numbers, biomass, and energy) for the same food chain. Each bar is labelled with the corresponding trophic level, starting with a wide base for producers.

Exam Traps

  • Do not assume all pyramids of numbers are pyramid-shaped — they can be inverted.
  • Avoid confusing biomass (mass of living material) with number of organisms.

Human impact on food webs

Overharvesting
Excessive removal of a species damages food chains, as predators may not have enough food to survive.
Foreign species
Introducing non-native species creates competition for resources. For example, the cane toad in Australia has destroyed native species and reduced biodiversity because it has toxic skin and few natural predators.

Exam Traps

  • Do not say introducing foreign species always increases biodiversity — it usually reduces it.
  • Avoid confusing overharvesting with sustainable resource management.

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