19.1Organisms and their Environment

Energy & Feeding Relationships — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

5 min read 12 parts Lesson 4 of 7 2 quizzes

📚 Cambridge IGCSE Biology 0610 Hub / Topical Revision

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Lesson summary

19.1 Energy and Feeding Relationships — IGCSE BIOLOGY notes: Describe how energy flows through food chains and webs, how pyramids represent it, how nutrients are cycled and how populations grow.The Sun — it is the principal source of energy input to biological systems.Light energ…

Energy flows through living organisms
Chemical elements
Removing or reducing one species
Effects are long-lasting
Pyramid of numbers
Pyramid of biomass

Part 1 of 10Energy input and flow

1The Sun

The Sun it is the principal source of energy input to biological systems.
Light energy is converted to chemical energy in organisms by photosynthesis.

2Energy flows through living organisms

Energy flows through living organisms it passes along food chains and is not recycled.
Energy is eventually transferred to the environment, for example as heat.

3Chemical elements

Chemical elements unlike energy, elements such as carbon and nitrogen are recycled.

Part 2 of 10Food chains and food webs

Food chain

Food chain a diagram showing the transfer of energy from one organism to the next, starting with a producer.

The arrows show the transfer of energy by ingestion (eating) from one trophic level to the next.
The source of all energy in a food chain is light energy from the Sun.

Food web

Food web a network of interconnected food chains.

Food webs are more realistic because animals rarely depend on one food source.
Food webs show interdependence, so a change in one population affects others.

Term	Meaning
Producer	Makes its own organic nutrients, usually by photosynthesis using light energy
Consumer	Gets energy by feeding on other organisms (primary, secondary, tertiary, quaternary)
Herbivore	Gets energy by eating plants
Carnivore	Gets energy by eating other animals
Decomposer	Gets energy from dead or waste organic material

Part 3 of 10Interdependence in food webs

Interdependence in food webs

Removing or reducing one species

Removing or reducing one species it changes the populations that feed on it or are eaten by it.
A species with only one food source decreases strongly if that food decreases.
A species with an alternative food source decreases only slightly.
The organisms that the reduced species used to eat usually increase.

Causes

Causes most changes are due to human impact.
Overharvesting of food species can reduce a population.
Introduction of foreign species to a habitat can disrupt the web.

Effects are long-lasting

Effects are long-lasting knock-on changes pass through the whole food chain or web.

Part 4 of 10Pyramids of number and biomass

Pyramid of numbers

Pyramid of numbers a diagram where the width of each bar shows the number of organisms at that trophic level.

It is not always pyramid-shaped because one large organism, such as an oak tree, can support many smaller organisms.
Organisms stay in food-chain order, with producers at the bottom, then primary, secondary and tertiary consumers.
Each bar is labelled with its trophic level and drawn to scale if graph paper is given.

Pyramid of biomass

Pyramid of biomass a diagram showing the mass of living matter (usually dry mass) at each trophic level.

It is always pyramid-shaped because mass decreases up a food chain.
It is more accurate than numbers because it takes the size of organisms into account.
Energy is stored in biomass, so biomass approximates the energy available at each level.

Part 5 of 10Energy loss and pyramids of energy (Extended)

Energy is lost between trophic levels

Energy is lost between trophic levels not all stored energy eaten by an organism becomes new tissue that can be eaten.
Energy is transferred to the environment by excretion of metabolic waste such as urine.
Energy is lost through movement and as heat.
Energy in uneaten parts (roots, bones) never enters the consumer.
Undigested waste (faeces) is removed and feeds decomposers.

Only about 10% of the energy stored at one trophic level is available to the next.

Food chains are rarely more than five organisms long

Food chains are rarely more than five organisms long too little energy remains to support the final consumer.

Pyramid of energy

Pyramid of energy a diagram of the energy stored at each trophic level.

It highlights the energy lost at each level.
It allows accurate comparison because different organisms store different amounts of energy per unit of biomass.

Longer chains lose more energy

Longer chains lose more energy the more trophic levels, the greater the energy loss between producer and final consumer.
In wheat → cow → human, more energy is lost than in wheat → human.
It is therefore more energy efficient for humans to eat plants than animals.

Real situations are more complex

Real situations are more complex animals are often fed on plants that humans cannot eat,
- such as grass,
- or that are too widely spread to collect,
- such as ocean algae.

Part 6 of 10Carbon cycle

1Photosynthesis

Photosynthesis plants remove carbon dioxide from the atmosphere to make glucose.

2Feeding

Feeding carbon passes to animals and microorganisms when they eat.

3Respiration

Respiration plants, animals and microorganisms return carbon dioxide to the atmosphere.

4Fossil fuels

Fossil fuels carbon in organisms that die where decomposers are absent is converted over millions of years under pressure into fossil fuels.
Combustion combines the carbon with oxygen and releases carbon dioxide.

5Human impact

Human impact increased fossil fuel use and mass deforestation raise atmospheric carbon dioxide.
Forests are often burnt to clear land, which releases even more carbon dioxide.

Part 7 of 10Water cycle

1Evaporation and transpiration

Evaporation and transpiration liquid water becomes vapour from surfaces and plant leaves.

2Condensation and precipitation

Condensation and precipitation vapour forms clouds and returns to Earth as rain.

3Run-off and infiltration

Run-off and infiltration water flows over or into the ground back to rivers, lakes and seas.

Part 8 of 10Nitrogen cycle (Extended)

1Nitrogen is needed for proteins

Nitrogen is needed for proteins plants and animals cannot absorb nitrogen gas because it is very stable.

2Nitrogen fixation

Nitrogen fixation conversion of nitrogen gas into ammonium compounds by nitrogen-fixing bacteria.

Nitrogen-fixing bacteria are free-living in soil or live in root nodules of some plants.
Lightning splits the N–N bond, forming nitrogen oxides that dissolve in rain and leach into soil.

3Uptake

Uptake plants absorb nitrates from the soil and build proteins, and animals obtain nitrogen by eating plant proteins.

4Decomposition

Decomposition decomposers break proteins in dead organisms, and urea in urine and faeces, into ammonium compounds.

5Nitrification

Nitrification conversion of ammonium compounds to nitrites and then nitrates by nitrifying bacteria.

6Denitrification

Denitrification conversion of nitrates to nitrogen gas by denitrifying bacteria.

It reduces soil fertility and plant growth.
Denitrifying bacteria are anaerobic, so aerating soil by ploughing or reducing waterlogging slows it.

Part 9 of 10Populations and the growth curve

Populations and the growth curve

Population

Population a group of organisms of one species, living in the same area at the same time.

Community

Community all of the populations of different species in an ecosystem.

Ecosystem

Ecosystem a unit containing the community of organisms and their environment, interacting together.

Population growth is controlled by food supply, predation and disease.

The best-adapted organisms generally increase at the expense of less well-adapted ones.

Part 10 of 10Sigmoid growth curve (Extended)

Phase	What happens
Lag	Organisms adapt to the environment and few individuals are reproducing
Log (exponential)	Food is abundant, birth rate is high, death rate is low and growth is exponential
Stationary	Birth rate equals death rate because a nutrient has become limiting
Death	Death rate exceeds birth rate because food is short or toxic wastes have built up

Natural populations rarely follow this curve
- temperature, light, predators, disease, immigration and emigration also affect them.

Key terms

Term	Meaning
Producer	An organism that makes its own organic nutrients, usually by photosynthesis.
Consumer	An organism that gets energy by feeding on other organisms.
Decomposer	An organism that gets energy from dead or waste organic material.
Food web	A network of interconnected food chains.
Pyramid of biomass	A diagram showing the dry mass of living matter at each trophic level.
Population	A group of organisms of one species living in the same area at the same time.
Community	All the populations of different species in an ecosystem.
Nitrogen fixation	Conversion of nitrogen gas into ammonium compounds by bacteria.

Avoid these mistakes

Saying energy is recycled in ecosystems — energy flows through and is lost as heat, whereas nutrients such as carbon and nitrogen are recycled.

Saying a species will die out when another falls — use increase or decrease and give the reason.

Thinking pyramids of numbers are always pyramid-shaped — a large producer such as a tree can make the pyramid inverted.

Confusing nitrification and denitrification — nitrification makes nitrates, denitrification removes them as nitrogen gas.

Last updated: 12 October 2026