Energy & Feeding Relationships

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Energy & Feeding Relationships (Biology, CIE) — use them in your lesson, or run the topic as a live class game.

Notes

Transfer of Energy

The **Sun** is the principal source of energy input to biological systems.
Energy flows through living organisms as **light energy** from the Sun and **chemical energy** in organisms.
Energy is eventually transferred to the environment, e.g. as **heat**.

Food Chains & Food Webs

A **food chain** shows the transfer of energy from one organism to the next, starting with a **producer**.
Arrows in a food chain represent the **transfer of energy** from one trophic level to the next.
Energy is transferred between organisms by **ingestion** (eating).
A **food web** is a network of interconnected food chains, showing **interdependence** of organisms.
Changes in one population can affect others; e.g. if earthworms decrease, grass increases, frogs and mice decrease, sparrows decrease slightly.

Pyramids of Number

A **pyramid of numbers** shows the number of organisms at each trophic level.
The width of each bar indicates the number of organisms.
Pyramids of numbers are **not always pyramid-shaped** because one large producer (e.g. oak tree) can support many consumers.
Rules: producers at bottom, then primary, secondary, tertiary consumers; label each bar; draw to scale if graph paper provided.

Pyramids of Biomass

A **pyramid of biomass** shows the mass of living matter (dry mass) at each trophic level.
Pyramids of biomass are **always pyramid-shaped** because mass decreases up the food chain.
Biomass gives a more accurate representation of living matter and stored energy than numbers.

Pyramids of Energy (Extended)

Energy is lost between trophic levels due to **excretion**, **movement**, **heat**, uneaten parts, and undigested waste (faeces).
Only about **10%** of energy stored at each trophic level is available to the next.
This explains why food chains are rarely more than **5 organisms** long.
A **pyramid of energy** visually represents energy stored at each level and highlights energy loss.

The Carbon Cycle

Carbon is taken from the atmosphere as **CO₂** by plants during **photosynthesis**.
Carbon is passed to animals and decomposers by **feeding**.
Carbon is returned to the atmosphere as CO₂ by **respiration** (plants, animals, microorganisms).
Fossil fuels form from dead organisms; **combustion** of fossil fuels releases CO₂.
Deforestation reduces CO₂ uptake and burning trees releases more CO₂.

The Nitrogen Cycle (Extended)

**Nitrogen-fixing bacteria** convert N₂ gas into ammonium compounds, which become nitrates.
**Nitrifying bacteria** convert ammonium compounds to nitrites then nitrates.
Plants absorb nitrates to build **proteins**; animals obtain nitrogen by eating plants.
**Decomposers** break down dead organisms and waste into ammonium compounds.
**Denitrifying bacteria** convert nitrates back to N₂ gas, reducing soil fertility.

Populations

A **population** is a group of organisms of one species living in the same area at the same time.
A **community** includes all populations of different species in an ecosystem.
An **ecosystem** includes the community and its environment interacting together.
Population growth is controlled by **food supply**, **predation**, and **disease**.
The **sigmoid growth curve** has four phases: **lag**, **log** (exponential), **stationary**, and **death**.
In natural environments, factors like temperature, light, predators, disease, immigration, and emigration affect growth.

Pyramid of energy showing energy transfer from producers to secondary consumers.

A simple food web showing interdependence of organisms.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.Which of the following is a correct example of a population?
Easy
- AAll the goldfish in a pond.
- BAll the animals in a forest.
- CAll the species of animals in Europe.
- DAll the people born in Scotland over a 150-year period.
2.Which process uses the principal source of energy input to biological systems?
Easy
- APhotosynthesis
- BIngestion
- CDecomposition
- DRespiration
3.What is an example of a community?
Easy
- AAll the populations of all species in a forest.
- BAll the goldfish in a pond.
- CAll the wheat and corn in a field.
- DAll the plants in a forest.
4.Which organism is a producer?
Easy
- AGrass
- BRabbit
- CHawk
- DFungus
5.The diagram shows part of the carbon cycle. Which process is shown by the arrow labelled Y?
Medium
- APhotosynthesis
- BFeeding
- CRespiration
- DCombustion
6.Which of the following diagrams show a pyramid of biomass for a forest?
Medium
- AA pyramid that is upright (wide base, narrow top)
- BA pyramid that is inverted (narrow base, wide top)
- CA pyramid that is rectangular (all bars same width)
- DA pyramid that is diamond-shaped (narrow base and top, wide middle)
7.A food chain is: tree → small insects → small birds → birds of prey. Which pyramid of numbers would be correct for this food chain?
Medium
- AA pyramid with a narrow base (tree) and widening upwards
- BA pyramid with a wide base (tree) and narrowing upwards
- CA pyramid with a wide base (tree), then narrower, then wider, then narrower
- DA pyramid with equal width at all levels
8.The table shows data for a food chain: Oak tree (1, 400,000 kg), Aphids (9,000, 900 kg), Predatory midges (120, 45 kg). What is the percentage biomass of the aphids transferred to the midges?
Hard
- A5%
- B0.05%
- C1.3%
- D0.225%

Slides

The Sun: The Principal Source of Energy

The Sun is the principal source of energy input to biological systems.
Energy flows through living organisms as light energy from the Sun and chemical energy in organisms.
Energy is eventually transferred to the environment, e.g. as heat.

What is the principal source of energy input to biological systems?

The Sun
Plants
Animals
Soil

Food Chains and Trophic Levels

A food chain shows the transfer of energy from one organism to the next, starting with a producer.
Arrows in a food chain show the direction of energy transfer.
Energy is transferred from one organism to another by ingestion (eating).
Example: Grass (producer) → Mouse (primary consumer) → Barn owl (secondary consumer).

In a food chain, what do the arrows represent?

The direction of energy transfer
The size of organisms
The number of organisms
The type of food

Food Webs and Interdependence

A food web is a network of interconnected food chains.
Food webs show interdependence: how a change in one population can affect others.
Example: If earthworms decrease, grass increases, frogs and mice decrease, sparrows decrease slightly.

Which of these are true about food webs?

A food web is a single linear chain.
Food webs show interdependence of organisms.
If one population changes, it can affect others in the web.
Food webs are less realistic than food chains.

Pyramids of Number

A pyramid of numbers shows how many organisms are present at each trophic level.
The width of each bar indicates the number of organisms.
Pyramids of number are not always pyramid-shaped because one large producer (e.g. oak tree) can support many smaller consumers.

Why might a pyramid of numbers not be pyramid-shaped?

Because producers are always the most numerous
Because one large producer can support many small consumers
Because energy is lost at each trophic level
Because biomass decreases up the food chain

Pyramids of Biomass

A pyramid of biomass shows the total living matter (dry mass) at each trophic level.
Pyramids of biomass are always pyramid-shaped because mass decreases as you go up a food chain.
Biomass is a good representation of the energy available at each level.

Which statement about pyramids of biomass is correct?

They can be any shape.
They are always pyramid-shaped.
They show the number of organisms.
They are the same as pyramids of numbers.

Energy Loss Between Trophic Levels (Extended)

Only about 10% of energy stored at one trophic level is available to the next.
Energy is lost through excretion, movement, heat, uneaten parts, and undigested waste.
This inefficiency explains why food chains rarely have more than 5 trophic levels.

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

The Carbon Cycle

Carbon is taken out of the atmosphere as CO₂ by photosynthesis in plants.
It is passed on to animals and decomposers by feeding.
Carbon is returned to the atmosphere as CO₂ by respiration (plants, animals, decomposers) and combustion of fossil fuels.

Which process removes carbon dioxide from the atmosphere?

Respiration
Combustion
Photosynthesis
Decomposition

The Nitrogen Cycle (Extended)

Nitrogen is needed to make proteins but cannot be absorbed as N₂ gas.
Nitrogen-fixing bacteria convert N₂ into ammonium compounds.
Nitrifying bacteria convert ammonium to nitrites then nitrates, which plants absorb.
Denitrifying bacteria convert nitrates back to N₂ gas, reducing soil fertility.

____ bacteria convert ammonium compounds into nitrites and then nitrates.

Populations and Communities

A population is a group of organisms of one species, living in the same area at the same time.
A community includes all the populations of different species in an ecosystem.
An ecosystem is a unit containing the community and its environment, interacting together.

Which is an example of a population?

All the animals in a forest
All the goldfish in a pond
All the species in a field
All the plants in a garden

Population Growth Curve (Extended)

A sigmoid (S-shaped) growth curve has four phases: lag, log (exponential), stationary, and death.
Lag phase: organisms adapt; few reproduce.
Log phase: abundant food, rapid birth rate, low death rate.
Stationary phase: population levels out due to limited resources.
Death phase: death rate exceeds birth rate due to food shortage or waste buildup.

During which phase of a sigmoid growth curve is food supply abundant and birth rate rapid?

Lag phase
Log phase
Stationary phase
Death phase

Unlock all 40 questions, slides & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

🗂️ Coming soon