6.1Organisms and their Environment
Energy & Feeding Relationships — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 11 parts Lesson 2 of 7 Unit quiz
Lesson summary
6.1 Energy & Feeding Relationships — IGCSE BIOLOGY notes: Explain energy flow, food chains and webs, ecological pyramids, nutrient cycles and population growth.Source — The Sun is the principal source of energy input to biological systems.Flow — Energy flows through living organi…
- Decomposer
- Cause of change
- Knock-on effects
- Pyramid of numbers
- Drawing rules
- Pyramid of biomass
Section 1 of 11Transfer of energy
1Source
- The Sun is the principal source of energy input to biological systems.
2Flow
- Energy flows through living organisms as light energy and as chemical energy in organisms.
3Loss
- Energy is eventually transferred to the environment, for example as heat.
Section 2 of 11Food chains and webs
Producer
- An organism that makes its own organic nutrients, usually using energy from sunlight.
Consumer
- An organism that gets energy by feeding on other organisms, classed as primary, secondary, tertiary or quaternary.
Herbivore
- An animal that gets its energy by eating plants.
Carnivore
- An animal that gets its energy by eating other animals.
Decomposer
- An organism that gets its energy from dead or waste organic material.
Food chain
- Shows the transfer of energy from one organism to the next, starting with a producer;
- arrows show the direction of transfer.
Food web
- A network of interconnected food chains that shows interdependence.
Section 3 of 11Interdependence in a food web
Cause of change
- Most changes in populations result from human impact, such as overharvesting or introducing foreign species.
Knock-on effects
- A change in one population affects others throughout the web.
- Fewer of a prey species means fewer of the predators that depend on it.
- Fewer consumers of a plant allow the plant population to increase.
Section 4 of 11Pyramids of numbers
Pyramid of numbers
- Shows how many organisms are present at each trophic level.
Shape
- It is not always pyramid-shaped because one large organism, such as an oak tree, can support many small ones.
Drawing rules
- Keep trophic levels in food-chain order with producers at the base.
- Label every bar with its trophic level.
- Draw to scale if graph paper is provided.
Section 5 of 11Pyramids of biomass
Pyramid of biomass
- Shows how much living matter, usually dry mass, is present at each trophic level.
Shape
- It is generally pyramid-shaped because mass decreases up a food chain.
Advantage
- It accounts for organism size and approximates the energy stored at each level.
Section 6 of 11Pyramids of energy (Supplement)
Transfer
- Only around 10% of the energy stored at one trophic level is available to the next.
Losses
- Energy goes to excretion, movement and heat, and uneaten or undigested parts such as roots, bones and faeces never pass it on.
Chain length
- Because of these losses food chains are rarely more than five organisms long.
Human food
- More energy reaches humans eating wheat than eating cows fed on wheat, since each extra trophic level loses energy.
Section 7 of 11Carbon cycle
1Removal
- Plants take carbon dioxide from the atmosphere by photosynthesis.
2Passing on
- Carbon passes to animals and microorganisms by feeding.
3Return
- Respiration by plants, animals and microorganisms returns carbon dioxide to the atmosphere.
4Fossil fuels
- Dead organisms that do not decompose can become fossil fuels over millions of years.
- Combustion of fossil fuels releases carbon dioxide.
5Deforestation
- It removes producers and, when trees are burnt, releases more carbon dioxide.
Section 8 of 11Nitrogen cycle (Supplement)
1Fixing nitrogen
- Nitrogen-fixing bacteria in soil or root nodules turn nitrogen gas into ammonium compounds, and lightning produces oxides that dissolve in rain.
2Uptake
- Plants absorb nitrates and build proteins, which animals obtain by feeding.
3Return
- Urea, faeces and dead organisms are converted to ammonium compounds by decomposers.
4Nitrification
- Nitrifying bacteria convert ammonium compounds to nitrites and then to nitrates.
5Denitrification
- Denitrifying bacteria convert nitrates to nitrogen gas, lowering soil fertility.
- They are anaerobic, so aerating the soil reduces their activity.
Section 9 of 11Populations and growth
Populations and growth
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.
Ecosystem
- A unit containing the community of organisms and their environment, interacting together.
Controls
- Population growth is controlled by food supply, predation and disease.
Sigmoid curve (Supplement)
- Lag phase, log phase, stationary phase and death phase.
- In the stationary phase birth rate equals death rate; in the death phase death rate is greater.
- Wild populations seldom follow it because of temperature, light, predators, disease, immigration and emigration.
Section 10 of 11Key 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. |
| Biomass | The dry mass of living matter in an organism or trophic level. |
| Nitrifying bacteria | Bacteria that convert ammonium compounds to nitrites and then nitrates. |
| Denitrifying bacteria | Bacteria that convert nitrates to nitrogen gas. |
| Population | A group of organisms of one species in the same area at the same time. |
Section 11 of 11Exam technique
1
Saying a species would die out when a food source falls — say its population would decrease and give the reason.
2
Saying energy is recycled — energy flows through ecosystems and is lost as heat, while nutrients are recycled.
3
Drawing a pyramid with levels out of order — producers form the base in food-chain order.
4
Thinking pyramids of numbers are always pyramid-shaped — one large producer can support many consumers.
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