How does energy flow through food chains, and how is carbon recycled in an ecosystem?
Food chains and food webs, the flow of energy from the Sun through trophic levels, why energy is lost at each level, pyramids of numbers and biomass, the role of decomposers, and the carbon cycle.
A focused CCEA GCSE Single Award Science answer on ecology, covering food chains and webs, the flow of energy from the Sun through trophic levels, why energy is lost at each level, pyramids of numbers and biomass, decomposers, and the carbon cycle.
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What this dot point is asking
CCEA wants you to read food chains and food webs, explain that energy enters from the Sun and flows through trophic levels, explain why energy is lost at each level, interpret pyramids of numbers and biomass, describe the role of decomposers, and describe the carbon cycle.
Food chains and food webs
Energy flow and trophic levels
Almost all the energy in an ecosystem comes from the Sun, captured by producers in photosynthesis. A trophic level is a feeding level in a food chain. Energy is lost between levels because:
- Organisms respire, releasing energy as heat.
- Mammals and birds use a lot of energy to keep warm.
- Energy is used for movement and life processes.
- Not all of an organism is eaten or digested, so energy is lost in waste and uneaten parts.
Only about 10 percent of the energy passes to the next level, which is why food chains are short.
Pyramids of numbers and biomass
Decomposers and the carbon cycle
Decomposers (bacteria and fungi) break down dead organisms and waste, recycling nutrients back into the soil.
Examples in context
Example 1. Why eating plants feeds more people. Because about 90 percent of energy is lost at each step, a field of crops can feed many more people directly than if the crops are first fed to cattle and the people eat the beef. Each extra level wastes most of the energy. This is why, in places where food is short, growing crops for people is far more efficient than raising animals.
Example 2. How burning fossil fuels adds to carbon dioxide. Fossil fuels are carbon locked away from living things millions of years ago. Burning them returns that carbon to the air as carbon dioxide much faster than photosynthesis can remove it, so the level of carbon dioxide in the atmosphere rises. This links the carbon cycle to the greenhouse effect and climate change, a common CCEA discussion point.
Try this
Q1. In a food chain, what name is given to a green plant that makes its own food? [1 mark]
- Cue. A producer.
Q2. Name the process that removes carbon dioxide from the air in the carbon cycle. [1 mark]
- Cue. Photosynthesis.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20204 marksExplain why a food chain rarely has more than four or five trophic levels.Show worked answer →
Four marks for the energy loss and the consequence.
At each trophic level most of the energy is lost, in respiration as heat, in keeping warm, in movement, and in waste and uneaten parts.
Only about 10 percent of the energy is passed on to the next level.
After four or five levels there is too little energy left to support another level of organisms.
So food chains are short, because the energy runs out. Markers reward the loss at each level, the small amount passed on, and the link to running out of energy.
CCEA SAS 20183 marksDescribe two processes that return carbon dioxide to the air in the carbon cycle.Show worked answer →
Three marks: name two processes and link them to carbon dioxide.
Respiration: plants, animals and microorganisms respire, releasing carbon dioxide back into the air.
Combustion: burning wood and fossil fuels releases carbon dioxide into the air.
Decomposition is a third valid answer: decomposers break down dead matter and respire, releasing carbon dioxide.
Markers reward two named processes that each return carbon dioxide to the atmosphere. Photosynthesis would be wrong here, because it removes carbon dioxide.
Related dot points
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)