5.2.2Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Energy and Biomass Transfers — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 6 parts Lesson 17 of 36 Unit quiz
Part 1 of 6Trophic Levels
Energy enters ecosystems as light and flows one way through feeding relationships.
Producers
- Plants and algae convert light energy to chemical energy and form the first trophic level.
Primary consumers
- Herbivores eat producers and obtain the energy stored in plant biomass.
Secondary consumers
- Carnivores eat herbivores and gain a smaller amount of energy again.
Decomposers
- Bacteria and fungi break down dead material, releasing nutrients back to the soil.
Part 2 of 6Why Transfer Is Inefficient
Energy is lost at every step.
Why Transfer Is Inefficient
Respiration
- Organisms use energy to respire, which is released as heat.
Uneaten parts
- Bones, roots or fur may not be eaten, so some biomass is never ingested.
Egestion
- Faeces contain energy that is not absorbed and passes to decomposers.
Excretion and movement
- Nitrogenous waste and movement use energy that is not stored as new biomass.
Part 3 of 6Pyramids
Different pyramids show different aspects of a food chain.
1
A pyramid of numbers counts organisms at each level and can be inverted, for example one tree supporting many insects.
2
A pyramid of biomass uses dry mass per area, which is more reliable but is a snapshot at one time.
3
A pyramid of energy uses energy per area per year and is always a pyramid shape.
4
Dry mass can be found by drying samples to a constant mass, and energy can be found by burning in a calorimeter.
Part 4 of 6Efficiency
Calculation
- Efficiency = energy transferred to the next level / energy available at the previous level x 100.
Typical value
- Transfer between trophic levels is often only around 10 percent, but varies widely.
Food chain length
- Few food chains have more than four or five levels, because too little energy remains.
Agriculture
- Eating plants rather than animals gives more people the same area of land.
| Level | Energy (kJ m⁻² year⁻¹) | Efficiency |
|---|---|---|
| Producers (NPP) | 15 000 | - |
| Primary consumers | 1 800 | 12 % |
| Secondary consumers | 180 | 10 % |
Part 5 of 6Worked Example
Question
- Producers have NPP of 15 000 kJ m⁻² year⁻¹ and primary consumers store 1 800 kJ m⁻² year⁻¹.
- Calculate the efficiency of transfer.
- Efficiency = energy in primary consumers / energy in producers x 100.
- = 1 800 / 15 000 x 100.
- = 12 percent.
Answer 12 percent.
Part 6 of 6Exam Checklist & Tips
5.2.2 – Key Points for the Exam
- Trophic level The position of an organism in a food chain.
- Biomass The mass of living material in an area, usually as dry mass.
- Decomposer An organism that feeds on dead organic matter and recycles nutrients.
- Transfer efficiency The percentage of energy passed from one trophic level to the next.
Common Mistakes
- Saying energy is recycled;
- energy flows through the ecosystem and is lost as heat, while nutrients are recycled.
- Stating that energy is destroyed; it is transferred to heat.
- Using wet mass for biomass instead of dry mass.
- Dividing the wrong way round when calculating efficiency.
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