OxfordAQA International AS and A-level Biology: Energy transfer, productivity and farming (9610) – Revision Notes

Quick recall for sections 3.3.4.1 and 3.3.4.2 of the OxfordAQA International AS and A-level Biology specification, Version 5.1 (Unit 3, International A-level only). For full explanations use the study guide. Then test yourself with the practice questions, tick off outcomes on the printable checklist, and see the course hub. The free diagnostics show which topics need most work.

3.3.4.1 Energy transfer

• Energy is transferred through the trophic levels of food chains and food webs.
• Transfer from producers to consumers, and between consumers, is inefficient.
• Food webs link chains. One species can feed at more than one trophic level.

Where the energy goes

Loss	What it is
Uneaten parts	Roots, bones, shells, fur, organisms that die uneaten
Faeces (F)	Undigested material that is egested
Urine (U)	Excretory waste such as urea
Respiration (R)	Used for movement, active transport and temperature control, then lost as heat

Energy flows one way through ecosystems. It ends as heat, so it is not recycled. Chains are short because too little energy remains to support many levels.

Efficiency of transfer

Efficiency (%) = energy passed to the next level ÷ energy available at the lower level × 100

Method in steps:

1. Pick out the two values, using the same units and the same area and time.
2. Divide the higher level by the lower level.
3. Multiply by 100.
4. For a chain of several steps, multiply the individual efficiencies, or divide the last value by the first.

Small worked reminder. A plant level stores 2 500 kJ m⁻² year⁻¹. A herbivore level stores 8% of it, then a carnivore stores 10% of that.

• Herbivore: 2 500 × 0.08 = 200 kJ m⁻² year⁻¹.
• Carnivore: 200 × 0.10 = 20 kJ m⁻² year⁻¹.
• Overall efficiency: 20 ÷ 2 500 × 100 = 0.8%.

Pyramids

Pyramid	Units	Inverted?	Reason
Number	Individuals per area	Yes, sometimes	A large producer such as a tree carries very many small consumers. Parasites on a host also invert it. Body size is ignored
Biomass	Dry mass per area, g m⁻²	Yes, sometimes	A snapshot. Fast-growing producers such as phytoplankton can have a smaller stock than the consumers that eat them
Energy	kJ m⁻² year⁻¹	Never over a full year	Each level gets its energy from the level below and some is lost at each step

Why dry mass? Fresh tissue has a variable water content. Drying removes the water so that masses can be compared.

Must-know distinction: biomass is a stock at one time. Energy transferred per year is a rate. A small stock can support a large flow if it is replaced quickly.

3.3.4.2 Energy and human food production

Definitions

Term	Definition
GPP	Chemical energy store in plant biomass in a given area or volume, in a given time
NPP	Chemical energy store in plant biomass after respiratory loss (R) has been taken into account

Equations

Quantity	Equation	Rearranged
Net primary production	NPP = GPP - R	R = GPP - NPP; GPP = NPP + R
Net production of a consumer	N = I - (F + U + R)	R = I - F - U - N; I = N + F + U + R

In the consumer equation, N is net production, I is the chemical energy of ingested food, F is the energy in faeces, U is the energy in urine and R is respiratory loss.

Small worked reminder (NPP). GPP = 15 600 kJ m⁻² year⁻¹ and R = 7 020 kJ m⁻² year⁻¹.

• NPP = 15 600 - 7 020 = 8 580 kJ m⁻² year⁻¹.
• NPP as a percentage of GPP: 8 580 ÷ 15 600 × 100 = 55%.

Small worked reminder (consumer). A calf ingests 840 kJ, faeces hold 168 kJ, urine 42 kJ and respiration 504 kJ.

• N = 840 - (168 + 42 + 504) = 126 kJ.
• N as a percentage of intake: 126 ÷ 840 × 100 = 15%.

Units: always carry the units through. Give area and time for primary production and for energy transfer between levels.

Farming practices that raise efficiency

Practice	How it helps	Possible drawback
Chemical pesticides (insecticide, herbicide, fungicide)	Fewer losses to pests, weeds and disease. Simpler non-human food web, so more energy stays in the crop	Harm to non-target species, resistance, residues
Biological agents (for example Encarsia formosa against whitefly, Bacillus thuringiensis against caterpillars)	Pest numbers fall without chemical residue	Slower, may not fully control the pest, an introduced species may become a pest itself
Integrated systems	Combine methods (rotation, resistant varieties, biological agents, chemicals only above a pest threshold)	More planning and monitoring
Reducing respiratory losses	Less movement and a warm environment mean less R, so more intake becomes N	Welfare concerns and higher housing costs

Simplifying food webs means fewer species compete with or feed on the crop or livestock, so a larger share of the energy ends up where people can use it.

Evaluating data: three lenses

• Economic: extra revenue minus cost, and return per pound spent.
• Environmental: non-target species, pollution, resistance, effect on biodiversity.
• Ethical: animal welfare, natural behaviour, food supply for people.

Must-know distinctions

• GPP against NPP: GPP counts everything the producers fix. NPP is the part left once their own respiration is paid for, and it is what feeders can eat.
• Efficiency and amount: a high efficiency at a low energy level can still give a small amount of energy. Quote both when comparing.
• Biological agent and pesticide: a biological agent is a living organism. A pesticide is a chemical.
• Pyramid of energy and pyramid of biomass: energy is a rate over time. Biomass is a stock at one moment.

Method in steps for an evaluation question:

1. Quote and compare figures, using calculations such as percentage increase.
2. Link the figures to energy: where was R, F or U reduced?
3. Give at least one economic, one environmental and one ethical point if asked to evaluate.
4. Reach a judgement and say what extra data would help.

Quick self-test

1. Define gross primary production.
2. Define net primary production and write the equation.
3. A forest has GPP 21 000 kJ m⁻² year⁻¹ and NPP 12 600 kJ m⁻² year⁻¹. Calculate R.
4. A stream has 4 800 kJ m⁻² year⁻¹ in algae and 576 kJ m⁻² year⁻¹ in mayfly larvae. Calculate the efficiency of transfer.
5. Name the four losses of energy between trophic levels.
6. A lamb ingests 2 400 kJ, loses 480 kJ in faeces, 120 kJ in urine and 1 560 kJ in respiration. Calculate N.
7. Why can a pyramid of energy never be inverted?
8. Why might a pyramid of number for an oak tree be inverted?
9. Explain why a biomass pyramid for the sea may be inverted.
10. Give two ways to cut respiratory loss in farmed animals.
11. A herbicide is used on a cereal crop. State how it increases productivity.
12. Give one ethical issue with reducing the movement of farm animals.

Answers

1. The chemical energy store in plant biomass in a given area or volume, in a given time.
2. The chemical energy store in plant biomass after respiratory loss (R) has been taken into account; NPP = GPP - R.
3. R = GPP - NPP = 21 000 - 12 600 = 8 400 kJ m⁻² year⁻¹ (40% of GPP).
4. 576 ÷ 4 800 × 100 = 12.0%.
5. Parts not eaten, indigestible material lost in faeces, excretion in urine, and respiration (lost as heat).
6. N = 2 400 - (480 + 120 + 1 560) = 240 kJ (10% of intake).
7. Energy at each level comes from the level below and part is lost at every transfer, so each level holds less energy per year.
8. One large producer supports many small consumers, so the first bar is narrower than the next.
9. Phytoplankton have a small standing biomass because they are eaten and replaced very quickly, so the zooplankton biomass at one moment can be larger.
10. Reduce movement (housing or restricting space) and keep the animals warm so less energy is used to maintain body temperature.
11. It kills weeds, so the crop has less competition for light, water and mineral ions, and more energy is stored in the crop biomass.
12. Animal welfare: the animals cannot show natural behaviour, so any gain in productivity has to be weighed against this.

Where marks are usually lost

• Saying energy is “used up” or “lost” with no mention of heat from respiration.
• Putting the higher trophic level on the bottom of an efficiency fraction.
• Adding efficiencies across two steps when they should be multiplied.
• Leaving out the urine term in N = I - (F + U + R).
• Giving a definition of GPP or NPP without “in a given area (or volume), in a given time”.
• Quoting a bare percentage without the working or units.
• Describing a farming practice but not linking it to energy flow or respiration.
• Evaluating with only economic points when the question asks for environmental and ethical ones too.

Official syllabus

OxfordAQA International AS and A-level Biology, Version 5.1, for International A-level exams May/June 2018 onwards, sections 3.3.4.1 and 3.3.4.2.