5.3.4Energy Transfers In and Between Organisms (A Level only)

Gross Primary Production — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 16 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision

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5.3.4 Gross Primary Production — AQA A Level Biology 7402 notes: Define gross primary production and explain what happens to the energy a producer captures.Gross primary production (GPP) is the total quantity of chemical energy stored in plant biomass, in a given area or volume, …

respiration
Exam-style question 1
Exam-style question 2
Respiratory loss
Limiting factor
Solar energy

Section 1 of 8Defining GPP

Gross primary production (GPP) is the total quantity of chemical energy stored in plant biomass, in a given area or volume, in a given time.

Defining GPP

It is the total amount of light energy fixed by photosynthesis in the form of organic molecules.

It is expressed in kJ m^-2 year^-1 when measured over a year in a terrestrial community, or kJ m^-3 year^-1 in an aquatic system.

GPP is a rate, not an amount present at one time. The units always include a time component, which distinguishes it from biomass. Two ecosystems may hold the same biomass but have very different GPP if one grows and turns over its tissue far faster than the other.

Section 2 of 8What limits GPP

Only a small percentage of the solar energy reaching an ecosystem ends up as GPP. Losses occur because:

Some light is reflected by leaf surfaces or by clouds and the atmosphere.

Some light is of wavelengths that pigments do not absorb, notably green and infrared.

Some light passes through leaves, or falls on non-photosynthetic tissue or bare ground.

Photosynthesis itself is inefficient; energy is lost as heat in the light-dependent and light-independent reactions.

Limiting factors reduce the rate: light intensity, carbon dioxide concentration, temperature, and water or mineral supply.

In many ecosystems GPP is around 1–3% or less of incident radiation, though crops managed intensively may reach higher figures.

Section 3 of 8Where does GPP go?

The energy trapped in GPP has two main fates in the producer.

Respiration

A large share is used in the plant's own respiration: this provides ATP for active transport, synthesis of macromolecules and growth,

and is eventually released as heat.

The remainder is stored in new biomass such as leaves, stems, roots, fruits and seeds.

This stored amount is the net primary production, covered in the next lesson.

Therefore:

GPP = net primary production (NPP) + respiratory losses (R)

Different ecosystems have different GPPs. Tropical rainforests have a high GPP because of warm temperatures, high light and plentiful water all year; deserts and polar regions have low values because of water shortage or low temperature and short growing seasons. Coastal and estuarine systems are productive because nutrients are abundant.

Section 4 of 8Interpreting data

When given data on GPP, compare values per unit area and per year.

Interpreting data

Look for the cause of differences: a tropical forest compared with a tundra shows how temperature, light duration and water availability set the upper limit.

Compare also how fertiliser, irrigation or crop choice can raise GPP in farmland.

Remember that GPP is not the energy available to the next trophic level; this is reduced by plant respiration,

which is why net production is the more meaningful measure for consumers.

Factor	Effect on GPP
Higher light intensity (up to saturation)	More photosynthesis, GPP rises
Low temperature	Enzyme activity falls, GPP falls
Water shortage	Stomata close, less CO₂, GPP falls
Mineral deficiency (e.g. Mg, N)	Less chlorophyll or enzymes, GPP falls

Section 5 of 8Worked comparison of ecosystems

Consider two sites.

A tropical forest might have a GPP around 45 000 kJ m^-2 year^-1 whilst a temperate grassland might have 10 000 kJ m^-2 year^-1.

The difference can be explained by a longer growing season, more constant warmth that keeps enzyme activity high, abundant rainfall that keeps stomata open,

and a multi-layered canopy that intercepts more of the incident light.

The percentage of solar energy captured is calculated as GPP divided by incident energy times 100.

If incident energy is 5 000 000 kJ m^-2 year^-1, a GPP of 45 000 is 0.9%, showing that even highly productive systems capture under one percent of the available light.

In an exam, quote the figures, make the comparison, and then explain it with named limiting factors.

Aquatic systems add a further limit

light is absorbed by water, so photosynthesis is restricted to the upper layer, and the supply of nitrate and phosphate often limits phytoplankton growth.
This is why upwelling zones with nutrient-rich water are among the most productive parts of the ocean.

Section 6 of 8Exam-style questions

Explain why gross primary production is less than the solar energy falling on an area. [4]4 marks
Give the units of GPP and explain why a time component is needed. [2]2 marks

Section 7 of 8Key terms

Term	Meaning
Gross primary production	Total chemical energy fixed by photosynthesis in producers per unit area per unit time.
Respiratory loss	Energy released by respiration, ultimately lost as heat.
Limiting factor	Factor in shortest supply that restricts the rate of a process.
Solar energy	Radiant energy from the sun that drives photosynthesis.
Incident radiation	Light energy that falls on a surface.

Section 8 of 8Exam technique

Giving GPP as an amount of biomass at a moment instead of a rate.

Saying GPP is all the energy used by the next trophic level. Plant respiration must first be subtracted.

Forgetting that respiration is carried out by producers as well as consumers.

Omitting units or the time period.

Last updated: 18 October 2026