Gross primary production ( GPP) is the chemical energy store in plant…

The idea

Plants convert light energy into chemical energy during photosynthesis, storing it in organic molecules. This total accumulated chemical energy in a specific area or volume is gross primary production. It represents the absolute maximum energy captured by producers before any metabolic use. Only one to three per cent of incident sunlight is successfully converted into this store. The rest is lost because it is the wrong wavelength, is reflected by the leaf surface, passes straight through the tissue, or misses the chloroplasts entirely. When expressed as a store, units are typically kJ m-2. If measured over time as a rate, units become kJ m-2 year-1.

What earns the marks

• Defining gross primary production as the total chemical energy store in plant biomass in a given area or volume.
• Identifying that only a small percentage of incident light energy is converted into this chemical energy store.
• Explaining light loss by stating it is the wrong wavelength, reflected, or transmitted through the leaf.
• Stating correct units for a store (e.g., kJ m-2) or a rate (e.g., kJ m-2 year-1) depending on the context.

Common mistakes and how to put them right

• Defining gross primary production as the energy available to herbivores.

  Put it right: It is the total energy fixed, not the energy available after plant respiration.

• Writing 'energy produced by the plant'.

  Put it right: Energy is transferred and stored in organic molecules, never created or produced.

• Assuming most light reaching a leaf becomes gross primary production.

  Put it right: Only one to three per cent is converted, as much misses the chlorophyll or is reflected.

Exam tips

• Read the printed units carefully: a kJ m-2 value represents a store, whereas a kJ m-2 year-1 value represents a rate.
• Use the words 'store' and 'transferred' rather than 'made' or 'produced' whenever you write about energy in ecosystems.