AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
These biological molecules form the biomass of the plants.
Full specification wording
Energy and ecosystems (A-level only): These biological molecules form the biomass of the plants.
During photosynthesis, plants synthesise organic compounds, primarily sugars like glucose, from carbon dioxide.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 11
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
During photosynthesis, plants synthesise organic compounds, primarily sugars like glucose, from carbon dioxide. While a significant proportion of these sugars is used as respiratory substrates to release energy for cellular processes, the remainder is used to synthesise other essential biological molecules. These include structural carbohydrates like cellulose for cell walls, storage carbohydrates such as starch, alongside lipids, proteins, and nucleic acids. Together, these organic molecules constitute the biomass of the plant. Consequently, plant biomass is not just physical matter; it represents the chemical energy store of the organism, which is subsequently available to higher trophic levels in the ecosystem.
What earns the marks
- Identify that plants synthesise sugars from carbon dioxide during photosynthesis.
- State that a proportion of these synthesised sugars is used by the plant as respiratory substrates.
- Explain that the remaining sugars are used to synthesise other biological molecules, such as cellulose, starch, proteins, and lipids.
- Define plant biomass as the total mass of these biological molecules, which represents the chemical energy store of the organism.
Common mistakes and how to put them right
Assuming all photosynthetic products become biomass.
Put it right: Explicitly state that a significant proportion of sugars is used in respiration and lost as carbon dioxide, so only the remainder forms biomass.
Confusing biomass with total plant mass including water.
Put it right: Biomass consists of the organic biological molecules, excluding water which does not store chemical energy.
Failing to link biological molecules to energy transfer.
Put it right: Explicitly state that these biological molecules act as a chemical energy store that can be passed to the next trophic level.
Exam tips
- When asked what happens to the products of photosynthesis, always distinguish between sugars used for respiration and those used to synthesise other biological molecules.
- Remember to explicitly use the phrase 'chemical energy store' when discussing the biological molecules that make up plant biomass.
Try the lesson
The start of the Energy and ecosystems (A-level only): These biological molecules form the biomass of the plants. lesson, made from checked teaching notes for this specification point.
Have a go first
Which class of biological molecules makes up the greatest proportion of the biomass of a plant?
Listen
During photosynthesis, plants synthesise organic compounds, primarily sugars like glucose, from carbon dioxide. While a significant proportion of these sugars is used as respiratory substrates to release energy for cellular processes, the remainder is used to synthesise other essential biological molecules. These include structural carbohydrates like cellulose for cell walls, storage carbohydrates such as starch, alongside lipids, proteins, and nucleic acids. Together, these organic molecules constitute the biomass of the plant. Consequently, plant biomass is not just physical matter; it represents the chemical energy store of the organism, which is subsequently available to higher trophic levels in the ecosystem. A strong answer earns marks for these points. Identify that plants synthesise sugars from carbon dioxide during photosynthesis. State that a proportion of these synthesised sugars is used by the plant as respiratory substrates. Explain that the remaining sugars are used to synthesise other biological molecules, such as cellulose, starch, proteins, and lipids. Define plant biomass as the total mass of these biological molecules, which represents the chemical energy store of the organism. A common slip: Assuming all photosynthetic products become biomass. Instead, explicitly state that a significant proportion of sugars is used in respiration and lost as carbon dioxide, so only the remainder forms biomass. Exam tip: When asked what happens to the products of photosynthesis, always distinguish between sugars used for respiration and those used to synthesise other biological molecules.
See it
Key words
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biological molecules
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photosynthesis
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chemical energy store
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See every definition
- biological molecules
- Organic compounds that together constitute the biomass of the plant.
- photosynthesis
- The process during which plants synthesise organic compounds, primarily sugars like glucose, from carbon dioxide.
- chemical energy store
- What plant biomass represents, being subsequently available to higher trophic levels in the ecosystem.
Label the diagram
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1Box 1
2Box 2
3Box 3
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