Describe photosynthesis as carbohydrate synthesis
Separate raw materials, light energy and newly made food.
Section B6.1 · Photosynthesis
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which input supplies carbon for the stated carbohydrate synthesis?
What you will learn
- Separate raw materials, light energy and newly made food.
- Use the stated evidence to explain the method and its limits.
Make carbohydrate rather than absorb ready-made food
Photosynthesis is the process in which plants synthesise carbohydrates from raw materials using light energy. Carbon dioxide and water supply matter; light supplies energy. A green plant does not obtain all its food as ready-made sugar from soil.
Carbon from carbon dioxide becomes part of carbohydrate. Water contributes material to the synthesis, while mineral ions have other important roles. The atoms in carbohydrate come from matter, not from light turning into atoms.
Distinguish two cell processes
Photosynthesis stores transferred light energy in chemicals. Respiration releases energy from nutrient molecules for cell activities. A photosynthesising plant still respires; these processes can occur in the same organism without being the same reaction.
Most familiar green leaves photosynthesise when appropriate conditions are available, but not every plant cell has chloroplasts. A root hair cell can receive organic nutrients transported from photosynthesising parts rather than photosynthesising itself.
Sources for this lesson
Pause and explain
Does photosynthesis mean a plant no longer respires?
Worked example
A green seedling grows in a supplied experiment with water, carbon dioxide, minerals and light but no added sugar. Explain how carbohydrate can become available.
Show the worked solution
- Identify carbon dioxide and water as raw materials.
- Identify light as the energy source.
- Explain carbohydrate synthesis by photosynthesis, without calling minerals ready-made food.
Answer The seedling can synthesise carbohydrate from carbon dioxide and water using light energy.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State what a plant makes by photosynthesis and the source of energy.
Give me a hint
Separate product from energy input.
Compare my reasoning
- Name carbohydrates.
- Name light energy.
Plants synthesise carbohydrates using energy from light.
Look for these in your work
- Include synthesis.
- Identify light energy.
A student says a growing leaf gains carbon atoms from sunlight. Correct the explanation.
Give me a hint
Name a carbon-containing raw material.
Compare my reasoning
- Light supplies energy.
- Carbon dioxide supplies carbon atoms.
Carbon atoms come from carbon dioxide; light provides energy for synthesis.
Look for these in your work
- Distinguish matter and energy.
- Name carbon dioxide.
A root hair lacks chloroplasts but receives sugar from the shoot. Why does that not contradict the plant photosynthesis model?
Give me a hint
An organism contains different specialised cells.
Compare my reasoning
- The shoot can make carbohydrate.
- Transport supplies organic nutrients elsewhere.
- Not every cell must photosynthesise.
Photosynthesis in suitable shoot cells can supply carbohydrate to non-photosynthetic root cells.
Look for these in your work
- Avoid claiming every plant cell has chloroplasts.
- Connect synthesis and transport.
Common mistakes
- Light supplies energy, not carbon atoms.
- Photosynthesis does not replace respiration.
What can you explain now?
Does photosynthesis mean a plant no longer respires?
Compare with the explanation
No; the two processes can occur together.
Plants still need energy release for cell activities.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain where carbohydrate matter and its stored energy come from.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.