Photosynthesis: reactants, products and light energy
Explain the Core word equation and keep chlorophyll and energy distinct from the material reactants.
Section C10.3 · Air quality and climate
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish reactants from products in a word equation.
- Explain an energy input without treating it as an atom source.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which side of a reaction arrow contains the stated products?
What you will learn
- Describe photosynthesis using carbon dioxide, water, glucose and oxygen.
- State the required word equation with chlorophyll and light-energy conditions.
State the direction and the conditions
Photosynthesis uses carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll, using energy from light. The Core word equation is carbon dioxide + water → glucose + oxygen, with the light and chlorophyll conditions stated. Those conditions are part of the explanation even when not written as substances on the left of the equation.
Carbon dioxide is a reactant in this process, so growing photosynthetic organisms can take up CO₂. The glucose product stores chemical energy supplied through the process. Oxygen is a product, not the same gas as carbon dioxide. The required word equation is different from the Extended balanced symbol equation in the later lesson.
Energy is not another element
Light supplies energy; it does not provide carbon atoms that replace the carbon dioxide input. Chlorophyll is present to enable the light-dependent process, rather than being written as the bulk glucose reactant in the simplified net equation. Separate material inputs, energy input and the condition that chlorophyll is present.
The word equation describes the stated photosynthetic process, rather than every process in a living plant. A plant can also respire, so its existence does not prove its net CO₂ uptake has a fixed positive rate at every time. A supplied comparison needs its light conditions and time period; for this Core objective, explain the named reaction correctly before inferring broader environmental effects.
Sources for this lesson
Pause and explain
What role is assigned to light?
Worked example
State the photosynthesis word equation and name the required energy input and presence condition.
Show the worked solution
- The material reactants are carbon dioxide and water.
- The products are glucose and oxygen.
- Light supplies energy and chlorophyll is present for the stated process.
Answer Carbon dioxide + water → glucose + oxygen, using light energy in the presence of chlorophyll.
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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.
Write the Core word equation and add the stated conditions.
Give me a hint
List two material inputs and two products, then state energy and chlorophyll separately.
Compare my reasoning
- Carbon dioxide and water are the reactants.
- Glucose and oxygen are the products.
- Use light energy in the presence of chlorophyll.
Carbon dioxide + water → glucose + oxygen; light energy and chlorophyll are required conditions.
Look for these in your work
- The direction and all four substances are correct.
- Both stated conditions are present.
A proposed word equation says glucose + oxygen → carbon dioxide + water and labels it photosynthesis. Correct the direction and explain the light input.
Give me a hint
The proposed direction corresponds to a different simplified process.
Compare my reasoning
- The required photosynthesis direction starts with carbon dioxide and water.
- It produces glucose and oxygen.
- Light provides energy in the presence of chlorophyll rather than being written as carbon matter.
Reverse to carbon dioxide + water → glucose + oxygen, using light energy with chlorophyll present.
Look for these in your work
- Reactants/products are corrected.
- Energy is not treated as a source of new elements.
A report claims that keeping a plant in darkness must give the same continuing photosynthesis rate as in light because water and CO₂ are available. Assess the missing condition without assigning a numerical rate.
Give me a hint
The named materials do not replace the required energy input.
Compare my reasoning
- Water and CO₂ are material reactants.
- The stated photosynthetic process also needs light energy with chlorophyll present.
- Their availability alone does not establish the same photosynthesis rate in darkness; no numerical rate follows from the supplied facts.
Reactants alone do not establish the same rate; the required light-energy condition is missing.
Look for these in your work
- Material and energy requirements are separated.
- No unsupported numerical rate is invented.
Common mistakes
- Reversing photosynthesis into the respiration direction.
- Omitting water or the oxygen product.
- Calling light a carbon-containing reactant.
- Giving a word equation without the light/chlorophyll conditions.
What can you explain now?
Which material reactants form glucose and oxygen in the required photosynthesis description?
Compare with the explanation
Carbon dioxide and water.
The process uses light energy in the presence of chlorophyll.
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Make it stick
State the word equation from memory and explain light, chlorophyll and carbon dioxide as three different roles.
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.