By the end of this lesson
- Recall the word equation for photosynthesis.
- Write and check the balanced symbol equation.
- Distinguish reactants, products and reaction conditions.
BIG QUESTION
What does a balanced equation reveal about photosynthesis—and what important biology does it leave out?
The equation conserves matter and names overall inputs and outputs; it compresses many enzyme-controlled steps into one line.
The word equation names substances
carbon dioxide + water → glucose + oxygen
Light and chlorophyll are written over or near the arrow as conditions. They are not placed on the reactant side because they are not consumed as materials in the overall equation.
The symbol equation conserves atoms
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The coefficients balance six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms on each side. Change coefficients, never the small numbers inside correct formulae.
An overall equation is a model
Photosynthesis is not one collision that instantly makes glucose. It includes light-dependent and carbon-fixation reactions controlled by enzymes and membranes in chloroplasts.
The equation is useful for matter accounting and predictions, but it does not show intermediate molecules, timing, cellular compartments or the source details of the released oxygen.
Direction and conditions matter
The photosynthesis equation resembles the reverse of aerobic respiration, but the pathways and energy transfers are not simply the same reaction running backwards.
Photosynthesis requires a continuing energy input. The arrow should therefore not be treated as evidence that products form under any conditions.
Reading each equation feature
Each notation carries a different kind of information.
| Feature | Correct interpretation | Common error |
|---|---|---|
| Coefficients | Relative numbers of molecules or moles. | Changing subscripts to balance atoms. |
| Arrow | Overall conversion from reactants to products. | Treating it as one-step chemistry. |
| Light | Energy input or condition. | Calling light a material reactant. |
| Chlorophyll | Light-absorbing pigment. | Writing chlorophyll as a consumed reactant. |
Language that earns precision
- reactant
- A starting substance changed during a reaction.
- product
- A substance formed by a reaction.
- coefficient
- A number placed before a formula to balance relative amounts.
- balanced equation
- An equation containing equal numbers of each atom on both sides.
Common misconceptions
Claim: The equation is balanced by changing CO₂ to CO₃.
Correction: Chemical formulae stay fixed; coefficients are changed.
Claim: Chlorophyll is a reactant.
Correction: It absorbs light and is not consumed in the overall equation.
Claim: The equation shows one reaction step.
Correction: It summarises many linked reactions.
Claim: Photosynthesis is simply respiration backwards.
Correction: The overall matter relationship is reversed, but mechanisms and energy pathways differ.
Match the answer to the command word
Write the substances in the correct order and spell them accurately.
Count C, H and O atoms on both sides after adding coefficients.
Keep conditions separate from material reactants.
Recall, apply, then justify
Write the word equation for photosynthesis.
Hint
Two reactants and two products.
Suggested marking points
- Carbon dioxide + water.
- Glucose + oxygen in the correct product positions.
Write the balanced symbol equation for photosynthesis.
Hint
Use coefficients of six around one glucose molecule.
Suggested marking points
- 6CO₂ + 6H₂O.
- C₆H₁₂O₆ + 6O₂.
Explain why light is not written as a reactant.
Hint
Distinguish energy from matter.
Suggested marking points
- Light supplies energy rather than atoms.
- It is not consumed as a material substance in the overall equation.
Use atom counts to verify that 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ is balanced, then state one limitation of the equation.
Planning hint
Count each element independently before evaluating the model.
What a strong design includes
- Both sides contain 6 carbon atoms.
- Both sides contain 12 hydrogen atoms.
- Both sides contain 18 oxygen atoms.
- The equation omits intermediate reactions, compartments or detailed energy carriers.