In photosynthesis, a plant uses carbon dioxide and water as raw materials and light energy as the energy source, and makes glucose and oxygen. The word equation is: carbon dioxide + water → glucose + oxygen, with light energy absorbed by chlorophyll.
This skill opens plant nutrition. It returns in every question that describes a plant experiment, because you must first know what is going in and what is coming out.
What goes in, what comes out, and where does it happen?
The reaction takes place in chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs light energy, and that energy drives the reaction.
The inputs arrive by different routes:
| Item | Direction | Route |
|---|---|---|
| Carbon dioxide | In | Diffuses from the air through the stomata into the leaf |
| Water | In | Absorbed by the roots, carried up in the xylem to the leaf |
| Light energy | In | Absorbed by chlorophyll in the chloroplasts |
| Glucose | Made | Used, stored as starch or moved as sucrose |
| Oxygen | Out | Diffuses out through the stomata (some is used in respiration) |
The balanced symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
You can check that it balances by counting atoms. Carbon: 6 on the left, 6 on the right.
Hydrogen: 12 on the left (6 × 2), 12 on the right. Oxygen: 18 on the left (12 + 6), 18 on the right (6 + 12).
Worked example
Question: A student says, “During photosynthesis the leaf takes in glucose and gives out carbon dioxide.” Correct the statement, then say how many oxygen molecules are released when 3 molecules of glucose are made.
Step 1, sort the substances. Glucose is a product, so it is made, not taken in. Carbon dioxide is a raw material, so it is taken in, not given out.
Step 2, rewrite. “During photosynthesis the leaf takes in carbon dioxide and gives out oxygen, and it makes glucose.”
Step 3, use the equation ratio. One glucose molecule is made with 6 oxygen molecules released. So 3 glucose molecules go with 3 × 6 = 18 oxygen molecules.
Step 4, check with the raw materials. Making 3 glucose needs 3 × 6 = 18 carbon dioxide molecules. Oxygen released equals carbon dioxide used, which is consistent with the equation (6 and 6).
Answer: The corrected statement is in step 2, and 18 oxygen molecules are released.
The mistake to watch for
A common slip is to mix up photosynthesis and respiration.
Mistaken answer: “Plants take in carbon dioxide in the day and oxygen at night, so photosynthesis happens in the day and respiration at night.”
This treats the two processes as shifts. Respiration never stops in a living plant cell.
The correction is that both processes run in the day, and only respiration runs at night. In bright light photosynthesis is faster, so the net result is oxygen out and carbon dioxide in. In the dark the net result reverses because only respiration is left. Always ask “which process, and is the question about net gas exchange?”
Check yourself
Try these first, then open each answer.
1. Name the two raw materials of photosynthesis and the two products.
Show answer
Raw materials: carbon dioxide and water. Products: glucose and oxygen. Light energy is the energy source, not a raw material.
2. Which part of a plant cell absorbs light, and which pigment does the absorbing?
Show answer
The chloroplast contains the pigment chlorophyll, which absorbs the light energy.
3. A leaf makes 2 glucose molecules. How many carbon dioxide molecules does it use?
Show answer
The equation uses 6 carbon dioxide per glucose, so 2 × 6 = 12 carbon dioxide molecules.
Where this leads next
Once inputs and products are secure, move on to reading a limiting-factor graph, where you decide which input is holding the rate back. You can then check your recall with the plant nutrition practice set.
Some students know the equation but freeze when a question hides it inside an experiment. A teacher in online one-to-one Biology tuition can practise that with you using fresh experiment questions.