These questions ask you to identify the materials and products of photosynthesis, trace where the materials come from, and explain what happens to the products. A reliable approach is to separate substances from energy and then follow each substance before and after the reaction.
Build the equation from roles
Start with the word equation: carbon dioxide + water → glucose + oxygen. The substances on the left are reactants, so they are used during the reaction. The substances on the right are products, so they are made. Carbon dioxide diffuses from the air into a leaf through stomata. Water is absorbed from the soil by roots and carried upwards to the leaves.
Light supplies the energy for the change, but it is not a chemical reactant in the word equation. Chlorophyll is the green pigment that absorbs this energy. Photosynthesis transfers light energy into chemical energy stored in glucose; it does not create energy. Without light, photosynthesis cannot continue, even if the leaf is warm and has plenty of water and carbon dioxide.
| Item | Role or immediate fate |
|---|---|
| Carbon dioxide | A reactant from the air |
| Water | A reactant absorbed from soil by roots |
| Glucose | A product used in respiration or changed into other substances |
| Oxygen | A product, most of which diffuses out of the leaf |
| Light and chlorophyll | The energy source and the pigment that absorbs it |
After the reaction, keep glucose separate from the substances made from it. A plant may convert glucose into insoluble starch for storage, use glucose in respiration to release usable energy, or join glucose units to make cellulose for cell walls. Starch and cellulose are therefore later fates of glucose, not direct products of photosynthesis.
Common wrong turns
| Tempting route | How to reject it |
|---|---|
| Reverse the photosynthesis equation or put soil minerals on the left | Ask whether sugar is being built. Photosynthesis consumes carbon dioxide and water; mineral ions have other roles in growth. |
| Treat heat or chemical energy from soil as the energy source | Only light provides the energy captured by chlorophyll. Warmth and substances from soil cannot replace it. |
| Use chloroplast when the question asks for the green pigment | A chloroplast is a cell structure; chlorophyll is the light-absorbing pigment inside it. |
| Send carbon dioxide through roots or take water through the leaf surface | Trace the routes separately: gas enters from air through stomata, while water travels from soil through roots. |
| Call starch a direct product or use it as cell-wall material | Glucose comes first. It can later become starch for storage or cellulose for structure. |
| Say that a plant creates energy or receives it from soil | Energy is transferred from light into glucose and can later be released by respiration. |
Try it
A plant converts some of the glucose it has just made into an insoluble substance and stores it in the leaf. Which substance is that?
- Cellulose
- Protein
- Chlorophyll
- Starch
What happens to most of the oxygen a leaf makes while it is photosynthesising?
- It is taken back in as one of the raw materials
- It passes out of the leaf into the air as a waste product
- It is stored in the leaf until the plant needs it at night
- It is carried down to the roots and used there
A leaf makes glucose in the light. What does the plant do with the part of that glucose it neither stores nor builds into structures?
- It respires the glucose to release energy for its life processes
- It passes the glucose back out through the stomata as a gas
- It saves the glucose until dark, because it can only respire at night
- It converts the glucose into chlorophyll to keep its leaves green