PSLE Science Photosynthesis Keywords for OEQ
Part 2 of 3 of the PSLE Science OEQ Guide: saying the science accurately, topic by topic, and why the everyday phrase gets it wrong.
In plants, "food" and "air" stop meaning what they mean at the dinner table. You know plants make food. You know plants need air. But which food? And which part of the air? Both matter here.
A plant does two things at once. It makes its own food. That is photosynthesis. It also uses oxygen to release energy. That is respiration, and every living thing does it. It's easy to read those as one process. That mix-up is what this topic tends to test.
It helps to hold the two side by side:
| Photosynthesis | Respiration | |
|---|---|---|
| Gas used | Carbon dioxide | Oxygen |
| Gas released | Oxygen | Carbon dioxide |
| When | Only in light | Day and night |
1. "makes food" → "makes glucose, stored as starch"
The plant makes glucose in its leaves during photosynthesis. It stores the extra as starch.
"Makes food" is correct. But a Science answer needs to name the food. During photosynthesis, a plant uses light energy, carbon dioxide and water. From those it makes glucose. It stores the extra as starch.
2. "needs air" → "needs carbon dioxide"
The plant needs carbon dioxide to make food.
Air contains several gases. To make food, a plant needs carbon dioxide. It also needs oxygen for respiration. Naming the gas is what shows which of the two processes you mean.
3. "needs air to breathe" → "needs oxygen for respiration"
Living things, plants included, need oxygen to release energy through respiration.
Living things break down glucose using oxygen. That releases energy. The name for it is respiration. Breathing isn't the same thing. Breathing is moving air in and out. Respiration is what the oxygen is then used for. "Needs oxygen for respiration" names the gas and the process it is used in. That is what the mark is for.
4. "breathes in carbon dioxide, breathes out oxygen" → "photosynthesises and respires at the same time"
The plant photosynthesises. It takes in carbon dioxide and gives out oxygen. At the same time it respires, taking in oxygen and giving out carbon dioxide.
Photosynthesis and respiration are two separate processes. They can happen at the same time. One is not the other in reverse. Photosynthesis stops in the dark. Respiration carries on, day and night. An answer here needs both processes named, along with the gas each one takes in and the gas it gives out.
5. "gets its food from the soil" → "absorbs water and minerals from the soil, and makes its own food"
The plant absorbs water and minerals from the soil through its roots. It makes its own food, glucose, in its leaves through photosynthesis.
The soil gives the plant water and minerals. The plant does not take food from the soil. It uses water, carbon dioxide and sunlight to make its own food in its leaves. A fungus is different. It takes in food from around it.
The swaps
| # | Say this | Why it earns the mark |
|---|---|---|
| 1 | Makes glucose, stored as starch | Names the substance the plant makes |
| 2 | Needs carbon dioxide | Names the gas used to make food |
| 3 | Needs oxygen for respiration | Names the process that releases the energy |
| 4 | Photosynthesises and respires at the same time | Says both processes run at the same time |
| 5 | Absorbs water and minerals, makes its own food | Says the plant makes its food, not takes it from the soil |
No wording is compulsory here. Name the gas you mean. Name the process. Then your own sentence can earn the mark.
If you want to try this on real questions, MCQ.SG marks open-ended questions point by point and gives feedback on every answer.
For more tips on improving your answers—from avoiding common mistakes to using precise science language—explore the complete PSLE Science OEQ Guide.