PSLE Science · Primary 5 and 6
Plant Transport and Photosynthesis
Plants cannot move around to find food, so they make their own. This topic covers how a plant carries water and food around its body, and how it makes food by photosynthesis. In the primary syllabus, plant transport belongs to the Systems theme and photosynthesis belongs to the Energy theme, and PSLE questions often combine the two. You will need these ideas again for food chains and energy conversion, because the energy in almost all food starts as light energy captured by plants.
What the syllabus expects
- State the functions of the roots, stem and leaves of a plant
- Describe how water and mineral salts travel from the roots to the leaves in the water-carrying tubes
- Describe how food made in the leaves travels to other parts of the plant in the food-carrying tubes
- State what a plant needs for photosynthesis and what it produces
- Explain experiments on photosynthesis, such as covering part of a leaf and counting bubbles from a water plant
- Explain the results of ring-barking and coloured-water experiments
Parts of a plant and their jobs
Each part of a plant has a job. The roots hold the plant firmly in the soil and take in water and mineral salts. Tiny root hairs help the roots to take in more water. The stem holds the plant upright and holds the leaves up so that they can get light. It also contains the tubes that carry water and food between the roots and the leaves. The leaves make food for the plant by photosynthesis. They are usually broad and flat, so they can trap a lot of light. Some plants store extra food in their roots, such as the carrot, or in their stems, such as sugar cane. Leaves also have many tiny openings called stomata, mostly on the underside. Gases such as carbon dioxide and oxygen pass in and out through them, and water vapour escapes through them.
Water-carrying tubes and food-carrying tubes
A plant has two kinds of transport tubes. Water-carrying tubes carry water and dissolved mineral salts from the roots, up through the stem, to the leaves, flowers and other parts. Food-carrying tubes carry food made in the leaves to all the other parts of the plant, including the roots, flowers, fruits and growing tips, so food can travel both upwards and downwards. In a woody stem, the food-carrying tubes are in the outer layer, just under the bark, and the water-carrying tubes are further inside. You can see the water-carrying tubes by standing a celery stalk or a white flower in water coloured with dye. After a few hours the dye appears in the leaves or petals, and a cut across the stem shows coloured dots where the water-carrying tubes are. Water evaporates from the leaves through the stomata, and this helps more water to be drawn up from the roots.
Photosynthesis
Photosynthesis is the process by which green plants make their own food. Chlorophyll, the green substance in leaves, traps light energy. The plant uses this light energy to make sugar (glucose) from water and carbon dioxide, and oxygen is given out. In words: water + carbon dioxide, with light energy and chlorophyll, gives sugar + oxygen. Water comes from the soil through the roots and the water-carrying tubes. Carbon dioxide comes from the air and enters the leaves through the stomata. Some of the sugar is used by the plant for energy and growth, some is carried to other parts in the food-carrying tubes, and some is stored as starch. During photosynthesis, light energy is converted into chemical potential energy stored in the food. Animals, including humans, depend on plants for food and for the oxygen that plants give out.
Experiments on photosynthesis
Two experiments appear often. In the first, a leaf is tested with iodine solution, which turns blue-black when starch is present and stays brown when there is no starch. A plant kept in the dark for two days uses up the starch in its leaves. If part of one leaf is then covered with black paper and the plant is placed in sunlight for some hours, only the uncovered part turns blue-black, which shows that light is needed to make food. In the second experiment, a water plant such as hydrilla is placed in water with a lamp beside it, and bubbles of oxygen are given off. When the lamp is moved closer, the light is brighter, more bubbles are given off per minute, and so the plant is photosynthesising faster. For a fair test, keep the same plant, the same amount of water and the same water temperature.
Worked example
Ahmad removed a ring of bark, including the outer layer of the stem, from the main stem of a plant. After a few weeks, the part of the stem just above the ring had become swollen, and the leaves stayed green and did not wilt. (a) Explain why the stem just above the ring became swollen. (b) Explain why the leaves did not wilt. (c) Predict what would happen to the plant after a long time, and explain.
- Identify what was removed: the food-carrying tubes are in the outer layer of the stem, so removing the ring cut them. The water-carrying tubes are further inside, so they were not removed.
- For (a), food made in the leaves travels down the stem in the food-carrying tubes. At the ring, the tubes are missing, so the food cannot pass. It collects just above the ring and the stem swells.
- For (b), the water-carrying tubes are still whole, so water from the roots still reaches the leaves. The leaves get enough water, so they do not wilt.
- For (c), the roots are below the ring, so food from the leaves can no longer reach them. Without food, the root cells cannot get energy to stay alive.
- Link to the whole plant: when the roots die, they can no longer take in water, so the whole plant will die.
Answer: (a) The food-carrying tubes were removed, so food from the leaves could not travel down past the ring and collected above it. (b) The water-carrying tubes were not removed, so water still reached the leaves. (c) The roots would die because no food reaches them, and then the plant would die because water can no longer be taken in.
Common mistakes
- Saying plants get their food from the soil. Plants make their own food by photosynthesis. The soil supplies water and mineral salts.
- Mixing up the two tubes. Water-carrying tubes carry water and mineral salts up from the roots. Food-carrying tubes carry food from the leaves to all parts, up and down.
- Saying plants only give out oxygen. Plants take in oxygen and give out carbon dioxide when they respire, day and night. In light, photosynthesis is faster, so more oxygen is given out overall.
- Saying chlorophyll makes the food by itself. Chlorophyll traps light energy, and the plant still needs water and carbon dioxide to make sugar.
- Forgetting that a lamp also gives off heat in the bubble experiment. If the lamp is very close, the water warms up, so the temperature should be checked and kept the same for a fair test.
Exam tips
- Use the exact terms 'water-carrying tubes' and 'food-carrying tubes', and say which way each substance travels.
- When you explain an experiment, state what was changed, what happened, and link it to photosynthesis, for example 'brighter light, so the plant photosynthesised faster and gave off more oxygen'.
- For ring-barking questions, say what happens above the ring, below the ring and to the leaves, each with its reason.
Practice questions
1. Easy: Name the things a plant needs for photosynthesis and the two things it produces.
Show answer
It needs light energy, water, carbon dioxide and chlorophyll (in its green parts). It produces sugar (food) and oxygen.
2. Medium: A white flower was placed in blue-coloured water. After six hours, its petals had turned blue. Explain why.
Show answer
The blue water was taken up by the water-carrying tubes in the stem and carried up to the petals. The dye was left in the petals, so they turned blue. This shows that water travels up the stem in the water-carrying tubes.
3. Harder: A water plant was placed 10 cm, 20 cm and 30 cm from a lamp. It gave off 24, 15 and 8 bubbles per minute. What can you conclude, and why can the number of bubbles be used to measure photosynthesis?
Show answer
The nearer the lamp, the more bubbles per minute, so the brighter the light, the faster the plant photosynthesises. The bubbles are oxygen, which is given out during photosynthesis, so more bubbles per minute means faster photosynthesis.
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