🌿 Plant Transport System
1. Plants have two sets of transport tubes inside the stem.
2. Food-carrying tubes: transport food made by the leaves to all parts of the plant. Also known as the phloem tubes.
3. Water-carrying tubes: transport water absorbed by the roots to all parts of the plant. Also known as the xylem tubes.
| Tube | Also known as | What it transports | Direction |
|---|---|---|---|
| Food-carrying tube | Phloem | Food (sugar) made by the leaves | Up or down |
| Water-carrying tube | Xylem | Water and mineral salts absorbed by the roots | Up only |
Food-carrying tubes carry food made by the leaves. Water-carrying tubes carry water and mineral salts taken in by the roots. Both sets of tubes are found in the stem.
1. Leaves make food in the form of sugar during photosynthesis.
2. Some of the sugars are used by the plant for growth and other life processes.
3. Excess sugars are transported to other plant parts such as the fruit, stem and roots for storage.
4. The excess sugars are usually converted into starch.
5. Sugars from the leaves may move up or down through the stem.
Food is the only thing that moves in two directions — up to the growing parts (young leaves, flowers, fruit) and down to the roots for storage as starch.
1. Water is absorbed by the roots, and the water-carrying tubes transport water to all parts of the plant.
2. Also known as the xylem tubes.
3. Mineral salts are dissolved in water and are also transported through the water-carrying tubes.
4. Water and mineral salts are transported up the plant from the roots.
5. Excess water evaporates and escapes from a plant through the stomata found on its leaves.
Water and mineral salts move in one direction only — upwards from the roots. Mineral salts must be dissolved in water to be transported.
Setup: A ring of food-carrying tubes is removed from the stem. Leaf X is attached above the cut.
1. After a few days, Leaf X remained healthy.
- The food-carrying tubes were removed but not the water-carrying tubes.
- Water absorbed by the roots could be transported to Leaf X through the water-carrying tubes.
- Hence, Leaf X could make food through photosynthesis, causing it to remain healthy.
2. After a few days, the part above the cut became swollen.
- Without the food-carrying tubes, food made by the leaves through photosynthesis could not be transported to the roots and accumulated above the cut, causing the part above the cut to become swollen.
3. After a few weeks, the plant died.
- The cut removed the food-carrying tubes of the plant.
- The food made by the leaves through photosynthesis could not be transported below the cut to the roots, causing them to die from the lack of food.
- When the roots died, water could not be absorbed by the roots, causing the plant to die from the lack of water.
Always follow the chain: no food to the roots → roots die → no water absorbed → plant dies. Markers look for both the food and the water link.
Setup: A ring of water-carrying tubes is removed from the stem. Leaf X is attached above the cut and Leaf Y is attached below the cut.
1. After a few days, Leaf X died.
- Without the water-carrying tubes, water absorbed by the roots could not be transported to the leaves above the cut.
- Without water, Leaf X could not make food through photosynthesis, causing it to die from the lack of water and food.
2. After a few days, Leaf Y remained healthy.
- Water absorbed by the roots could be transported to Leaf Y through the water-carrying tubes.
- Leaf Y could make food through photosynthesis, causing it to remain healthy.
The leaf above the cut always suffers first, because water comes up from the roots. Compare this with Experiment A, where the leaf above the cut stayed healthy and it was the roots that starved.
1. Leaves have tiny openings called stomata.
2. They allow gaseous exchange between the plant and the surrounding air to occur.
3. Water loss: water vapour escapes through the stomata.
4. Each stoma is opened and closed by two guard cells.
Stomata are found mainly on the leaves. They let gases in and out, and they are also the openings through which water vapour escapes (water loss).
| In the day (open stoma) | At night (closed stoma) |
|---|---|
| In the presence of sunlight, the plant undergoes photosynthesis. | When sunlight is absent, photosynthesis cannot take place. |
| The stomata open to allow gaseous exchange to take place. | The stomata close to reduce the amount of water vapour lost through the stomata, leaving a small gap for gaseous exchange. |
| To undergo photosynthesis, carbon dioxide is taken in while oxygen is given out. | To undergo respiration, oxygen is taken in while carbon dioxide is given out. |
| To undergo respiration, oxygen is taken in while carbon dioxide is given out. |
Day = open stoma: photosynthesis needs carbon dioxide in, and oxygen leaves as a waste gas. Night = closed stoma: no photosynthesis, so the plant closes its stomata to save water — only respiration continues.