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B2 S) Transpiration
B2 S) Transpiration
Transpiration is the loss of water from a plant through the stomata by evaporation and diffusion. Most of the transpiration happens in the leaves of plants because the leaves are adapted for photosynthesis. The bottoms of leaves have tiny holes that are known as stomata. The stomata are responsible for gas exchange. Carbon dioxide enters a leaf through the stomata where it is used for photosynthesis. Oxygen exits a leaf through the stomata – oxygen is one of the products of photosynthesis. Water also leaves a leaf through the stomata (transpiration). All of these molecules move due to diffusion, which is the net movement of molecules from an area of high concentration to an area of low concentration. A diagram of a leaf is shown below.
All of the stomata are surrounded by two guard cells. On the above diagram, you can see that there is a higher concentration of water inside the leaf compared to outside the leaf. This means that water will diffuse from inside the leaf to outside the leaf.
The evaporation and diffusion of water out of leaves creates a shortage of water in the leaves. This results in water being drawn up the xylem tubes to replace the water that has been lost in the leaves by diffusion through the stomata. Water is drawn up through all of the xylem tubes to the leaves through the stems and the roots; you can think of the xylem tubes like a straw. The water that is taken up the xylem tubes contains mineral ions. This process of leaves being constantly supplied with water and mineral ions is known as a transpiration stream. Transpiration streams allow water to overcome the forces of gravity.
The evaporation and diffusion of water out of leaves creates a shortage of water in the leaves. This results in water being drawn up the xylem tubes to replace the water that has been lost in the leaves by diffusion through the stomata. Water is drawn up through all of the xylem tubes to the leaves through the stems and the roots; you can think of the xylem tubes like a straw. The water that is taken up the xylem tubes contains mineral ions. This process of leaves being constantly supplied with water and mineral ions is known as a transpiration stream. Transpiration streams allow water to overcome the forces of gravity.
There are many benefits of transpiration, such as:
- The obvious one is that the leaves have a constant supply of water, which is needed for photosynthesis (the equation for photosynthesis is: carbon dioxide + water –> glucose + oxygen). However, only about 5% of the water that is taken up via transpiration is used for photosynthesis – the rest exits the leaves through the stomata and enters the atmosphere.
- The water that is taken through the xylem tubes contains mineral ions, which the plant needs for healthy growth and development. Nitrate ions are taken up through transpiration streams.
- Transpiration keeps plants cool. 80% of the cooling effect of shade from a tree is because of transpiration.
- Transpiration maintains the turgid pressure in plant cells, which means that the leaves and stems of the plant are upright and firm. Turgidity is important so that the plant is able to gain sufficient quantities of light and therefore able to effectively photosynthesise. Turgidity also ensures that the guard cells around the stomata work effectively ensuring that enough carbon dioxide is taken in for photosynthesis.