Transpiration: is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by the diffusion of water vapour through the stomata.
- The main force that draws water from the soil and through the plant is caused by this.
- Water evaporates from the leaves and causes a kind of ‘suction’, which pulls water up the stem.
- The water travels up the xylem vessels in the vascular bundles and this flow of water is called the transpiration stream.
- Root → Stem → Leaf
Factors affecting the rate of transpiration:
Describe how water vapour loss is related to cell surfaces, air spaces and stomata:
- Transpiration is the loss of water vapour from the leaf;
- Water in the mesophyll cells form a thin layer on their surfaces;
- The water evaporates into the air spaces in the spongy mesophyll;
- This creates a high concentration of water molecules in the air spaces.
- Water vapour diffuses out of the leaf into the surrounding air, through the stomata, by diffusion.
The mechanism of water uptake and movement in terms of transpiration producing a tension (“pull”) from above, creating a water potential gradient in the xylem, drawing cohesive water molecules up the plant:
Mechanism of water uptake:
- Water enters root hair cells by osmosis (as the water potential in the soil surrounding the root is higher than in the cell);
- As the water enters the cell, its water potential becomes higher than in the cell next to it, e.g. in the cortex;
- So the water moves by osmosis, into the next cell;
- This process is repeated until water reaches the xylem.
Mechanism of water movement through a plant:
- Transpiration continuously removes water from the leaf;
- Thus water is constantly being taken from the top of the xylem vessels, to supply the cells in the leaves;
- This reduces the effective pressure at the top of the xylem vessels;
- This creates a transpiration stream or ‘pull’, pulling water up;
- Water molecules have a strong tendency to stick together. This is called cohesion;
- When the water is ‘pulled’ up the xylem vessels, the whole column of water stays together;
- Roots also produce a root pressure, forcing water up the xylem vessels.
Occurs when the transpiration rate is faster than the rate of water absorption. The amount of water in the plant keeps on decreasing. The water content of cells decreases and cells turn from turgid to flaccid. The leaves shrink and the plant will eventually die.