Transpiration and Translocation Flashcards
What is the benefit of xylem tissue being dead?
It means there is little resistance to water movement.
Describe the structure of a xylem.
Made of dead cells joined end to end, forming a hollow tube. Strengthened with lignin.
Explain cohesion - tension theory.
The theory that states movement of water in plants caused by a continuous pull of water up from the roots and out to the leaves.
Define cohesion.
Water molecules stick to each other due to attraction formed by hydrogen bonds.
Define adhesion.
Water molecules stick to other things (such as the xylem vessel).
Describe how cohesion/tension work in a plant.
Explain in terms of pressure.
Water molecules stick to the xylem vessel resisting the force of gravity. As water is constantly being lost in evaporation from leaves, this draws more water up from the roots, through the xylem. This forms a continuous unbroken column called the transpiration pull.
Transpiration puts the xylem under a negative pressure.
Describe how water moves through leaves.
Water evaporates from mesophyll cells and lands in air spaces between leaves. This lowers the water potential in cells and raises it in the air spaces. In turn, cells take water from other cells by osmosis to raise their water potential.
How is water lost from the plant through stomata?
Humidity of atmosphere is usually less than in the plant so water potential lower in atmosphere. This draws water out of the plant by diffusion through the stomata in evaporation.
How does increased wind speed affect transpiration?
Causes air molecules to be blown away from leaves, resulting in a steeper concentration gradient and a faster diffusion rate.
How does increased temperature affect transpiration?
Increase in temperature raises the kinetic energy of particles, so they move faster.
How does increased humidity affect transpiration?
Flattens the concentration gradient, so diffusion is slower.
How can sunlight lead to water loss?
Sunlight means more photosynthesis. This means there are more waste gases that need to be lost from the plant, so stomata open. But if more stomata are open then more water can be lost.
Give 3 reasons why plants need water.
As a metabolite
To keep leaves cool
For use in hydrolysis
How can changes in tree trunk diameter support cohesion-tension theory?
Explain what would happen if transpiration is high and low.
When transpiration is at its highest, there is more tension in the xylem. This pulls the walls of the xylem inwards and causing the tree trunk to shrink.
When transpiration is low, there is less tension in the xylem. Therefore, the walls push outwards, causing the diameter to increase in length.
What happens if the xylem column breaks?
Air enters, so water molecules can no longer form a tight stick to the walls.
As water does not leak out when they xylem column breaks, how does this support cohesion tension theory?
Compare this to if pressure were to maintain the transpiration pull.
Instead air is drawn in.
If the system was under pressure instead, then water would leak.
