4.2.3.2 Plant organ system

What is translocation?

The movement of food substances made in the leaves up or down the phloem, for use immediately or storage

What is transpiration?

The loss of water or water vapour from the leaves and stems of the plant

How does the transpiration stream work?

Water evaporates at the open stomata and as water molecules are attracted to each other, when some molecules leave the plant the rest are pulled up through the xylem which results in more water being take up from the soul resulting in a continuous transpiration stream

What factors affect the rate of transpiration?

Increase in temperature, increase in humidity, increased wind, increase in light intensity

How does an increase in temperature affect the rate of transpiration?

The molecules move faster which results in evaporation happening at a faster rate so the rate of transpiration increases

How does an increase in humidity affect the rate of transpiration?

If the humidity is high there will be a reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion and decreasing the rate of transpiration

How does increased wind affect the rate of transpiration?

If more air is moving away from the leaf because of it being blown away, then the concentration of water vapour surrounding the leaf will be lower meaning there will be a steeper concentration gradient resulting in diffusion happening faster, increasing the rate of transpiration

How does an increase in light intensity affect the rate of transpiration?

The rate of photosynthesis will increase so more stomata open to allow gas exchange to happen, meaning more water can evaporate leading to an increased rate of transpiration

How are guard cells adapted to do their role?

When lots of water is available to the plant, the cells fill and change shape, opening stomata which allows gases to be exchanged and more water to leave the plant via evaporation.

Why is more stomata found on the bottom of the leaf?

It allows gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler