Transport in Plants- Xylem

What does the xylem do?

Transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves.

What does the phloem do?

Transports organic substances like sugars both up and down the plant.

What is the structure of xylem vessels?

• Xylem vessels are part of the xylem tissue

• They are very long, tube-like structures formed from dead cells joined end to end

• There are no end walls, making an uninterrupted tube that allows water to pass up through the middle easily.

What helps water move up a plant against the force of gravity?

• Cohesion-tension theory

• Water evaporates from the leaves at the ‘top’ of the xylem which creates tension, which pulls more water into the leaf.

• Water molecules are cohesive, so when some are pulled into the leaf others follow. This means the whole column of water in the xylem moves upwards

What is transpiration?

• Transpiration is the evaporation of water from the plant’s surface, especially in the leaves

• Water evaporates from the moist cell walls and accumulates in the spaces between the cells in the leaf.

• When the stomata open, it moves out of the leaf down the concentration gradient

How does light affect transpiration rate?

• The lighter it is the faster the transpiration rate

• This is because the stomata open when it gets light to let in CO₂ for photosynthesis.

• When it is dark, the stomata are usually closed, so there is little transpiration

How does temperature affect transpiration rate?

• The higher the temp the faster the transpiration rate

• Warmer water molecules have more energy so they evaporate from the cells in the leaf faster

• This increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

How does humidity affect transpiration rate?

• The lower the humidity, the faster the transpiration rate

• If the air around the plant is dry, the concentration gradient between the leaf and the air is increased, which increases transpiration.

How does wind affect transpiration rate?

• The windier it is, the faster the transpiration rate.

• Lots of air movement blows away water molecules from the stomata.

• This increases the concentration gradient, which increases the rate of transpiration.