Transport In Xylem

Question 1

Why are xylem vessels known as mass transport systems?

Answer 1

They transport water and ions over large distances

Question 2

Describe and explain the structure of xylem vessels?

Answer 2

• dead tissue
• no organelles so an unobstructed flow
• hollow tubes for minimal resistance to flow
• cell wall is strengthened by lignin which is impermeable to water and keeps the tubes rigid
• no end walls so flow is uninterrupted
• have bordered pits which allow water and ions to move laterally

Question 3

Explain the Cohesion-Tension theory.

Answer 3

1. Water evaporates from the leaves by transpiration
2. This lowers water potential of the cells in the leaves
3. This draws water up from the xylem into the leaf along a water potential gradient causing tension
4. Water is pulled upwards in a continuous column as water molecules are joined together by cohesive hydrogen bonds
5. Water column is maintained by adhesion between water molecules and the walls of the xylem

Question 4

Why does the rate of water movement change with the rate of transpiration?

Answer 4

If transpiration rate increases,
- rate of water movement increases as the absorption of water from the roots increases

Question 5

How does light intensity affect the rate of transpiration?

Answer 5

1. During the day, stomata are open to allow carbon dioxide to enter for photosynthesis so, water evaporates from the leaf faster through the stomata.
2. At night no photosynthesis occurs so stomata are closed to prevent water loss so, less water evaporates from the leaf through the stomata.

Question 6

How does air movement affect the rate of transpiration?

Answer 6

1. More air movement removes water from leaves which lowers water potential outside of the leaf , so more water leaves the leaf down the water potential gradient.
2. Still air movement means less water is removed from the leaves so no water potential gradient is firmed, so the rate of transpiration is less.

Question 7

How does temperature affect the rate of transpiration?

Answer 7

1. At higher temperatures, water molecules have more kinetic energy so there is a higher rate of transpiration.
2. At lower temperatures, water molecules have lower kinetic energy so the rate of water movement by osmosis is lower

Question 8

How does humidity affect the rate of transpiration?

Answer 8

1. Lower humidity means less water vapour outside of the leaf so the water potential gradient is maintained so the rate of transpiration is high.
2. Higher humidity means there is a higher concentration of water vapour outside of the leaf which decreases the water potential gradient so the rate of transpiration is lower.