Topic 3---B: More exchange and Transport Systems- 6. Transport in plants (xylem)

Question 1

What does the xylem tissue do?

Answer 1

• transport water and mineral ions in solution
• these substances move up the plant from the rots to the leaves

Question 2

What does the phloem tissue do?

Answer 2

• transports organic substances like sugars (also in solution) both up and down the plant

Question 3

Structure of the xylem

Answer 3

• xylem vessels are part of the xylem tissue that actually transports the water and mineral ions.
• xylem vessels are very long, tube like structured formed from dead cells joined end to end
• there are no end walls on these cells making an uninterrupted tube that allows water to pass up through the middle easily.

Question 4

Water movement up a plant

Answer 4

• water moves up a plant against the force of gravity from roots to leaves

Question 5

Cohesion-tension theory

Answer 5

1. Water evaporates from the leaves at the top of the xylem (transpiration)
2. This creates more tension (suction) which pulls more water into the leaf
3. Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow so this means the whole column of water in the xylem from the leaves down to the roots moves upwards
4. Water then enters the stem through the roots

Question 6

What is transpiration ?

Answer 6

evaporation of water from a plants surface (especially the leaves)
water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf
when the stomata open it moves out of the leaf down a water potential gradient as there is more water inside the leaf than in the air ourside

Question 7

Factors affecting transpiration rate

Answer 7

• light intensity
• temperature
• humidity
• wind

Question 8

Light intensity

Answer 8

• the lighter it is the Faster the transpiration rate
• because the stomata open when its light to let in carbon dioxide for photosynthesis and when its dark the stomata are usually closed so there is little transpiration

Question 9

Temperature

Answer 9

• the higher the temperature the Faster the transpiration rate
• warmer water molecules have more energy so they evaporate from the cells inside the leaf faster
• this increases the water potential Gradient between the inside and outside of the leaf making water diffuse out of the leaf faster

Question 10

Humidity

Answer 10

• the lower the humidity the Faster the rate of transpiration
• if the air around the plant is dry the water potential Gradient between the leaf and air is increased which increases transpiration rate

Question 11

Wind

Answer 11

• the windier it is the Faster the transpiration rate
• lots of air movement blows away water molecules from around the stomata
• this increases the water potential Gradient which increases the rate of transpiration