section 2 (6) describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissue

Question 1

What does Xylem do?

Answer 1

moves water and dissolved mineral ions UP the plant (roots to leaves)

Question 2

Outline the transpiration stream theory

Answer 2

• also known as the cohesion-adhesion theory
• water enters the roots by osmosis (mineral ions enter the roots by diffusion and active transport)
• Root pressure, which is increased by the constant inflow of water and mineral ions forces the solution up the plant
• transpiration creates a concentration gradient across the leaves and hence transports materials across the leaves without xylem
• dissolved minerals are carried by the transpiration stream and a transported across the leaves by active transport

Question 3

What is the transpiration stream?

Answer 3

The flow of water through a plant, from the roots to the leaves, via the xylem vessels.

Question 4

What is the structure of Xylem vessels?

Answer 4

• hollow and narrow

- do not provide much resistance to water flow

Question 5

What are cohesive forces?

Answer 5

• the force of attraction between water molecules, causing them to form a continuous stream

Question 6

What are adhesive forces?

Answer 6

• the force of attraction between the water molecules and the xylem vessels

Question 7

What does the combination of adhesive and cohesive forces in xylem vessels result in?

Answer 7

• results in the capillary action of water

- water column does not break when water is pulled up

Question 8

What type of transport does phloem facilitate?

Answer 8

movement of sugars produced by photosynthesis DOWN the plant (leaves to rest of plant/roots)

Question 9

What is the active flow theory?

Answer 9

• also known as the source-path-sink theory
• a process that requires active transport, driven by osmotic pressure
• involves loading sugar into one end of phloem and unloading into tissues at the other end
• ‘source’→ ‘sink’