3.3 Exchange in plants

Question 1

What is the function of xylem tissue?

Answer 1

1. carries dissolved minerals and water up the plant
2. Structural support
3. Food storage

Question 2

What is the structure in roots?

Answer 2

• The vascular bundle is in the centre
• The centre-core is made of xylem tissue
• On the edges of the centre core is the phloem tissue

Question 3

What is the structure in stems?

Answer 3

• vascular bundles are located around the outside
• xylem tissue is found on the inside (closest to the centre of the stem) to help support the plant
• phloem tissue is found on the outside

Question 4

What is the structure in leaves?

Answer 4

• the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line.
• The xylem tissue is found on the upper side of the bundles (closest to the upper epidermis)
• phloem tissue is found on the lower side of the bundles

Question 5

What is the function of the phloem?

Answer 5

• Transport organic compounds (sucrose) from the source to the sink
• The transport can occur up and down the plant

Question 6

What does transpiration refer to?

Answer 6

he loss of water vapour from a plant to its environment by evaporation and diffusion

Question 7

What is an advantage of transpiration?

Answer 7

1. It provides a means of cooling the plant via evaporative cooling
2. The transpiration stream is helpful in the uptake of mineral ions

Question 8

What does transpiration stream mean?

Answer 8

the movement of water from the roots to the leaves

Question 9

What are the key steps to the process of transpiration?

Answer 9

1. Water moves up the xylem - it is under tension and pulled up in the transpiration stream
   —> cohesion between water molecules and adhesion between molecules and the walls of the xylem pulls the water up in one continuous column
   —> it also moves by capillary action and mass flow

Question 10

Explain the cohesion-tension theory

Answer 10

1. Evaporation of water from mesophyll cells in leaf (transpiration) responsible for water potential gradient from root to leaf.
2. This causes negative hydrostatic pressure in xylem (tension)
3. Due to its polar nature water molecules are cohesive.
4. This cohesive force results in the transpiration stream, continuous column of water.

Question 11

What factors affect transpiration?

Answer 11

1. Surface area
2. Concentration gradient
3. Length of diffusion path

Question 12

How does the plant prevent excess water loss?

Answer 12

1. Thick waxy cuticle layer
2. Stomata on cooler underside of the leaf
3. Thick leaves - increased diffusion distance
4. Stomata sunken and in pits

Question 13

What is the Apoplast pathway?

Answer 13

Water moves through the mesophyll cell wall

Question 14

What is the Symplast pathway?

Answer 14

Water moves out of the mesophyll cytoplasm