transport in plants

Question 1

what is the prime gas exchange surface in a leaf and why

Answer 1

the spongy mesophyll layer becasue it has large air spaces and thin walls. It is in close contact with pores and stomata which gases enter and leave via diffusion down the concentration gradient.

Question 2

How does the leaf minimise water loss ?

Answer 2

thick waxy cuticle on epidermis
action of guard cells can close the stomata

Question 3

Describe movement of water though the leaf

Answer 3

1. When stomata is open water vapour diffuses from air spaces out the stomata down the gradient - transpiration
2. To replace this water evaporates from the walls of mesophyll cells into air spaces forming water vapour
3. The water in the mesophyll cells is replaced by water in the xylem

Question 4

what are the 4 structural features of xylem

Answer 4

1. contains lignin - provides strength and makes them waterproof
2. dead cells - hollow lumen no cytoplasm little resistance to mass flow
3. contain tiny holes- if a vessel becomes blocked the water can be diverted laterally
   4.lose their end walls - continuous column

Question 5

suggest 2 advantages of the plant having a spiral structure

Answer 5

used less material and therefore less wasteful
allows xylem to be flexible/ have a lower mass

Question 6

describe cohesion tension theory

Answer 6

1. water molecules are attached by weak hydrogen bonds (cohesion) and attach to the wall (adhesion)
2. as transpiration occurs through open stomata water is evaporated
   3.This creates a low pressure at the top of the xylem
   4: water in the xylem is under tension and is pulled up toward leaves
3. continuous columns of water are maintained due to cohesion between water molecules
4. adhesion created an inward pull on vessel walls causing a decrease in diameter.

Question 7

how are xerophytic plants adapted to minimise water loss

Answer 7

thick wavy cuticle - increased length of diffusion pathway
stomata sunken in pits - held above stomatal pore reducing WP gradient
hairs on lower epidermis of leaf -water vapour trapped between hairs
trapped humid air with high water potential - trapped within rolled lead reduces WP gradient

Question 8

describe the potometer procedure

Answer 8

Procedure:
1. The potometer was immersed in water to remove air bubbles. 2. The screw clip was opened to fill the capillary tube with water.
3. The branch of the plant was cut in the water.
4. The end of branch was pushed through the hole in the rubber stopper under water.
5. A beaker of water was placed at the end of the capillary tube.
6. The screw clip was closed and the potometer was removed from the water.
7. The surface of the leaves and stems was wiped with tissue paper.
8. Vaseline was spread around every joint to ensure that they are airtight.
9. The potometer was left until the air bubble in the capillary tube moved steadily.
10. Two points, P and Q, of length 10 cm apart were marked on the capillary tube. 11. The time taken for the air bubble to move from P to Q was recorded.
12. The position of the air bubble was reset at point P by opening the screw clip. 13. Steps 11 and 12 were repeated twice.
14. Steps 11 to 13 were repeated by placing the potometer below a moving fan. 15. The rate of transpiration in this case was calculated by using the following formula:
rate of transpiration = 10/t cm min1 where t = time taken for air bubble to move from P to Q

Question 9

describe the mass flow theory

Answer 9

1. increase concentration of sucrose in the sieve tubes lowers the WP so water enters from the xylem via osmosis.
2. this creates high hydrostatic pressure within the sieve tubes
   3.sucrose concentration at the sinks is low as it’s used up on respiration
3. sucrose is actively transported from sieve cells through companion cells to sink
4. This reduces the WP of those cells so water leaves via osmosis entering sink
5. hydrostatic pressure is lowered
6. hydrostatic pressure gradient between the source and sink
7. mass flow of sucrose down the gradient