3.4.2: Mass transport in plants

Question 1

Root hair cells structure and function (3)

Answer 1

1) extensions that increase surface area - to increase uptake of water and increase the area for channel and carrier proteins
2) Thinner cellulose walls - shorter diffusion pathway
3) Many mitochondria - actively transports ions using energy from the hydrolysis of ATP, from soil across their membrane into cytoplasm by carrier proteins

Question 2

Structure and function of xylem

Answer 2

1) Dead cells that form hollow tubes with no organelles - Faster water flow
2) End walls break down forming a continuous tube with no end walls - continuous column of water
3) Cell wall strengthened with lignin - makes xylem waterproof and provides support to enable adhesion of water
4) Xylem pits - little holes which allow water to move laterally around blocked vessels

Question 2

What is symplastic pathway

Answer 2

slower movement of water through cytoplasm across membranes by osmosis

Question 3

What is apoplastic pathway?

Answer 3

faster movement of water along cellulose walls by diffusion until it reaches the endodermis

Question 4

How does water enter the root? (3)

Answer 4

1) Casparian strip blocks apoplast pathway
2) Active transport by endodermis
3) of ions into xylem
4) lower water potential in xylem

Question 5

How does water go from the xylem to the leaf? (4)

Answer 5

1) Transpiration (loss of water from leaf by opening of stomata) creating water potential gradient across leaf to the xylem vessels
2) Creates cohesion (water pulled up by tension from roots. Water molecules form weak hydrogen bonds between molecules)
3) Adhesion - (attracted to the walls of the xylem)
4) Creates continuous columns of water

Question 6

What causes root pressure? (4)

Answer 6

1) Active transport by endodermis
2) of ions into xylem
3) Lowers water potential
4) water ents by osmosis

Question 7

Why is the leafy shoot cut diagonally under water?

Answer 7

Prevents damage to the xylem and ensures continuous column of water

Question 8

Why is care taken to prevent water getting on the leaves?

Answer 8

The water could prevent the loss of water vapour through the stomata

Question 9

Describe the mass flow hypothesis for the mechanism of translocation in plants (5)

Answer 9

1) In the source/ leaf sucrose is actively transported to sieve tubes into phloem
2) By companion cells
3) Lowers water potential of sieve tubes and water enters by osmosis
4) Increase in hydrostatic pressure causes mass movement towards the sink
5) Sucrose is converted in root for respiration or storage

Question 10

What is the observation for the ringing experiment?

Answer 10

• if ring of bark is removed (phloem) from stem a bulge forms above ring

Question 11

Evidence for ringing experiment (3)

Answer 11

• Fluid from bulge has higher concentration of sugars that fluid below the ring
• Sugars can’t move past where bark is removed - downward flow of sugars
• Concentration of sucrose high in source that in sink.

Question 12

Evidence against ringing (

Answer 12

• Sieve plate function unclear
• A lot of pressure would be needed for solutes to get through
• Not all solutes move at the same rate

Question 13

What is the observation for tracers?

Answer 13

• Leaves are supplied with 14C which is converted to glucose
• Radioactive 14C to be tracked as its transported through the plant

Question 14

Evidence for tracers (3)

Answer 14

• Increase in sucrose levels in leafs followed by phloem later
• Metabolic poison inhibits translocation showing active transport is involved