Mass Transport In Plants

Question 1

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves. (6)

Answer 1

(In the root)
1. Casparian strip blocks apoplast pathway / only allows symplast pathway;

2. Active transport by endodermis;
3. (Of) ions/salts into xylem;
4. Lower water potential in xylem / water enters xylem by osmosis /down a water potential gradient;

(Xylem to leaf)
5. Evaporation / transpiration (from leaves);

6. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure;
7. Adhesion / water molecules bind to xylem;
8. (Creates continuous) column of water

Question 2

Root pressure moves water through the xylem. Describe what causes root pressure. (4)

Answer 2

1. Active transport by endodermis;
2. ions/salts into xylem;
3. Lowers water potential (in xylem);
4. (Water enters) by osmosis;

Question 3

Name a factor that can affect transpiration

Answer 3

Light (intensity) / temperature / air movement / humidity;

Question 4

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.

Answer 4

1. Seal joints / ensure airtight / ensure watertight;
2. Cut shoot under water;
3. Cut shoot at a slant;
4. Dry off leaves;
5. Insert into apparatus under water;
6. Ensure no air bubbles are present;
7. Shut tap;
8. Note where bubble is at start / move bubble to the start position;

Question 5

Describe the mass flow hypothesis for the mechanism of translocation in plants.

Answer 5

1. In source/leaf sugars actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve cell/tube and water enters by osmosis;
4. Increase in pressure causes mass movement (towards sink/root);
5. Sugars used/converted in root for respiration or storage;

Question 6

The mass flow hypothesis is used to explain the movement of substances through phloem.

Use your understanding of the mass flow hypothesis to explain how pressure is generated phloem tube.

Answer 6

1.Sucrose actively transported (into phloem);

2. Lowering/reducing water potential

OR

More negative water potential;

Water moves (into phloem) by osmosis (from xylem);

Question 7

Describe the cohesion tension theory of water transport in the xylem

Answer 7

1.Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells;

OR
Transpiration / evaporation / diffusion of water (molecules) through stomata / from leaves;

2.Lowers water potential of mesophyll / leaf cells;

3.Water pulled up xylem (creating tension);

4.Water molecules cohere / ‘stick’ together by hydrogen bonds;

5.(forming continuous) water column;

6.Adhesion of water (molecules) to walls of xylem;

Accept W’ or WP

Question 8

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.

Suggest one reason why.

Answer 8

Plant has roots
OR
xylem cells very narrow;

Ignore references to air bubbles / mass flow / photosynthesis

Accept xylem damaged when cut

Question 9

Describe the mass flow hypothesis for the mechanism of translocation in
plants.

Answer 9

1.In source / leaf sugars actively transported into phloem;

2.By companion cells;

3.Lowers water potential of sieve cell / tube and water enters by osmosis;

4.Increase in pressure causes mass movement (towards sink / root);

5.Sugars used / converted in root for respiration for storage.

Accept starch

Question 10

suggest and explain ways in which sieve cells are adapted for mass transport.

Answer 10

No / few organelles / very little cytoplasm / cytoplasm at edge / more room / hollow / large vacuole / large space / thick walls;

Accept strong walls for thick walls
(So) easier / more flow / (thick / strong walls) resist pressure.

Easier flow may be expressed in other ways e.g. lower resistance to flow

Question 11

suggest and explain one way in which companion cells are adapted for the transport of sugars between cells.

Answer 11

1.Mitochondria release energy / ATP / site of respiration;

Q Reject: ‘produce energy’
but accept produce energy in form of ATP

2.For active transport / uptake against concentration gradient

Question 12

Describe how a high pressure is produced in the leaves.

Answer 12

1.Water potential becomes lower / becomes more negative (as sugar enters phloem);

2.Water enters phloem by osmosis;

3.Increased volume (of water) causes increased pressure.

Question 13

Xylem adaptations:

Answer 13

1.dead hollow

2. Contain lignin which makes them rigid and water proof

3.thick walls to withstand negative water pressure

4.narrow to enable water to travel up via capillary action

5. Pits enable water to move laterally between xylem vessels (important if parts of stem become damaged)

Question 14

Adaptations of the root hair cells:

Answer 14

1. Hair like extensions SO increases surface area SO increases uptake of water AND increases the area for channel and carrier proteins
2. Thin cell wall SO shorter diffusion pathway

Question 15

Define osmosis:

Answer 15

Movement of water from a region of highER water potential to a region of lowER water potential

ACORSS a partially permeable membrane

Through AQUAPORINS