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

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 for storage;

Question 6

Describe the transport of carbohydrate in plants

Answer 6

Sucrose actively transported into phloem
By companion cells
Lowers water potential and water enters from xylem by osmosis
Produces higher hydrostatic pressure
Mass flow to respiring cella
Unloaded by active transport