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 for storage