Mass Transport in Plants

Question 1

Describe the structure of xylem cells and explain the function of each structure (4)

Answer 1

1. Dead cells form hollow tubes SO allows easier water flow
2. Xylem forms continuous tube with no end walls SO water can form a continuous column
3. Cell wall strengthened with lignin SO can withstand tension
4. Xylem pits SO allow water to move laterally and get around blocked vessels

Question 2

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

Answer 2

(IN THE ROOT)
1. Casparian strip blocks apoplast pathway
2. Active transport by endodermis
3. Of ions into xylem
4. Lowers water potential of xylem / water enters xylem via osmosis
(XYLEM TO LEAF)
5. Evaporation / transpiration of water from leaves
6. Creates cohesion tension
7. Creates adhesion (between water molecules and xylem)
8. Creating a continuous column of water

Question 3

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

Answer 3

1. Active transport by endodermis
2. Of ions into xylem
3. Lowers water potential of xylem
4. Water enters by osmosis

Question 4

Describe how a high pressure is produced in the leaves (3)

Answer 4

1. Water potential becomes lower
2. Water enters phloem by osmosis
3. Increased volume of water causes increased pressure

Question 5

Name 4 factors affecting the rate of transpiration (4)

Answer 5

1. Light Intensity
2. Temperature
3. Air Movement
4. Humidity

Question 6

Give two precautions the students should take when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot (2)

Answer 6

1. Seal joints / Ensure airtight
2. Cut shoot under water
3. Cut shoot at 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 start position

Question 7

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

Answer 7

1. In source/leaf sucrose actively transported into the phloem
2. By companion cells
3. Lowers water potential of sieve cell and water enters by osmosis
4. Increase in pressure causes mass movement
5. Sucrose converted in root for respiration for storage of starch

Question 8

1. Describe the cohesion-tension theory of water transport in the xylem. (5)

Answer 8

1. Water lost from leaf by evaporation
2. Lowers water potential of leaf cells
3. Water pulled up xylem
4. Causing cohesion of water by hydrogen bonds
5. Forms continuous column of water
6. Adhesion of water to wall of xylem