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;

Question 6

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

Answer 6

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 7

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

Answer 7

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

Question 8

Name 4 factors affecting the rate of transpiration (4)

Answer 8

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

Question 9

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

Answer 9

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