Mass Transport in Plants 3.4.2

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 into xylem;
4. Lower water potential in xylem so water enters by osmosis/down water potential gradient;

(Xylem to leaf)
5. Transpiration (from leaves);
6. (Creates) cohesion / tension / H bonds between H2O / NEGATIVE PRESSURE;
7. Adhesion / H20 binds 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 into xylem;
3. Lowers water potential (in xylem);
4. Water enters by osmosis.

Question 3

Name a factor that can affect transpiration (1)

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 (2)

Answer 4

• Seal joins / ensure watertight / airtight
• Cut shoot under water
• Cut shoot at slant
• Dry off leaves
• Insert into apparatus under water
• Ensure no air bubbles are present
• Shut tap
• Move bubble to start position / note where bubble is to start

Question 5

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

Answer 5

1. Sugars actively transported from source/leaf into phloem;
2. By companion cells;
3. Lowers water potential of sieve cells so water enters by osmosis;
4. Increase in pressure caused mass movement (to sink/root);
5. Sugars used in root for respiration / converted (to sucrose) for storage.

Question 6

How is water transported in plants?

Answer 6

Through xylem vessels: long, continuous columns that provide structural support to stem.