mass transport in planst

Question 1

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

Answer 1

Water evaporates from leaves, lowers water potential of leaf cells; Water pulled up xylem creating tension; Water molecules are cohesive due to hydrogen bonds between water molecules; forming a continuous water column. Adhesion of water molecules to walls of xylem also occurs; reducing the trunk diameter.

Question 2

2. Describe the mass flow hypothesis for the
   mechanism of translocation in plants.

Answer 2

At source sucrose is actively transported into the phloem; By companion cells; this lowers water potential in phloem and water enters by osmosis; This produces high hydrostatic pressure; Causing the mass flow towards sink tissue; At sink sucrose is removed; and used in respiration or stored as starch, water moves out by osmosis so the hydrostatic pressure is lower. Mass flow is down a hydrostatic pressure gradient.

Question 3

3. Explain how sieve cells are adapted for mass transport.

Answer 3

They have few organelles and very little cytoplasm; So easier flow. End walls have perforations to allow movement of substances between cells.

Question 4

4. Explain how companion cells are adapted for mass transport

Answer 4

Mitochondria release energy / ATP; For active transport. Ribosomes produce proteins; for carrier proteins for active loading of sucrose into sieve tube elements.

Question 4

5. Explain how xylem is adapted for mass transport.

Answer 4

Xylem vessels have no end walls so form a series of unbroken tubes from root to leaf. Cells are dead so no cytoplasm or organelle which would restrict flow. Spirals of lignin withstand changes in pressure and prevent collapsing of the tube. Pits in the lignin allow lateral movement of water around blocked vessels.

Question 5

6. Describe and explain the precautions you should take when using a photometer, to obtain reliable results.

Answer 5

1. Seal joints to ensure watertight; 2. Cut shoot under water to prevent air entering the xylem; 3. Cut shoot at a slant; 4. Dry off leaves to increase the diffusion gradient; 5. Insert into apparatus under water to prevent air getting into the xylem; 6. Ensure no air bubbles are present; 7. Shut tap; 8. Note where bubble is at start to find the distance it moves;

Question 6

7. A potometer measures the rate of water uptake rather than the rate of transpiration. Give two reasons why the potometer does not truly measure the rate of transpiration.

Answer 6

Water used for support of plant; Water used in photosynthesis; Water produced in respiration;

Question 7

8. Explain why transpiration rate is highest around midday.

Answer 7

There is more photosynthesis at midday as there is more sunshine, so rate of water uptake will be higher. It is usually warmer, so more evaporation will take place, increase transpiration rate.

Question 8

9. What does a ringing experiment show about translocation?

Answer 8

The phloem is involved in translocation, (bulge at top of ring) shows mass flow down the stem.

Question 9

10. Why can radioactively labelled CO2 be used to investigate translocation.

Answer 9

The CO2 is made into glucose/sucrose in the leaf via photosynthesis, then transported round the plant in translocation. It can be seen using an x-ray of the plant.