Unit 3 Transport in plants

Question 1

Describe the cohesion tension theory

Answer 1

1. Solar energy causes water to evaporate out of the stomata
2. This lowers the waterpotential of the air spaces
3. Water evaporates from the mesophyll cells into the air spaces
4. The water potential of these mesophyll cells is reduced compared to inner mesophyll cells.
5. Water moves from inner to outer mesophyll cells by osmosis along a water potential gradient.
6. This water potential gradient extends across the leaf mesophyll cells to the xylem vessels.
7. Water is drawn from the xylem creating a tension (‘negative pressure’)
8. The water column is maintained in the xylem by cohesive forces and adhesive forces.
9. Cohesion refers to the attraction of the water molecules to each other by hydrogen bonding.
10. Adhesion refers to the attraction of the water molecules to the xylem walls.

Question 2

Name the four factors that increase transpiration

Answer 2

1. Increased Light intensity
2. increased Air Movement
3. Increased Temperature
4. Decreased Humidity

Question 3

Explain how Light intensity affects transpiration

Answer 3

In high light intensity stomata open for photosynthesis.
Water can evaporate faster through the open stomata

Question 4

Explain how air movement affects transpiration

Answer 4

Wind removes evaporating water
maintaining the water potential gradient,
so transpiration increases

Question 5

Explain how humidity affects transpiration

Answer 5

Decreased humidity means air has less water vapour.
This maintains the water potential gradient
so transpiration increases.

Question 6

Explain how temperature affects transpiration

Answer 6

Higher temperatures
increased kinetic energy
so water evaporates faster

Question 7

Xerophytes are plants adapted to reduce transpiration. How do hairy leaves and pitted stomata reduce the rate of transpiration?

Answer 7

Hairs/pits trap air saturated with water.
This increases humidity/water vapour in air surrounding stomata
and decreases water potential gradient.

Question 8

A student used a potometer to measure the movement of water through the shoot of a plant. As water is lost from the shoot, it is replaced by water from the capillary tube.

In one experiment, the air bubble moved 7.5 mm in 15 minutes. The diameter of the capillary tube was 1.0 mm.

Calculate the rate of water uptake by the shoot in this experiment.

Give your answer in mm3 per hour. Show your working. (The area of a circle is found using the formula, area = πr 2)

____________________ mm3 hour−1

Answer 8

Correct answer 23.55 – 24 two marks;

For one mark 5.9 OR 94.2;

Working:
3.14 x 0.5 x 0.5 x7.5 = volume = 5.9
x 4 to get from 15mins to 1 hr

Question 9

Lignin is a polymer found in the walls of xylem vessels in plants. Lignin keeps the xylem vessel open as a continuous tube.

Explain the importance of the xylem being kept open as a continuous tube.

Answer 9

1. Allows unbroken water column so continuous flow OR stream of water
2. Cohesion from H bonds between all water molecules
3. Evaporation/transpiration creates tension at the top of the Xylem

Question 10

Describe the transport of carbohydrate in plants.

Answer 10

1. Sucrose actively transported into phloem cell/sieve tube
2. (By) companion/transfer cells;
3. Lowers water potential in phloem and water enters from xylem by osmosis;
4. Produces higher hydrostatic pressure;
5. Mass flow to respiring cells/storage organs

Accept buds/young leaves/fruit/seeds/shoot tip/root tip/ meristems/root

6. Unloaded/removed from phloem by active transport;

Accept facilitated diffusion

Question 11

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

Answer 11

1. Water lost from stomata because of transpiration / evaporation
2. Lowers water potential of mesophyll / leaf cells;
3. Water pulled up xylem creating tension;
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;