Transport in Plants

Question 1

Describe the role of phloem

Answer 1

Transports sucrose and amino acids between the leaves and other parts of the plant

Question 2

Describe the role of xylem

Answer 2

Transports water and mineral salts from the roots to other parts of the plant

Question 3

Explain how water is absorbed by root hair cells

Answer 3

Long, thin outer projections of root hair cells penetrates between the soil particles, reacting the soil water. The water in the soil has some solutes dissolved in it, such as mineral ions, but their concentrations are much lower than the concentrations of solutes inside the root hair cel.
The soil water has a higher water potential than the inside of the cell which allows water to enter the root hair cell by osmosis. This water movement dilutes the contents of the cell, increasing its water potential. Water moves out of the root hair cell into the root cortex. A gradient of water potential is set up across the root cortex, kept going by water being taken up by the xylem.

Question 4

What is transpiration?

Answer 4

The evaporation of water from the surface of a plant

Question 5

Explain how the rate of transpiration is affected

Answer 5

Light intensity: rate of transpiration increases in the light because of the opening of the stomate in the light
Temperature: high temperatures increase the rate by increasing the rate of evaporation of water from the mesophyll cells
Humidity: when air is humid, this reduces the diffusion gradient between the air sacs in the leaf and the external air. The rate therefore decreases in humid air and speeds up in dry air.
Wind speed: increases with fast air movements across the surface of the leaf. The moving air removes water vapour near the stomata. The moist air would otherwise reduce the diffusion gradient.

Question 6

Describe experiments to investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot

Answer 6

Volume photometer: whole apparatus is placed in a sink of water and any air in the tubing removed. A shoot is taken form a plant and the end of the stem is cut at an angle which makes it easier to push the stem into the rubber tubing, which is done under water to stop air entering. The apparatus is removed form the sink and vaseline used to seal any joints. The movement of water column in the capillary tube can be timed.