Transport in Plants

Question 1

Describe a Xylem?

Answer 1

• made up of thick walled cells which contain no cytoplasm, they are completely hollow and arranged end to end
• cell wall contains lignin, waterproof
• the end walls have broken down so they form a continuous transport system throughout the plant

Question 2

What does xylem transport?

Answer 2

Transports water and mineral ions from the roots to the leaves.

Question 3

What are nitrate and magnesium ions needed for in xylem?

Answer 3

Nitrate
- nitrate ions are needed to make amino acids and proteins

Magnesium
- magnesium ions are needed to make chlorophyll

Question 4

Describe a Phloem?

Answer 4

• consists of living cells
• tubes are formed by cells arranged end to end but have cell walls made of cellulose
• has a two way flow

Question 5

What is the sugar in photosynthesis in phloem converted to?

Answer 5

sucrose (another sugar)

Question 6

What does phloem transport?

Answer 6

Phloem transports sucrose and amino acids from the leaves to the growing points (e.g. the tips of the shoot and flowers) or storage areas (such as the roots or bulbs).

Question 7

What is moving sucrose and amino acids around the plant called?

Answer 7

Translocation

Question 8

What are the characteristics of root hairs?

Answer 8

They are:
- thin walled
- have hair like extensions that increase the surface area of the root epidermis
- the main site of water absorption

Question 9

What is the function of root hair?

Answer 9

To absorb water and minerals from the soil.

Question 10

What is transpiration?

Answer 10

Transpiration is the evaporation of water from the surface of a plant.

Question 11

What is the transpiration stream?

Answer 11

Water pulled up the xylem in the stem and roots in a continuous flow.

Question 12

What are the functions of the transpiration stream?

Answer 12

• to carry mineral ions to the leaves for the synthesis of amino acid and chlorophyll
• to keep turgor pressure high in leaf cells, holding the leaves up
• evaporation cools the leaf
• supplies water for photosynthesis

Question 13

Effect of Environmental Conditions on Transpiration: Temperature

Answer 13

• on a hot day, water will evaporate quickly from the leaves of a plant as the water
  molecules have more kinetic energy
• transpiration therefore will increase as temperature increases

Question 14

Effect of Environmental Conditions on Transpiration: Humidity

Answer 14

• Very humid air contains a great deal of water vapour – there is a smaller concentration gradient, so transpiration slows down
• In dry air the diffusion of water vapour from the leaf to the atmosphere will be rapid
• Transpiration therefore increases if humidity decreases.

Question 15

Effect of Environmental Conditions on Transpiration: Wind speed

Answer 15

• In still air, the region around a transpiring leaf will become saturated
  (full) with water vapour so that no more can escape from the leaf –
  causing transpiration to slow down
• In moving air, the water vapour will be blown away from the leaf as fast
  as it diffuses out. This will speed up transpiration.
• Transpiration therefore increases as wind speed increases

Question 16

Effect of Environmental Conditions on Transpiration: Light intensity

Answer 16

• Light itself does not affect evaporation, but in daylight the stomata of leaves are open to supply carbon dioxide for photosynthesis
• This allows more water to diffuse out of the leaves and into the atmosphere

Question 17

Effect of Environmental Conditions on Transpiration: Water supply

Answer 17

• If water is in short supply, and the plant is losing water by transpiration faster
  than it is being taken up by the roots
• Then the plant may start to wilt
• Before this happens the guard cells become flaccid and stomata start to close,
  therefore reducing transpiration and delaying wilting

Question 18

What is the stomata?

Answer 18

The stomata are holes in the surface of the leaf. They are mainly found on the bottom surface.

Question 19

What is the function of guard cells?

Answer 19

There are two guard cells on either side of the stomata - these are sausage shaped cell which control the opening and closing of the stomata.

Question 20

What feature does each guard cell have?

Answer 20

A thickened, inflexible inner cell wall (on the stomatal side)

Question 21

What happens when guard cells photosynthesise?

Answer 21

• the concentration of sugars increases
• here water potential in the guard cells falls –> so water moves into the guard cells by osmosis
• they become turgid(swollen)
• guard cells become banana shaped, due to the inflexible inner cell wall
• opens the stomata

Question 22

What happens when guard cells stop photosynthesising?

Answer 22

• sugar concentration falls (due to respiration)
• water potential increases and water moves out of guard cells
• they become flaccid and the stomata close

Question 23

What is the function of the sieve tube?

Answer 23

• it has a thin layer of cytoplasm at the edge of the cell but is mainly filled with sap
• the sap flows through the sieve plates and through the phloem

Question 24

What apparatus is needed for measuring the rate of transpiration?

Answer 24

• potometer
• cut shoot
• reservoir
• ruler
• air bubble
• capillary tube

Question 25

What is a potometer?

Answer 25

A potometer is used to measure the rate of water uptake from a leafy shoot - as 90% of its water is transpired, we use this as a measure of the rate of transpiration

Question 26

How to set up a potometer:

Answer 26

1. The potometer must be set up under water - this prevents any air bubbles from
   entering the system and blocking the xylem
2. Cut the stem of a shoot
3. Put the shoot stem into the bung, grease the joint with plenty of petroleum jelly
   
   • this prevents water loss and air entry
4. Put the bung into the potometer.
5. Make sure the tap is closed and it is full of water (no bubbles). Then lift the
   potometer out of the water.
6. Leave the end of the capillary tube out of the water until an air bubble forms
   then put the end into a beaker of water.
7. You can measure the transpiration rate as distance the bubble travels in five
   minutes (or the time taken for the bubble to travel a set distance). You should
   take a number of readings and calculate a mean rate.