Chapter 9: Transport in plants

Question 1

State and explain the function of xylem

Answer 1

1. Conducts water and dissolved mineral salts from the roots to the stem, to the leaves

• Xylem has an empty lumen without protoplasm
• This reduces the resistance of water flowing through the xylem

2. Provide mechanical support for the plant

• The inner walls of the xylem vessel is thickened with lignin to prevent the collapse of the xylem vessel
  So that the xylem vessel can provide mechanical support for the plant together

Question 2

State and explain the function of phloem

Answer 2

1. Transports manufactured food substances from the leaves to other parts of the plant

• It has companion cells which contains many mitochondria to provide energy to load sugars up from mesophyll cell into sieve tubes by active transport
• Sieve tube plates between sieve tube cells have holes
• This allows the rapid flow of manufactured food flowing through the sieve tube

Question 3

Define transpiration

Answer 3

Transpiration is the loss of water vapour from aerial parts of the plant, but mainly through the stomata of the leaves

Question 4

Define transpiration pull

Answer 4

The drawing force of water and mineral salts up the plant. It is a suction force caused by transpiration

Question 5

Explain how the roots absorb water

Answer 5

Water enters the root hair cell through the partially permeable cell membrane by osmosis

Question 6

Explain how the roots absorb mineral salts

Answer 6

Mineral salts is absorbed by the root hair cell by diffusion. When the mineral salt concentration is higher in the soil solution than in the cell sap. It moves down the concentration gradient from the soil into the roots

Question 7

Define translocation

Answer 7

Translocation is the transport of manufactured food substances in the plant

Question 8

Define wilting and explain how it happens

Answer 8

Wilting occurs when the rate of transpiration is faster than the rate of absorption

This results in loss of water in vacuole, resulting in loss of turgor pressure

This causes the leaves to dry up and wilt.

Question 9

Explain how temperature affects transpiration

Answer 9

1. Increased temperature increases the rate of evaporation of water from mesophyll cells into intercellular air spaces
2. The water vapour concentration increases between the intercellular air spaces in the leaves and the surrounding air
3. This increases the rate of diffusion of water vapour leaving the leaf through the stomata
4. This thus increases the rate of transpiration

Question 10

Explain how humidity affects transpiration

Answer 10

1. Decreased / reduced humidity increases rate of transpiration
2. This increases the water vapour concentration gradient between the intercellular air spaces in the leaves and the surrounding air
3. This increases the rate of diffusion of water vapour leaving the leaf through the stomata
4. This thus increases the rate of transpiration

Question 11

Explain how wind affects transpiration

Answer 11

1. Increased wind increases the rate of transpiration
2. This increases the rate of removal of water vapour outside the leaf / stomata
3. This increases the water vapour concentration gradient between the intercellular air spaces and the surrounding air
4. This increases the rate of diffusion of water vapour leaving the leaves through the stomata
5. This thus increases the rate of transpiration

Question 12

Explain how light intensity affects transpiration

Answer 12

1. Increased light intensity increases the rate of transpiration
2. This causes the stoma to open, open wider or to close
3. This increases the water vapour concentration gradient between the intercellular air spaces in the leaf and the surrounding air
4. This increases the rate of diffusion of water vapour leaving the leaves through stomata
5. This thus increases the rate f transpiration