Chapter 10 - Adaptations for Transport in Plants

Question 1

What are the main types of cells in xylem?

Answer 1

Vessels and Tracheids.

Question 2

What are the 2 functions of xylem?

Answer 2

• Transport water and dissolved minerals.

- Provide mechanical strength and support.

Question 3

What is the material in xylem vessels that gives it their mechanical strength?

Answer 3

Lignin.

Question 4

What are the gaps within the xylem vessel called?

Answer 4

Pits.

Question 5

What is the loss of water from a plant through the leaves called?

Answer 5

Transpiration.

Question 6

What are the 3 pathways water can take through the root?

Answer 6

• Apoplast pathway, water moves in the cell walls.
• Symplast pathway, water moves through the cytoplasm and plamodesmata.
• Vacuolar pathway.

Question 7

What is the single layer of cells that surround the endodermis called?

Answer 7

Endodermis.

Question 8

What is the impermeable waterproof barrier in the cell wall of the endodermis cells called?

Answer 8

Casparian strip (Band of suberin).

Question 9

What is the function of the casparian strip?

Answer 9

To block the movement of water in the apoplast pathway driving it into the cytoplasm (Symplast pathway). Active transport allows the plant to absorb the ions selectively at the endodermis.

Question 10

How are minerals from the soil absorbed into the cytoplasm?

Answer 10

By active transport against a concentration gradient.

Question 11

What are the 3 main mechanisms for the movement of water from roots to leaves?-

Answer 11

• Cohesion tension, as water leaves xylem clees, they pull up other water molecules behind them in the xylem. The charges on the water also cause attraction to the hydrophilic lining of the vessels, adhesion.
• Root pressure, a consequence of osmotic movement of water into the xylem pushing water already there further up.
• Capillarity, movement of water up narrow tubes.

Question 12

What is transpiration?

Answer 12

The evaporation of water vapour from the leaves or other above-ground parts of the plant,out through stomata into the atmosphere.

Question 13

What factors affect the rate of transpiration?

Answer 13

• Genetic factors (Density of stomata).
• Environmental factors (Temperature, Humidity, Wind Velocity, Light intensity- Most of these affect the water potential gradient between the water vapour in the leaf and the atmosphere).

Question 14

How can rates of transpiration be measured?

Answer 14

Using a potometer.

Question 15

What are the three ways plants can be classified, depending on the prevailing water supply?

Answer 15

• Mesophytes.
• Hydrophytes.
• Xerophytes.

Question 16

How do mesophytes survive unfavourable times of the year?

Answer 16

• Many shed their leaves in winter so they don’t lose water by transpiration.
• The aerial parts die off.
• Most annual mesophytes survive over winter as dormant seeds.

Question 17

What adaptations do xerophytes have that makes them suitable to living in conditions with very little water?

Answer 17

• Rolled leaves, reduces leaf area exposed.
• Sunken stomata, reduces water potential gradient as a local humidity is created.
• Hairs, trap water vapour.
• Thick cuticle, reducing water loss.
• Fibres of sclerenchyma so leaf shape is maintained.

Question 18

How are hydrophytes adapted?

Answer 18

• No lignified support tissues as water is a supportive medium.
• Leaves have no cuticle, no need to prevent water loss.
• Stomata on upper leaf surface.
• Stems and leaves have large air sacs providing buoyancy.

Question 19

What is translocation?

Answer 19

The movement of the soluble products of photosynthesis, such as sucrose and amino acids, through phloem, from source to sink.

Question 20

What cells are phloem comprised of?

Answer 20

• Sieve tube.

- Companion cells.

Question 21

How are sieve tubes adapted for the flow of material?

Answer 21

They compromise of end-to-end cells calle sieve tube elements. The end walls and sometimes parts of the sidewall are perforated in areas called sieve plates. They lose most of their organelles and have cytoplasmic filaments extending from one to the next.

Question 22

What techniques have been used to show that organic substances are translocated through the phloem?

Answer 22

• Ringing experiments.
• Radioactive tracers and autoradiography.
• Aphid experiments.
• Aphids and radioactive tracers.

Question 23

How does the mass flow hypothesis explain translocation.

Answer 23

The source has high levels of sucrose, making the water potential very high causing water to move into the cells. Hydrostatic pressure increases forcing sucrose solution into the phloem and along from high to low hydrostatic pressure into the sink. This increased pressure causes water to be pushed out of the sink back into the xylem, back to the source.

Question 24

What aspects of translocation does mass flow not explain?

Answer 24

• The rate of phloem is much to fast for it to happen by diffusion alone.
• It doesn’t take into account the sieve plates,
• Sucrose and aa move at different rates.
• Companion cells are very biochemically active but mass flow hypothesis doesn’t suggest a role for them.

Question 25

What do other theories suggest happens during translocation?

Answer 25

• An active process may be involved.
• Protein filaments pass through the sieve pores so perhaps different solutes are carried along different routes through the same sieve tube element.
• Cytoplasmic streaming, could be responsible for movement in different directions in individual sieve tube elements.