Transport in plants

Question 1

What does the cohesion-tension theory state?

Answer 1

Water moves through the xylem because:
Cohesion between water keeps it together
Adhesion means that water sticks to the walls, which means that water moves via capillary action, aka due to the pressure
Tension is created by water transpiring out of the leaves.

Question 2

What are the key features of xylem?

Answer 2

Long tubes
Strong
Dead
Pits

Question 3

What are the 3 cell types that make up phloem

Answer 3

Sieve tubes
Sieve plates
Companion cells

Question 4

What are there a lot of in companion cells and why?

Answer 4

Mitochondria to produce energy for the sieve tubes and for active transport

Question 5

What is the general direction of translocation?

Answer 5

Source to sink

Question 6

What is a source?

Answer 6

Any part of the plant that is producing or releasing assimilates for use in other plant tissues

Question 7

What is a sink?

Answer 7

Any part of the plant that requires assimilates for metabolic reactions.

Question 8

What are assimilates?

Answer 8

Photosynthesis products

Question 9

What are the steps of translocation?

Answer 9

1. Sucrose from photosynthesis is co-transported with hydrogen ions into companion cells
2. Sucrose therefore decreases Ψ in phloem, so water enters from xylem. This creates hydrostatic pressure that pushes fluid along the phloem.
3. Companion cells actively transport sucrose out of the phloem
4. The surrounding cells then have a more negative Ψ than the phloem, so water leaves the phloem and enters the cells.
5. Cells either use the sucrose or convert it into starch for storage, and thus the concentration remains low, keeping a Ψ gradient.

Question 10

What are the 4 environmental factors effecting transpiration?

Answer 10

1. Hot temperatures increase evaporation
2. Low relative humidity increases Ψ gradient between air spaces
3. Air movement (wind) blows away the water from the leaf, thus increasing the Ψ gradient.
4. Plentiful water means that plants do not limit water loss

Question 11

What adaptations can plants have to help them reduce water loss?

Answer 11

1. Photosynthesis in stem - Reduces water loss through leaves
2. A thick waxy cuticle increases diffusion distance
3. Large volume for storage
4. Hairs can trap moist air
5. Roll leaves into cylinders?
6. Needle leaves reduce SA:Vol ratio

Question 12

What adaptations can plants have to help them exchange gases easier?

Answer 12

1. Thin/no cuticle
2. Stomata on leaf upper side
3. Hollow aerenchyma for diffusion to stem/ roots

Question 13

What are the three pathways of water?

Answer 13

Apoplast = Water moves between/around cells. Dissolved substances are transported this way.
Symplast = Water enters cells and moves through cytoplasm via plasmodesmata
Vaculoar = Same as symplast but also moves through vacuoles.

Question 14

What is the casparian strip?

Answer 14

A band of suberin or lignin that is impermeable to water.

Question 15

What does the casparian strip cause to happen?

Answer 15

It causes the water and substances in the apoplast pathway to move into the cells. Dissolved nitrates are then moved into the xylem to lower the water potential,