Module 3:Translocation and Transpiration

Question 1

Which vessel does translocation occur in?

Answer 1

the phloem

Question 2

Define translocation

Answer 2

the movement of assimilates in cell sap by mass flow up and down the phloem from sources to sinks

Question 3

Describe the process of active loading of sucrose(assimilates)into the phloem
(include why its required)

give the first 3 points

Answer 3

-active loading is required for sucrose to move into the phloem vessels via the apoplast pathway
-companion cells pump H+ ions into their cell walls via a proton pump
-this creates a concentration gradient for H+ ions

Question 4

Describe the process of active loading of sucrose(assimilates)into the phloem

give the last 2 points

Answer 4

-H+ ions then diffuse back into the cytoplasm of companion cells via a cotransporter protein , carrying sucrose into the cytoplasm aswell
-sucrose can then diffuse down its concentration gradient via the symplast pathway through plasmodesmata into the phloem

Question 5

Describe the Mass Flow Hypothesis

give the first 3 points

Answer 5

1) sucrose is loaded into companion cells from its source and is then loaded from the companion cells into the phloem by diffusion
2)the addition of sucrose reduces the water potential in the phloem
3)water is drawn into the phloem by osmosis from the xylem and companion cells

Question 6

Describe the Mass Flow Hypothesis

give the final 3 points

Answer 6

4)the addition of water creates a high hydrostatic pressure at the top of the phloem so the cell sap will moves down the sieve tubes down its hydrostatic pressure gradient
5)sucrose is unloaded into the companion cells by diffusion and then diffuses into the sink
6)this increases the water potential in the phloem so water moves back into xylem and companion cells by osmosis

Question 7

Why is the hydrostatic pressure smaller at the end of the phloem that is near to the sink?

Answer 7

because the assimilates(sucrose) are being unloaded so the water potential is greater so less water is being drawn in and causing a pressure increase

Question 8

What does active loading of assimilates into the phloem require?

Answer 8

energy from ATP

Question 9

What is transpiration?

Answer 9

the loss of water vapour from the leaves and stems of plants

-its an inevitable consequence of gas exchnage

Question 10

Why are leaves covered in a waxy cuticle?

Answer 10

to make them waterproof which prevents leaves losing water rapidly and constantly by evaporation

Question 11

What are stomata and what are they opened and closed by?

Answer 11

they are microscopic pores in the lower epidermis of the leaf

by guard cells

Question 12

Describe the transpiration stream

5 points

Answer 12

1)water enters the roots by osmosis
2)water is transported up the xylem to the leaves
3)water moves by osmosis across membranes and by diffusion in apoplast pathway from the xylem through leaf cells
4) water evaporates from the freely permeable wall of mesophyll cells
5)water vapour leaves through stomata and enters external air

Question 13

What are the 4 factors that affect transpiration rate?

Answer 13

-humidity
-wind speed
-temperature
-light intensity

Question 14

What affect does low and high humidity have on transpiration rate?

Answer 14

low-lowers rate of transpiration because of reduced water vapour potential gradient between the inside and outside of the leaf

high-increases rate

Question 15

What affect does high wind speed have on transpiration rate?

Answer 15

wind moves the layer of air trapped around surface of the leaf which decreases water vapor potential gradient and increases diffusion gradient which will increase transpiration rate

Question 16

What affect does high temp have on transpiration rate?

Answer 16

increases kinetic energy of water molecules which increases rate of evaporation from spongy mesophyll cells into air spaces of leaf

Question 17

What affect does high light intensity have on transpiration rate?

Answer 17

increases no of open stomata which increases the rate of water vapor diffusing out so increases evaporation from surface of leaf , so increases rate of transpiration