Module 3:Translocation and Transpiration Flashcards
Which vessel does translocation occur in?
the phloem
Define translocation
the movement of assimilates in cell sap by mass flow up and down the phloem from sources to sinks
Describe the process of active loading of sucrose(assimilates)into the phloem
(include why its required)
give the first 3 points
-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
Describe the process of active loading of sucrose(assimilates)into the phloem
give the last 2 points
-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
Describe the Mass Flow Hypothesis
give the first 3 points
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
Describe the Mass Flow Hypothesis
give the final 3 points
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
Why is the hydrostatic pressure smaller at the end of the phloem that is near to the sink?
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
What does active loading of assimilates into the phloem require?
energy from ATP
What is transpiration?
the loss of water vapour from the leaves and stems of plants
-its an inevitable consequence of gas exchnage
Why are leaves covered in a waxy cuticle?
to make them waterproof which prevents leaves losing water rapidly and constantly by evaporation
What are stomata and what are they opened and closed by?
Give two ways this structure is adapted to its role
they are microscopic pores in the lower epidermis of the leaf
by guard cells
-they are able to bend/change shape
-they contain many chloroplasts to provide them with energy in form of ATP
Describe the transpiration stream
5 points
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
What are the 4 factors that affect transpiration rate?
-humidity
-wind speed
-temperature
-light intensity
What affect does low and high humidity have on transpiration rate?
low-lowers rate of transpiration because of reduced water vapour potential gradient between the inside and outside of the leaf
high-increases rate
What affect does high wind speed have on transpiration rate?
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
