Chapter 9 Transpiration and Translocation

Question 1

Define translocation

Answer 1

The movement of organic compounds, assimilates, between sources and sinks

Question 2

What are sources and sinks? Give examples

Answer 2

Sources- tissues containing the assimilates
e.g green leaves and stems, tubers, food stores in seeds
Sinks- tissues needing the assimilates
roots, meristems, food stores

Question 3

Can the sources and sinks switch roles?

Answer 3

Yes. e.g food sources such as tubers

Question 4

Define assimilates

Answer 4

The products of photosynthesis that are transported in the plants (mostly sucrose, some fructose, glucose)

Question 5

Why is sucrose used in transport?

Answer 5

It is less likely to be metabolised in the process of transport, not like glucose

Question 6

What are the two possible routes of phloem loading? Active or passive?

Answer 6

Symplast- passive
Apoplast- Active

Question 7

Describe in detail the process of phloem loading

Answer 7

Companion cells actively pump out hydrogen ions into source cells via the H+ATPase pump, producing an electrochemical gradient
H+ ions diffuse via cotransport protein, use facilitated diffusion to move into the companion cell with the sucrose molecule down the concentration gradient
The sucrose then diffuse into the sieve tube element down the concentration gradient, reducing the water potential in the sieve tube element
Water moves into the sieve tube element from the xylem/ source cell increasing hydrostatic pressure
The solutes then move down the phloem due to the pressure difference, to low pressure sinks

Question 8

How are companion cells adapted for phloem loading?

Answer 8

Many infoldings to increase the surface area to increase the amount of active transport
Many mitochondria to meet the ATP demand of active transport

Question 9

What is the evidence for the mechanism of translocation?

Answer 9

Microscopy shows companions cells are adapted for active transport
If mitochondria poisoned, translocation stops
The flow is 10000x faster than if it relied on diffusion alone
Aphids show high pressure in phloem which pushes out sap, lower near sinks, as well as lower solute concentration near sinks

Question 10

Why is water important to plants?

Answer 10

Hydrostatic pressure acts as skeleton to support stems/leaves, and allows roots to expand, penetrate ground
Loss of water= cooling
Transporting mineral ions
Reactant in photosynthesis

Question 11

What are adaptations of root hairs on root hair cells which help with water uptake?

Answer 11

(=Protrusion)
Microscopic size to penetrate soil particles
Large SA:V
Thin surface layers, short diffusion distances
High solute concentration in cytoplasm

Question 12

How does water move via the symplast pathway?

Answer 12

Through the continuous cytoplasm of living plant cells, connected through plasmodesmata
Moves from one cell to another via osmosis

Question 13

How does water move via the apoplast pathway?

Answer 13

Movement of water through the apoplast= the cell walls and intercellular spaces
Water fills spaces in cellulose fibre network, more water pulled by cohesive forces, creating tension and continuous flow

Question 14

Describe the movement of water in the roots in terms of pathways

Answer 14

Water moves across roots via symplast and apoplast until endodermis
At endodermis, cells surrounded by waxy suberin, forced into symplast path
Once in the vascular bundle, both paths resume

Question 15

What is the Casparian strip and why is it useful?

Answer 15

Band of waxy suberin around endodermal cells which prevents the apoplast pathway
Forces water into cytoplasm to follow symplast path
However, must pass through selectively permeable cell membrane, removing toxic solutes

Question 16

What is root pressure and why does it occur?

Answer 16

The enhanced movement of water in the xylem, caused by active transport of mineral ions into the xylem

Question 17

What is evidence of root pressure?

Answer 17

If mitochondria are poisoned, root pressure disappears
Increases with high temps and then falls, suggesting chemical reactions
Oxygen required
Guttation- xylem sap spills even at night when transpiration doesn’t occur

Question 18

What is transpiration and why does it occur?

Answer 18

The loss of water vapour from the leaves and stems via the stomata due to concentration gradients
As stoma are needed for gas exchange, water must be lost as a consequence

Question 19

Outline the transpiration stream

Answer 19

Water evaporates from the mesophyll into air spaces, diffusion down the concentration gradient
This produces a water potential gradient between the leaves and top of the xylem, with water moving down this gradient via osmosis from the xylem to the leaf, through symplast and apoplast pathways
Reduces the hydrostatic pressure of the xylem
Water exhibits capillary action, using mass flow, moving as a continuous column of water, pressure gradient. Movement caused by tension of cohesion between molecules, and adhesion with the surface (H bonds)
Water potential gradient between the endodermal cells and bottom of the xylem, osmosis through the symplast pathway only due to the casparian strip
Water potential gradient with root hair cells, then with soil, using both pathways

Question 20

What is the evidence for the cohesion-tension theory of water?

Answer 20

The diameter of a tree decreases with transpiration and tension increases, measure at different times (as adhesion of water molecules pulls the xylem inwards)
When xylem broken, air pulled in rather than water leaking out, then continuous flow broken,

Question 21

What is a potometer and how is it used? Any equation?

Answer 21

Measures transpiration by distance an air bubble moves
Vaseline to prevent loss of water by evaporation
Reservoir to reposition bubble
Rate of diffusion = distance/ time

Question 22

How do guard cells control the opening of the stomata?

Answer 22

Guard cells pump in K+ by active transport, reduce water potential, water moves in via osmosis
Turgor increases, due to thicker inner cell wall, bean shape and so open stomata
Closed by hormonal signals from the root

Question 23

What factors affect transpiration and why?

Answer 23

Light intensity- opens stomata
Humidity- High humidity decreases w.p gradient, so lower rate of diffusion and then transpiration
Temperature- increase rate of evaporation, increases amount of air capable of held, increasing gradient and transpiration
Air movement- removes water vapour in air still, increasing w.p gradient

Question 24

What is a limitation of using a water potometer?

Answer 24

Measures water uptake, with not all the water taken used for transpiration instead for cooling, photosynthesis…
Mass potometer directly measures transpiration

Question 25

Improvements to potometer: precautions and when in use?

Answer 25

Cut shoot under water and insert into apparatus underwater - reduce chance of air bubbles
Ensure the system is air tight/ water tight so the joints are sealed, as air bubbles could block xylem, use vaseline/petroleum jelly
Cut shoot at an angle- greater surface area for water uptake
Dry leaves

When using:
Ensure the bubble does not move too far and use the syringe/reservoir to move the bubble
Place open end in water to prevent more air bubbles
Keep shoot still to prevent a break in the seal.

Question 26

What are the units of a potometer?

Answer 26

mm3 cm–2 min–1

Question 27

How do you prepare a sample of xylem/phloem for microscopy?

Answer 27

Soak the plant in a dye or stain
Using a sharp blade, cut a very thin section of either a transverse or longitudinal depending on what is needed

Question 28

How do you actually set up a potometer?

Answer 28

Fill the potometer with water
Cut a shoot from a plant at an angle under water. Insert it into the potometer underwater and dry the leaves
Ensure the system is air tight by dealing with Vaseline
Raise the potometer out of the water and back into the water to allow an air bubble in
Move this bubble by opening the reservoir, then ensure it is shut
Measure the about the bubble has moved in a given time