Translocation

Question 1

What is translocation?

Answer 1

The movement of sucrose around the plant

Question 2

Why is sucrose the main carbohydrate transported?

Answer 2

It is not used up in metabolism

Question 3

How is sucrose moved?

Answer 3

From storage areas around the plant so can also travel up trunk

Question 4

What are sources?

Answer 4

Sources of sucrose

Question 5

What are sinks?

Answer 5

Where the sucrose goes

Question 6

What are examples of sources and sinks?

Answer 6

Green leaves and stem - growing roots, fruits and seeds, storage organs - meristems that are actively dividing and food stores in seeds when they germinate - shoots and roots of germinating seeds and meristems

Question 7

What is phloem loading?

Answer 7

How sucrose gets from the source into the phloem

Question 8

How is phloem loaded?

Answer 8

Active transport or passively by diffusion

Question 9

How is the phloem loaded by active transport?

Answer 9

From the apoplast

Question 10

How is the phloem loaded by passive diffusion?

Answer 10

From the symplast

Question 11

What is the symplast route for sucrose?

Answer 11

Sucrose moves from the source and into the sieve tube elements by diffusion through plasmodesmata

Question 12

How does phloem loading in symplast affect water?

Answer 12

Water follows by osmosis as water potential goes down in sieve tube

Question 13

How is sucrose moved along in the symplast route?

Answer 13

Hydrostatic pressure builds up and moves the sucrose along by mass flow as water moves from and area of high to low pressure

Question 14

How is sucrose moved in the apoplast route?

Answer 14

1. Sucrose is loaded into phloem side tube by an active process.
2. Companion cells have proton pump which use ATP to actively transport H+ ions out of cytoplasm into surrounding tissue
3. H+ diffusion gradient is set up.
4. H+ ions diffuse back into companion cells via cotransporter proteins in the cell surface membrane, bringing sucrose across with them.
5. Sucrose builds up, then diffuses across into sieve tube elements through plasmodesmata

Question 15

What is the structure of companion cells?

Answer 15

They have many infoldings in cell membrane and many mitochondria

Question 16

What is the function of many infoldings in cell membrane?

Answer 16

Increase the SA for proton pumps, cotransporter proteins and for sucrose loading into the phloem sieve tube

Question 17

What is the function of many mitochondria?

Answer 17

Provide ATP for active transport for the proton pups

Question 18

What is sucrose unloading?

Answer 18

Sucrose molecules diffuse from phloem sieve tube into active cells then is converted into starch or glucose

Question 19

What is the effect of sucrose unloading on sucrose?

Answer 19

Sucrose concentration in the cells go down and sucrose concentration gradient is maintained

Question 20

What is the effect of sucrose unloading on water?

Answer 20

Water potential in phloem sieve tube goes up and water follows the sucrose into cells by osmosis so hydrostatic pressure in phloem at sink is reduced

Question 21

What is the hydrostatic pressure gradient?

Answer 21

It is a gradient set up between the sources and sink which produces a flow of water from source to sink

Question 22

What is the effect of mass flow?

Answer 22

Sucrose is carried along

Question 23

What is the direction of mass flow?

Answer 23

Either up or down plant, according to where sugar is needed

Question 24

How do water molecules move as a result of mass flow?

Answer 24

Some of the water molecules are released from the sieve tube elements at the sink and joins the transpiration stream in the xylem

Question 25

What is the impact of water molecules joining the transpiration stream?

Answer 25

It helps keep the movement in the phloem going