Translocation

Question 1

what is translocation?

Answer 1

translocation is the movement of assimilates.

Question 2

Define the term assimilates:

Answer 2

Assimilates = sugars and other chemicals made by the plant

Question 3

what form are sugars often transported in the phloem as?

Answer 3

Sugars are often transported in the phloem as sucrose.

Question 4

Why is sucrose transported and not glucose?

Answer 4

sucrose is a more efficient, compact and water soluble means of storing and transporting energy around the plant.

Question 5

what is sucrose made of?

Answer 5

glucose and fructose

Question 6

what are the sugars transported in?

Answer 6

assimilates are transported in sieve elements which are found in phloem tissue together with other types of cells called companion cells.

Question 7

what do the sieve elements and companion cells work closely together to achieve?

Answer 7

sieve elements and companion cells work closely together to achieve translocation.
They also move substances from ‘sources’ to ‘sinks’

Question 8

define the term ‘source’:

Answer 8

Source = A site where sucrose/assimilates are loaded into the phloem

Question 9

give an example of a source?

Answer 9

the leaves

Question 10

define the term ‘sink’:

Answer 10

A site where sucrose/assimilates are unloaded from the phloem

Question 11

give an example of a sink:

Answer 11

food storage organs - fruit, growing points - root, stems/ meristems

Question 12

How does the direction of transportation in the phloem differ to that in the xylem?

Answer 12

in the phloem sap flows both up and down, whereas, in the xylem flow is always upwards

Question 13

How does the process of loading of sucrose into the phloem differ from the loading of water into the xylem?

Answer 13

sucrose is loaded into the phloem by an active process, unlike the loading in the xylem which is a passive process

Question 14

summarise the loading of sucrose into the phloem:

Answer 14

1. first there is the active transport of hydrogen ions out of the companion cells - this process uses ATP
2. This creates a hydrogen concentration gradient - high concentration of H+ ions outside of the cell
3. There is then facilitated diffusion of H+ ions back into the companion cells - down a concentration gradient.
   This diffusion occurs through co-transporter proteins
   These enable sucrose/assimilates to move into the companion cell with the H+ ions
4. as the concentration of sucrose/assimilates increases it can then diffuse through the plasmodesmata - from companion cell to sieve element

Question 15

How does sucrose move along the phloem?

Answer 15

1. at the source: sucrose entering the sieve tube reduces the water potential inside the sieve tube
   - As a result water moves into the sieve tube element via osmosis. (the water comes from surrounding tissues)
   - This therefore increases the hydrostatic pressure in the sieve tube at the source.
2. Along the phloem - water which carries sucrose and other assimilates moves down the phloem down a hydrostatic pressure gradient
3. At the sink: Sucrose is removed from the sieve tube and is used by the surrounding cells e.g. in respiration or converted into starch
   - This movement of sucrose out of the sieve tube element increases the water potential in the sieve tube
   - water moves out of the tube thus reducing the hydrostatic pressure

Question 16

Define the term hydrostatic pressure:

Answer 16

the pressure that a fluid exerts when pushing against the sides of a vessel or container.

Question 17

Where does unloading of the sucrose occur in plants?

Answer 17

unloading occurs in any tissues that require sucrose

Question 18

describe the process of unloading of sucrose from the phloem to any tissue that requires it:

Answer 18

• sucrose moves out of the phloem and into the tissue by facilitated diffusion
• in the tissues, sucrose is converted into something else by enzymes

Question 19

why is sucrose converted into something else by enzymes in the tissues?

Answer 19

converting sucrose into something else in the tissues decreases the concentration of sucrose which then allows a concentration gradient from the phloem into the tissue to be maintained

Question 20

what is the evidence for the phloem being used?

Answer 20

1. ringing a tree causes sugars to collect above the ring. This is because sugars cannot pass the cut. This decreases the water potential so water moves into cells. Increased cell division to produce cells to store sugars.
2. An aphid feeding on the plant stem can be used to show mouthparts are taking food from the phloem.
3. If a plant is supplied with radioactively labelled carbon dioxide, the radioactively carbon dioxide soon appears in the phloem

Question 21

what do we know the phloem transport needs ATP energy?

Answer 21

1. translocation can be stopped using a metabolic poison that inhibits the formation of ATP
2. Flow rate is about 10,000 than diffusion
3. The companion cells have many mitochondria
4. Phloem sap always has a relatively high pH - approx. 8 - that is what you would expect if hydrogen ions are being actively transported out of the neighbouring companions cells

Question 22

What is the evidence against the phloem transportation mechanism?

Answer 22

• the sieve plates would create a barrier to mass flow - a lot of pressure would be needed for the sugar solution to get through
• sugars travel to many different sinks - not just the ones with the highest water potential which the model would suggest

Question 23

what is meant by the term secondary active transport when referring to the loading of sucrose into the phloem?

Answer 23

secondary active transport is used as the cotransportation of sucrose along with protons into the companion cell is a result from the active transport of protons out of the cell and it moves sucrose against its concentration gradient.

Question 24

in early spring give an example of what the source could be?

Answer 24

in early spring this could be the roots where energy stored as starch is converted into sucrose and moved to other parts of the plant in order to enable growth in spring.

Question 25

What occurs when the leaves are still green during LATE SPRING/SUMMER/EARLY AUTUMN?

Answer 25

sugars made during photosynthesis are converted to sucrose and loaded into the phloem sieve tubes. This occurs in late spring, summer and early autumn, whilst the leaves are green. the sucrose is transported from the sieve tube to other areas of the plant that may still be growing (meristems) or to other areas e.g. roots for storage

Question 26

define the term meristem?

Answer 26

a region of plant tissue, found chiefly at the growing tips of roots and shoots and in the cambium, consisting of actively dividing cells forming new tissue.