4.11 Transpiration and Translocation (not done)

Question 1

What is translocation?

Answer 1

the transport of organic solutes/substances in a plant through the phloem

Question 2

Does translocation involve active transport?

Answer 2

yes

Question 3

What is the mass flow hypothesis:

Answer 3

the mass flow of organic substances from the source (of production) to the sink (where they are used up)

Question 4

What is the source and sink in translocation?

Answer 4

• source = where the organic substances (e.g. glucose) are produced – in the leaves by photosynthesis
• sink = the site where organic substances (e.g. glucose and sucrose) are used up in respiring tissues

Question 5

Describe the mass flow model:

Answer 5

• the source cell (leaf cell) produces organic solutes through photosynthesis
• water enters the source cell by osmosis, as the concentration of organic solutes increases in the cell
• this increases hydrostatic pressure in the source cell
• the sink cell uses up organic solutes (e.g. glucose) through respiration
• water leaves the sink cell by osmosis, as the concentration of organic solutes decreases in the cell
• this decreases hydrostatic pressure in the sink cell
• liquid always moves from high to low pressure, so solution containing organic solutes will move from the source cell to the sink cell through the phloem

Question 6

Describe the movement of organic solutes from the source cell into the sieve tube element:

Answer 6

1. photosynthesis occurring in the chloroplasts of leaf cells produces organic substances (e.g. sucrose)
2. this generates a high concentration of organic solutes in the source cell (leaf cell)
3. therefore, the solutes diffuse down their concentration gradient into the companion cell via facilitated diffusion
4. organic solutes from the companion cell move into the sieve tube element by active transport

Question 7

Describe the movement of organic solutes within the sieve tube element:

Answer 7

1. the increase of organic solutes in the sieve tube element lowers the concentration of water in the sieve tube element
2. water enters the sieve tube elements from the surrounding xylem vessels via osmosis
3. the increase of water volume in the sieve tube element increases the hydrostatic pressure, forcing the solution to move towards the sink cell

Question 8

Describe the transport of sucrose from the sieve tube element into the sink cell:

Answer 8

1. organic solutes (e.g. sucrose) are used in respiration at the sink, or stored as insoluble starch
2. sucrose is actively transported into the sink cell, causing the concentration of water in the cell to decrease
3. this means water moves from the sieve tube element into the sink cell by osmosis (some water also returns to the xylem)