Transport Of Organic Substances In The Phloem

Question 1

What’s translocation

Answer 1

The Movement of solutes to where they are needed in a plant

Question 2

Describe the mass flow hypothesis

Answer 2

1)Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source

Lowering water potential inside sieve tubes, so water enters tubes by osmosis from the xylem and companion cells

Creating a high pressure inside the sieve tubes at the source end of the phloem

2) at the sink end, solutes are removed from the phloem to be used up

This increases water potential inside the sieve tubes, so water also leaves the tubes my osmosis

This lowers the pressure inside the sieve tubes

3) the result is a pressure gradient from the source end to sink end

This gradient pushes solutes along the sieve tubes towards the sink

When they reach the sink the solutes will be used or stored

Question 3

Describe the transfer of sucrose into sieve events from photosynthesising tissue

Answer 3

Sucrose manufactured from the products of photosynthesis in cells with chloroplasts

Sucrose diffuses Down a conc gradient by facilitated diffusion from photosynthesising cells into companion cells

H ions are actively transported from companion cells into the spaces within cell walls using ATP

H ions diffuse Down a conc gradient through carrier proteins into the sieve tube elements

Sucrose molecules are transported along with the H ions in co transport

Question 4

What’s the evidence supporting the mass flow hypothesis

Answer 4

Pressure in sieve tubes, as shown by sap being released when they are cut

The conc of sucrose is higher in leaves (source) than in roots (sink)

Downwards flow of phloem occurs in daylight, but ceases when leaves are shaded or at night

Increase in sucrose levels in the leaf are followed by similar increases in sucrose levels in the Phloem a little later

Metabolic poisons or lack of oxygen inhibit translocation of sucrose in the phloem

Companion cells possess many mitochondria and readily produce ATP

Question 5

Evidence questioning the mass flow hypothesis

Answer 5

Function of the sieve plates is unclear, as they would seem to hinder mass flow

Not all solutes move at the same speed- they should do so if movement is by mass flow

Sucrose is delivered at more or less same rate to all regions, rather then going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest