Transport of organic substances in the phloem

What are sources?

Sites of production of sugars (normally photosynthesising cells)

What are sinks?

Where the sugars produced from the sources are used or stored

(Normally respiring cells)

How is sucrose transferred into sieve elements from photosynthesising tissue?

Sucrose manufactured from glucose

Sucrose diffuses down a concentration gradient by facilitated diffusion from photosynthesising cells into companion cells

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

Hydrogen ions diffusion down a concentration gradient through carrier proteins into the sieve tube elements

Sucrose molecules co-transport with the hydrogen ions by protein carriers

What happens during the mass flow of sucrose?

Sucrose being transported into the sieve tubes lowers the water potential so water from the xylem moves by osmosis - creates a high hydrostatic pressure

At the sinks, sucrose is used up for respiration or converted to starch for storage - sucrose is actively transported into them from the sieve tubes

Lower water potential in the sinks - water moves into the respiring cells by osmosis

Higher water potential in the sieve tube elements - water moves to xylem by osmosis

Low hydrostatic pressure at the sink: mass flow of sucrose down the hydrostatic gradient

What evidence is there for mass flow hypothesis?

There’s pressure within sieve tubes - sap is released when they’re cut

Concentration of sucroses is higher in leaves (sources) than in roots (sinks)

Downward flow in the phloem during the day but ceases when shaded or at night

Increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later

What evidence questions the mass flow hypothesis?

Function of sieve plates is unclear - hinder mass flow

Not all solutes move at the same speed - they should if mass flow

Sucroses is delivered at a similar rate to all regions rather than quicker to low sucrose concentration

Sieve plates have what?

Pores

Sieve plates are accompanied by what cell?

Companion cell

How is pressure generated in the phloem 3 MARKS

Sucrose actively transported (into phloem);

Lowering/reducing water potential

OR

More negative water potential;

Water moves (into phloem) by osmosis (from xylem);