9.4 translocation

What is glucose converted to for transport in the plant

Sucrose

Why is glucose converted when being transported through phloem

Sucrose is not used in metabolism as readily as glucose and is therefore less likely to be metabolised during the transport process

What is translocation

Movement of sugars (assimilates) through phloem in plants from 'source' to 'sink'

What are assimilates

the products of photosynthesis that are transported around a plant

What is the main assimilate transported around the plant

sucrose

What are the main 'sources' of assimilates in a plant (3)

-green leaves and green stems

-storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period

-food stores in seeds when they germinate

What are the main 'sinks' in a plant (3)

-roots that are growing and/or actively absorbing mineral ions

-meristems that are actively dividing

any parts of the plant that are laying down food stores e.g. developing seeds, fruits or storage organs

What is phloem loading

When surcrose is moved by active transport from source leaf cells through companion cells to sieve-tube elements in the phloem

What are the 2 ways in which plants load assimilates into the phloem

-symplast route (largely passive)

-apoplast route (active and move common)

How does sucrose travel in the apoplast route (2)

-sucrose from the source travels through the cell walls and inter-cell spaces to the companion cells and sieve elements by diffusion down a concentration gradient

-this concentration gradient is maintained by the removal of sucrose into the phloem vessels

What happens to sucrose once it reaches companion cells from the apoplast route (3)

-sucrose is moved into the cytoplasm across the plasma membrane in an active process

-H+ is pumped out of the companion cells into the surrounding tissue using ATP by proton pumps

-H+ return to the companion cells into down a concentration gradient via a co-transport protein, and sucrose is the molecule that is co-transported into the cell

Adaptations of companion cells to aid sucrose transport (2)

-many infoldings on their plasma membranes to give an increased SA for the active transport of sucrose into the cytoplasm

-many mitochondria to supply the ATP needed for the proton pumps

What happens after sucrose enters companion cells

-the build up of sucrose in the companion cell and sieve tube elements causes water to move in via osmosis

-this leads to an increase in turgidity, and the water carrying the assimilates moves into the tubes of the sieve elements, reducing pressure in the companion cells

-this leads to an increase in turgidity, and the water carrying the assimilates moves into the tubes of the sieve elements, reducing pressure in the companion cells

-water moves up or own the plant by mass flow to areas of lower pressure

How is sucrose unloaded from phloem

-diffusion of sucrose from the phloem into the surrounding cells

What happens to sucrose once it enters cells that need it

Why

it is converted into glucose or other energy stores, to maintain a concentration gradient

that the concentration gradient of sucrose is maintained between the phloem and the surrounding cells

What does a loss of solutes from the phloem lead to

-a rise in water potential of the phloem, causing water to move out into the surrounding cells by osmosis

Evidence for translocation (3)

-advances in microscopy allow us to see the adaptions of the companion cells for active transport (e.g. large SA and lots of mitochondria

-if the mitochondria of the companion cells are poisoned, translocation stops (as no ATP is supplied for the proton pump and co-transporter)

-the flow of sugars in phloem is faster than it would be by diffusion alone, suggesting an active process is driving the mass flo

How is mass flow achieved? talk about at sources and at sinks

the build-up of sucrose concentration in the companion cell and phloem tube elements, causes water to move in by osmosis, therefore increasing hydrostatic pressure

At sinks, since sucrose diffuses into other cells and leaves the phloem, it increases the water potential of the phloem

This means that water moves out of the phloem into neighbouring tissue like xylem or moves into the cells that have taken the sucrose. This lowers hydrostatic pressure

water moves from high pressure to low pressure areas through mass flow