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translocation and loading

hey, i'm just revising for my AS and i really don't understand translocation active loading and all that stuff in plants, i was wondering if anyone could help me understand it because i know sometimes they ask 9 mark questions on this.

thank you xxx

Reply 1

Excalibur

15

Translocation is the transport of organic solutes up and down the plant through phloem tissue and is an active process. (the fact that translocation is active is proven by the fact that metabolic poisons can stop translocation.)

Sieve tube elements are accompanied by companion cells. Some of these companion cells have very folded cell walls and cell membranes, which gives them a large surface area; these are called transfer cells. These cells move sucrose in and out of the sieve tube element by active transport, using energy from ATP.

First, at the 'source' where sucrose is made (typically leaves), the companion cells actively transport sucrose into the sieve tube element. This increases the solute potential inside the sieve tube element and thus the water potential is lowered; water is then drawn into the sieve tube element by osmosis from nearby cells and xylem vessels.

At the 'sink', where sucrose is being used (e.g. roots where sucrose is being converted into starch for storage), the companion cells are actively, or passively transporting sucrose out of the sieve tube element. This decreases the solute potential inside the sieve tube element and thus raises the water potential. This makes water move out of the sieve tube element into nearby cells by osmosis.

Thus, there is a high hydrostatic pressure near the source, as water is drawn in; while at the sink there is a low hydrostatic pressure as water is moved out. Thus, the organic solutes move from source to sink down the pressure gradient by mass flow.

Hope that helps.

Reply 2

adrenaline1989

2

Excalibur

Translocation is the transport of organic solutes up and down the plant through phloem tissue and is an active process. (the fact that translocation is active is proven by the fact that metabolic poisons can stop translocation.)

Sieve tube elements are accompanied by companion cells. Some of these companion cells have very folded cell walls and cell membranes, which gives them a large surface area; these are called transfer cells. These cells move sucrose in and out of the sieve tube element by active transport, using energy from ATP.

First, at the 'source' where sucrose is made (typically leaves), the companion cells actively transport sucrose into the sieve tube element. This increases the solute potential inside the sieve tube element and thus the water potential is lowered; water is then drawn into the sieve tube element by osmosis from nearby cells and xylem vessels.

At the 'sink', where sucrose is being used (e.g. roots where sucrose is being converted into starch for storage), the companion cells are actively, or passively transporting sucrose out of the sieve tube element. This decreases the solute potential inside the sieve tube element and thus raises the water potential. This makes water move out of the sieve tube element into nearby cells by osmosis.

Thus, there is a high hydrostatic pressure near the source, as water is drawn in; while at the sink there is a low hydrostatic pressure as water is moved out. Thus, the organic solutes move from source to sink down the pressure gradient by mass flow.

Hope that helps.

What an explanation mate!

Would you be able to answer the question I posed on water potential in the thread I just created?

Reply 3

vi_26

OP

woah thanks but what about the hydrogen ions thats the bit i'm really stuck on. ever thought bout becoming a bio teacher? :p:

Reply 4

Excalibur

15

vi_26

woah thanks but what about the hydrogen ions thats the bit i'm really stuck on. ever thought bout becoming a bio teacher? :p:

Noooo, too impatient to be a teacher :p:

What exactly do you mean by hydrogen ions? Afaik there are none involved with translocation.

Reply 5

adrenaline1989

2

Excalibur: What are the main ways/mechanisms by which material crosses through a plant?

One of them is translocation, which is what you described above.

Reply 6

Excalibur

15

Uhhh... movement of water through xylem? :s-smilie:

I'm not really sure what you mean.

Reply 7

Tes

4

There is transpiration using cohension-tension where water is pulled up the stem and with it pulls up soluble ions and compounds but i cant think of anymore

Reply 8

vi_26

OP

Excalibur

Noooo, too impatient to be a teacher :p:

What exactly do you mean by hydrogen ions? Afaik there are none involved with translocation.

with active loading how something transports h+ and sucrose together

Reply 9

jkatherine

3

Hydrogen ions are used in translocation. Sucrose moves into the phloem sive tube element via the apoplast or symplast route.
Using the apoplast route involves the sucrose moving in the cell wall down a concentration gradient. This concentration gradient is created by the companion cells releasing hyrdogen ions into the cell wall, by active loading, so the sucrose moves by osmosis from a low water potential to a high water potential.

Reply 10

seanclancy

The use of H+ ions some of you guys were asking about is in sucrose loading.

Cotransporter proteins can move sucrose into the companion cell but the protein has a secondary site for H+ ions which must be filled before the sucrose can cross.
ATP is used to pump loads of H+ ions from the companion cell to the source cell against the conc. gradient. The H+ ions and sucrose molecules are then transported into the companion cell down the conc. gradient by facillitated diffusion.
The lack of H+ ions in the companion cell also creates a high pH which is one peice of evidence in favour of mass flow :wink:

Hope thats helped someone...

Reply 11

zzz123

Excalibur

Translocation is the transport of organic solutes up and down the plant through phloem tissue and is an active process. (the fact that translocation is active is proven by the fact that metabolic poisons can stop translocation.)

Sieve tube elements are accompanied by companion cells. Some of these companion cells have very folded cell walls and cell membranes, which gives them a large surface area; these are called transfer cells. These cells move sucrose in and out of the sieve tube element by active transport, using energy from ATP.

First, at the 'source' where sucrose is made (typically leaves), the companion cells actively transport sucrose into the sieve tube element. This increases the solute potential inside the sieve tube element and thus the water potential is lowered; water is then drawn into the sieve tube element by osmosis from nearby cells and xylem vessels.

At the 'sink', where sucrose is being used (e.g. roots where sucrose is being converted into starch for storage), the companion cells are actively, or passively transporting sucrose out of the sieve tube element. This decreases the solute potential inside the sieve tube element and thus raises the water potential. This makes water move out of the sieve tube element into nearby cells by osmosis.

Thus, there is a high hydrostatic pressure near the source, as water is drawn in; while at the sink there is a low hydrostatic pressure as water is moved out. Thus, the organic solutes move from source to sink down the pressure gradient by mass flow.

Hope that helps.

nice explanation, but whats hydrostatic pressure?

translocation and loading

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