Phloem. Watch

The Smeezington
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I missed a week so missed the end of the xylem part and all of phloem. I've taught myself the rest of the xylem parts but need help on the components of the phloem (functions of them) and how assimilates are transported.

There is also a part about a proton/ H+ pump that I have NO idea about.


btw this os for Biology OCR F211
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The Smeezington
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If anyone can be bothered to answer i'd also like to know how and when something can be a source and a sink. and is it at the same time
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jlebb
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Let me whip my notes out, i will be with you in a second!
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pastpaper-guy
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source- the place where sucrose is PRODUCED (the source of the sucrose)

SINK- The part where the sucrose is Removed and used (Sinks in)

A example of a source can be the leaf. photosynthesis takes place their producing glucose. this travels to the sink via the phloem in a process called translocation.

The sink can be either the place where food is made e.g in fruits of a plant or the roots which require the energy to grow etc.
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The Smeezington
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(Original post by jlebb)
Let me whip my notes out, i will be with you in a second!
sitting impatiently by my computer waiting for your return. my desperation is insane.
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jlebb
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Phloem tissue is responsible for the transport of sugars in plants, they are adapted for their function in a number of ways:
.They are living cells with a thin cytoplasm but no nucleus
.Cells are joined end to end, but the end walls are perforated, this is called a sieve plate
.There are companion cells which contain a large amount of mitochondrion, therefore provide ATP as an energy source which is needed for the translocation of sugars
.Interconnections between sieve elements and companion cells are called plasmodesmata

Translocation if the process of moving photosynthetic products (sucrose) through the phloem
1. Hydrogen ions are pumped out of the companion cells via active transport (using ATP)
2. Hydrogen opns return to companion cell with sucrose, down the concentration gradient. This is known as cold transport as two things are moving together via the same protein carrier.
3. Sucrose diffuses into the sieve tube elements through the plasmodesmata
4. The phloems water potential lowers, so water from the xylem into the sieve tube elements. This creates a high hydrostatic pressure at the top of the phloem vessel, the the sugar moves down to an area of low hydrostatic pressure (moving from source to sink)
5. Sucrose molecules move from sieve tube to surrounding cells by facilitated diffusion/active transport, into the sink where they are either used or stored as starch.
6. Water moves out of the sieve tube by osmosis back into the xylem, making the hydrostatic pressure drop so that the process can repeat.
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pastpaper-guy
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what do you mean 'when something can be a source and sink'?
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giraffegiraffe
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(Original post by The Smeezington)
There is also a part about a proton/ H+ pump that I have NO idea about.
At the source (where sugars are made):

- hydrogen ions pumped out of companion cell (actively)
- this sets up a diffusion gradient
- therefore H+ ions diffuse back in, by co-transporter proteins, which bring sucrose molecules into the companion cell too
- so concentration of sucrose increases in companion cells
- so sucrose diffuses from companion cells to sieve tubes via plasmodesmata
- so water potential in sieve tube decreases
- and water enters sieve tube by osmosis
- this increases the hydrostatic pressure at the source
- the water and assimilates (sucrose etc) flow down the sieve tube, down a pressure gradient = MASS FLOW
- the sink has a low hydrostatic pressure (lower than the source) as water moves out of sieve tubes at sink and sucrose is converted into something else at sink (therefore maintaining the gradient)

this can happen in any direction (up or down a plant)

this picture might help

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Philbert
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Do you not have a course text book that you can make notes from?
I remember using this site when I was doing A-Level Biology: http://www.s-cool.co.uk/a-level/biol...port-in-plants . I'm sure there are plenty of other sites that explain it differently.
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L'Amour Toujours
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Scroll down and have a look at the information on this website for a basic overview of the components of phloem and this one too, if you don't find the first website detailed enough. Hope that helps a little towards your cause OP, good luck.
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The Smeezington
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my teacher said something about a potato being a source and a sink. I'm unsure whether it can be both at the same time or it alternates
(Original post by pastpaper-guy)
what do you mean 'when something can be a source and sink'?
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jlebb
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During the winter months plants are not able to conduct photosynthesis effectively, if at all, so the starch that plants store in their roots during the translocation of sugars is then used as their energy source.

...I think.
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jlebb
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(Original post by giraffegiraffe)
sorry jlebb didnt realise you posted the H+ ion explanation
its cool, yours has a diagram so is a lot easier to understand!
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The Smeezington
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scribbling down SO many notes. Thanks for the help. Would rep you all if I hadn't run out
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giraffegiraffe
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(Original post by jlebb)
its cool, yours has a diagram so is a lot easier to understand!
haha nah mine is a bit brief i prefer your explanation to be honest! you know when you mentioned plasmodesmata, are they holes in the cell walls between the sieve tubes and comp. cells or parts of cytoplasm that connect the cells..? my teacher said one and the book says another (i think) a bit confused?
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jlebb
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(Original post by giraffegiraffe)
haha nah mine is a bit brief i prefer your explanation to be honest! you know when you mentioned plasmodesmata, are they holes in the cell walls between the sieve tubes and comp. cells or parts of cytoplasm that connect the cells..? my teacher said one and the book says another (i think) a bit confused?
Well, im not entirely sure, but this is what my book says:

"The cytoplasm of the companion cells and the sieve tube elements are linked through many plasmodesmata. These are gaps in the cell walls allowing communication and flow of minerals between cells"

I dont think they would ask what they specifically are, just remember to namedrop them if a question on the translocation of sugars appear
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S.A.S
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Well, during the summer, the plant photosynthesises and produces sugar (in the leaves)- so the leaves are the source. During winter, the leaves fall off and the sugars are stored in the roots (e.g potato) so at the start of spring, the leaves are now the sink.

I think people have pretty much covered translocation, soooo

Evidence For and Against the Mechanism of Translocation in the Phloem:

We know that the Phloem is used in Translocation because:
- An mouthpart of an aphid feeding on a plant stem can be used to show that it is taking food (sugar) from the phloem.
- Ringing a tree to remove the bark (phloem) results in sugars collecting above the ring.
- If a plant is supplied with radioactively labelled CO2 (which will be used in photosynthesis), the radioactive CO2 will soon appear in the phloem.

How we know Translocation needs Energy:
- The companion cells have lots of mitochondria.
- The rate of flow of sugars in the phloem is so high that energy must be needed to drive the flow. It has been shown that the sugars move up to 10 000 times faster by this (active) mass flow than if the process was by diffusion alone.
- Translocation can be stopped by using a metabolic poison that inhibits the formation of ATP.

How we know that the mechanism involves a H+ pump in loading Sucrose and the Source:
- The concentration of the sucrose is higher in the source than the sink.
- the pH of the companion cells is higher than that of the surrounding cells.

Evidence Against the mechanism:
- The role of the sieve plates is unclear
- Not all the solutes in the sap move at the same rate.
- Sucrose is moved to all parts of the plant at the same rate rather than going more quickly to areas with a low concentration.

Yes, sadly, we need to know this

Good luck tomorrow, guys!
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giraffegiraffe
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thanks jlebb and SAS too that does help
- the pH of the companion cells is higher than that of the surrounding cells.
is that because of the H+ ions, increasing pH?
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S.A.S
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(Original post by giraffegiraffe)
thanks jlebb and SAS too that does help
is that because of the H+ ions, increasing pH?
Yup!
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S.A.S
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(Original post by The Smeezington)
I missed a week so missed the end of the xylem part and all of phloem. I've taught myself the rest of the xylem parts but need help on the components of the phloem (functions of them) and how assimilates are transported.

There is also a part about a proton/ H+ pump that I have NO idea about.


btw this os for Biology OCR F211
How'd you do in this then?! I got a B. Haha, I always get B's in Biology.
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