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h26
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"Describe the role of the companion cells and the phloem in transporting excess sugars from the leaf during the summer"
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I don't understand what the companion cells are doing at the sink and what they mean by offloaded by the companion cells?
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Callicious
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Heyo!

Goodmorning. I know, it's the afternoon, but hey I just woke up, that's how life is.

Okay, so,

Companion cells are cells that are necessary for the function of the phloem tissue; they actively onload sugars and offload them, too. You're fine with onloading, so, offloading it is.

At the sink, there is an inflow of sap through the phloem: it has liquid content as well as solute content, i.e. water, and sugars such as sucrose. To get this sucrose to the cells that need it, the sap can't just directly flow through cytoplasmic pathways/etc to the cells, that's a heck of a lot of sap, and all you'd have is a hydrostatic pressure gradient, not really as much a concentration gradient, so the companion cells offload the sugars inside the phloem through a partially active partially passive process: these sugars accumulate in the companion cells and through various diffusive means make their way through to the cells that need them.

Another thing worth noting is that if phloem just delivered sap to the cells and companion cells didn't do the offloading, there's no guarantee the phloem sap would reach the base of the plant with high enough pressure to deliver anything, I mean it might as well just all get pushed by the pressure into whatever way the cells would get it without this companion cell offloading system. Either ways, stuff just doesn't work well without them doing it.
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h26
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(Original post by Callicious)
Heyo!

Goodmorning. I know, it's the afternoon, but hey I just woke up, that's how life is.

Okay, so,

Companion cells are cells that are necessary for the function of the phloem tissue; they actively onload sugars and offload them, too. You're fine with onloading, so, offloading it is.

At the sink, there is an inflow of sap through the phloem: it has liquid content as well as solute content, i.e. water, and sugars such as sucrose. To get this sucrose to the cells that need it, the sap can't just directly flow through cytoplasmic pathways/etc to the cells, that's a heck of a lot of sap, and all you'd have is a hydrostatic pressure gradient, not really as much a concentration gradient, so the companion cells offload the sugars inside the phloem through a partially active partially passive process: these sugars accumulate in the companion cells and through various diffusive means make their way through to the cells that need them.

Another thing worth noting is that if phloem just delivered sap to the cells and companion cells didn't do the offloading, there's no guarantee the phloem sap would reach the base of the plant with high enough pressure to deliver anything, I mean it might as well just all get pushed by the pressure into whatever way the cells would get it without this companion cell offloading system. Either ways, stuff just doesn't work well without them doing it.
Thanks!!! And is the mechanism by which companion cells load sugars into the phloem and offload sugars in the phloem the same? I know we need to know about how companion cells load sugars into the sieve tube element(through co-transporter proteins, hydrogen ions actively transported out of cell etc and then sucrose build up in companion cell results in it diffusing into sieve tube element at the source) but do we need to know about the specifics in offloading at A level?
Anyways, so for offloading
I assumed that since the sieve tube element is connected to the companion cell by plasmodesmata, the assimilates just diffuse into the companion cell at the sink just like they did at the source? However, as you kindly advised, sap can't just directly flow through cytoplasmic pathways -but then there is plasmodesmata which allows the free passage of these assimilates?
Thanks again
Sorry couldn't reply soon enough as wasn't home
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Callicious
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(Original post by h26)
Thanks!!! And is the mechanism by which companion cells load sugars into the phloem and offload sugars in the phloem the same? I know we need to know about how companion cells load sugars into the sieve tube element(through co-transporter proteins, hydrogen ions actively transported out of cell etc and then sucrose build up in companion cell results in it diffusing into sieve tube element at the source) but do we need to know about the specifics in offloading at A level?
Anyways, so for offloading
I assumed that since the sieve tube element is connected to the companion cell by plasmodesmata, the assimilates just diffuse into the companion cell at the sink just like they did at the source? However, as you kindly advised, sap can't just directly flow through cytoplasmic pathways -but then there is plasmodesmata which allows the free passage of these assimilates?
Thanks again
Sorry couldn't reply soon enough as wasn't home
Been a while, so I don't remember the specifics for offloading. Don't take this as entire fact, I just remember it being a form of facilitated diffusion: the sugars pass through carrier proteins and do the reverse process, transporting solutes from the sink into the phloem which are then utilised for another process (sorry, I can't remember )

You don't need to know about offloading, thankfully, at least not for any of the major exam boards.

The assimilates (which I assume regard nutrients, sugars, etc) can diffuse through the plasmodesmata, i.e. the cytoplasmic pathway, the apoplastic pathway (not the xylem here, so, works relatively well) and vacuolar pathways; they can simply pass through. The sap can't simply do this because it wouldn't make sense in the grand scheme of things: sure, originally this liquid water was taken up through processes in the roots, but that's going from a large area to a small one, to generate enough flow upward, i.e. source of water to a sink (analogous, but not exact, I know) via evapotranspiration in a biological system designed for the transport of fluids, whereas phloem is designed to deliver solutes via the sap to the cells, not essentially to deliver hydration. So, yes, it's safe to say that the assimilates, i.e. nutrients, sugars, etc, are offloaded and pass through free passage.
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h26
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thanks!!!!!!!
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