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Phloem

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Phloem

Postby Navin » Thu Jun 09, 2005 2:26 pm

Hi

I want to know is there is Starch present in the phloem.

From what I know, the phloem transports sucrose.

So, would the phloem turn blue-black when flooded with iodine solution?

Also, would the pith of a stem turn blue-black when added with iodine solution?
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Postby James » Thu Jun 09, 2005 3:10 pm

Sucrose is transported in the phloem, and converted to starch at sink cells. The phloem doesn't contain starch, but the pith does.
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Postby ERS » Thu Jun 09, 2005 4:24 pm

they should stain blue-black or thereabouts.
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Postby MrMistery » Thu Jun 09, 2005 8:01 pm

Actually it is a bit more complicated than that... Starch can not disolve into water, so it is moved around under the form of sucrose(or zaharose). Now the floem contains 25% organic substances and 75% water. From the total of organic substances, 90% is sucrose, the other 10% is made up of other sugars, aminoacids and lipids. So it is false to say that the role of the phloem is to transport sucrose, as some textbooks say.
Now, the phloem does not contain starch, if you put Lugol on it it will not turn blue.
I tried to do the experiment to test this last year, but no book tells us how to isolate the phloem from the plant. The pith will no doubt turn blue as there is starch present there
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Postby lohita » Fri Jun 10, 2005 2:43 am

and also phloem doesn't carry only starch. it carries all the products of photosynthesis, this process is called translocation.
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Postby Navin » Fri Jun 10, 2005 3:38 am

Hey thanks a bunch!
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Postby MrMistery » Fri Jun 10, 2005 8:29 pm

lohita wrote:and also phloem doesn't carry only starch. it carries all the products of photosynthesis, this process is called translocation.

Actually the phloem doesn't carry starch at all because starch is not water-soluble. The stuff stranslocated through the phloem is called sucrose or sacharose
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Postby b_d_41501 » Fri Jun 10, 2005 10:54 pm

It carries disaccharides, correct? It doesn't carry polysaccharides (starches) due to this insolubility, right?
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Postby MrMistery » Sat Jun 11, 2005 4:30 pm

Yes that is true... It also carries fructose(something like 0.000001% but it is there) and other molecules such as aminoacids
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Postby b_d_41501 » Sat Jun 11, 2005 6:37 pm

I thought so
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