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The Rate of Starch Synthesis in Destarchred Leaf Discs

Asked by 00kay00 | Apr 13, 2007 | A Level > Biology > Coursework
00kay00
00kay00 asks:

I'm doing my coursework on the rate of starch synthesis on destarched leaf discs, investigation 2 variables, but varying them at different times.
The first variable is concentration of sugar solution. This thing is I dont know what sugar concentration the effect of sugar concentration will be and why. I can only guess that an increase in sugar concentration would lead to an increase in rate of starch synthesis until the conc is too high and the cells in the disc start to plasmolyse. But I wouldnt know which concentrations to use.
The other variable is the type of sugar solution. I have no idea which sugar solution to use and why. I need at least 5 different sugars but I can only think of 3 that may work but I dont know the reasons for why any of them will work.
Please may you help me because I'm really stuck on what to do.

etutor answers:
Your 'guess' about the efect of suhgar concentration is probably correct. You could find out the best range of sugar conentrations to use by doing a trial experiment. In order to make starch, glucose is needed. Beside glucose itself, the following sugars contain some glucose - maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose). Fructose is found naturally in plants and so could be used  (I'm not sure if plants can inter-convert fructose and glucose). lactose may not work as it is found in milk (which plants don't come across) so they may not have enzymes that break it down. It does not matter if all the sugars don't work - this is an 'investigation', after all!

7 student responses

bmack
bmack

Apparently lactose doesn't work, so you can use it as a comparison group

responded Apr 29, 2007 2:35:23 PM BST
dnnah1
dnnah1
you are planning an experiment to determine the effect of this variables on the rate of starch synthesis, you don't have to actually know the result- but  make a justifiable prediction
responded Apr 30, 2007 8:29:14 PM BST
azimaizie
azimaizie

I'm doing this same planning exercise at the moment, and those two variables seem like the most logical ones to choose. BUT I don't know how to measure the rate of starch synthesis, cos surely if you test the leaf for starch by boiling it in ethanol etc, you've killed said leaf, so if it doesn't have any starch after however many minutes then you have to start again with another disc? So it'll be biased?

And I don't think it matters if some of the sugars don't work, thats the whole point of it

responded May 5, 2007 1:41:20 PM BST
Whisper447
Whisper447

Hey alex. (found you on biomad as well :P).
Well from what ive been thinking and reading about maybe u don't boil the disc's in alcohol until after the leaf has been in the sugar solution.
So the leaf disc's been in the solution for a while, take it out, boil it to remove chorophyll and then test for starch- therefore allowing teh leaf to respire and tehrefore making starch from glucose and then u can boil it and then get the result?
Sound logical? maybe meigh was making it sound harder and mroe complicated then it actually is... ?

responded May 6, 2007 9:25:57 PM BST
azimaizie
azimaizie

Heya! Yeah you don't boil them til after it's been in the sugar, but how do you know how long to leave them in there for before you test them?! But I spose she is good at makin things sound too complicated....

responded May 6, 2007 11:52:02 PM BST
Its_impossible
Its_impossible

Well it seems I'm not the only one struggling with this ... how do you measure the rate? The best I could come up with was taking a leaf disc out of the solution every hour or whatever, but apart from getting a tape measure and measuring the blue black areas of the leaf, I cant come with a way of showing the change. Unless you simply say theres more blue black after two hours than 1?? My 'teacher' suggested using a colorimeter ... so what wash the the leaf in water and measure the different values, but surely the iodine solution would have a similar colorimeter value to the blue black solution ...? (or I am now making it too complicated?)

responded May 14, 2007 11:31:20 AM BST
Jenbo_17
Jenbo_17

im sure you could use a different colour filter so that you only measure the percentage transmission of the one colour of light therfore eliminating the values for the iodine solution. if you use a red filter then only red light can enter the solution. as the blue/black solution will absorb this light the more starch present the stronger the solution and the less red light transmitted!  the iodine will allow the red light to be transmitted and so should not affect the results.

responded May 15, 2007 12:33:22 PM BST
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Aleya
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