Biologyn00b
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#1
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hi, I am resitting biology aqa unit 1 as and in a revision session me and the teacher have been doing a experment to show diffusion. shes put some pink jelly in acid and then i have to calculate how long the blocks take to change color. not all the blocks are the same size and shes set me some questions whihch i was wondering if you could help me with.

1. how does the cube with the largest surface area to vol ratio show that cells can only be a certain size?

I wrote about how materials in teh body diffuse into cells but, the jelly shows that cells with a larger surface area to vol ratio takes longer for the materials to diffuse into the cell. .

2. should you have kept the volume of acid the same for each cube

I wrote yes as a higher voluem of acid means that theres more acid to diffuse into the cube

3. in the experiemtn some of teh cube touchde the bottom of the beaker how coule this be improved.

i Put suspend it in acid so the cube's fulll surface aera is available for the acid to diuffse into

4. the agar contained phph a dye which changes colour in acid. explain why the colour cahnged?

i put becase the phph is an indicator which chanegs color in when acid id present to identify that acid is there.

5. explian why multicellular organisms cannot surviev by diffusion alone but single celled oragnisms can

i put that sing celled oganisms have a larger surface aera volmue ratio and so all sbstances it needs cna just dfiisue into the cell

6. smoeone did the experiment again btu acid conc changed for smoe blocks. why did the diff conc mean it took dffierent time for blocsk to chegna colour?
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Llewellyn
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(Original post by Biologyn00b)
hi, I am resitting biology aqa unit 1 as and in a revision session me and the teacher have been doing a experment to show diffusion. shes put some pink jelly in acid and then i have to calculate how long the blocks take to change color. not all the blocks are the same size and shes set me some questions whihch i was wondering if you could help me with.

1. how does the cube with the largest surface area to vol ratio show that cells can only be a certain size?

I wrote about how materials in teh body diffuse into cells but, the jelly shows that cells with a larger surface area to vol ratio takes longer for the materials to diffuse into the cell. .

2. should you have kept the volume of acid the same for each cube

I wrote yes as a higher voluem of acid means that theres more acid to diffuse into the cube

3. in the experiemtn some of teh cube touchde the bottom of the beaker how coule this be improved.

i Put suspend it in acid so the cube's fulll surface aera is available for the acid to diuffse into

4. the agar contained phph a dye which changes colour in acid. explain why the colour cahnged?

i put becase the phph is an indicator which chanegs color in when acid id present to identify that acid is there.

5. explian why multicellular organisms cannot surviev by diffusion alone but single celled oragnisms can

i put that sing celled oganisms have a larger surface aera volmue ratio and so all sbstances it needs cna just dfiisue into the cell

6. smoeone did the experiment again btu acid conc changed for smoe blocks. why did the diff conc mean it took dffierent time for blocsk to chegna colour?
Could you please post with the exact problems you are having. Your answers to some of these questions are good, so outline what you want help on and I'll go through the theory with you (I won't just tell you the answers).
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Biologyn00b
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It's mainly the questions above. I mean I'm unsure whether the amount of acid one makes a different. I mean as long as the block is covered surely it doesn't matter. Also, will the acid concentration make a difference. Surely the higher acid concentration means diffusion is quicker as there's a steeper conc grad. Also does the jelly show cells have to be a certain size badly be a use cells are significantly smaller and have some acid in it (from R groups or wgatever) and so different is slower so it's a bad example. Thanks again for your help.
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Llewellyn
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(Original post by Biologyn00b)
It's mainly the questions above. I mean I'm unsure whether the amount of acid one makes a different. I mean as long as the block is covered surely it doesn't matter. Also, will the acid concentration make a difference. Surely the higher acid concentration means diffusion is quicker as there's a steeper conc grad. Also does the jelly show cells have to be a certain size badly be a use cells are significantly smaller and have some acid in it (from R groups or wgatever) and so different is slower so it's a bad example. Thanks again for your help.
It is always best to keep as many things the same as possible. So try to keep the volume/ amount of acid the same (this may mean that for small blocks you have a silly volume surrounding the cube).

Your concentration reason is correct.

The experiment is clearly an oversimplification of what happens in a cell. But the basic principles should still apply to both processes (surface area to volume ratio, rate of diffusion, etc.) And your thoughts on this should give you the reason why (living) cells cannot or should not exceed a certain size.
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Biologyn00b
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Okay Tnahkns. So for qusteion 1 then it woudl be a good moedl as it shows how oenc an organsim reaches a certain size (SA:V) that diffusion of substances i so flow that it cannot sustain lif and so a specialised system eg lungs for gas exchange is needed to supply each cell with the amount of O2 needed. Would you say there are any bad points to this model?

Why should you keep the acid conc the same is it because then there will be the same conc grad of acid for all cubes, or is acid conc determines by the concentration of the acid and not the volume. I know that doesn't make sense btw. I mean, if you have 100ml of acid at one conc and 50ml at the same conc will the 100ml of acid have higher conc because there's more acid or not because the concentration is the same?

For 3 that is the only way that I think the experiment could be improved. Do you think there are any other ways of improveing it?
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Llewellyn
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(Original post by Biologyn00b)
Okay Tnahkns. So for qusteion 1 then it woudl be a good moedl as it shows how oenc an organsim reaches a certain size (SA:V) that diffusion of substances i so flow that it cannot sustain lif and so a specialised system eg lungs for gas exchange is needed to supply each cell with the amount of O2 needed. Would you say there are any bad points to this model?

Why should you keep the acid conc the same is it because then there will be the same conc grad of acid for all cubes, or is acid conc determines by the concentration of the acid and not the volume. I know that doesn't make sense btw. I mean, if you have 100ml of acid at one conc and 50ml at the same conc will the 100ml of acid have higher conc because there's more acid or not because the concentration is the same?

For 3 that is the only way that I think the experiment could be improved. Do you think there are any other ways of improveing it?
The model is obviously oversimplified as a cell is not a cube and a cell's membrane behaves different, but the model is pretty good.

The rate of diffusion depends on the concentration difference. So the concentration difference needs to be constant (concentration of acid needs to be constant) if you are just investigating how the shape of the jelly affects the rate of diffusion.

Apart from suspending the jelly in the jar, I don't see an obvious way to improve the experiment.
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trina3101
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Does anyone have any results from this experiment as I have to find some secondary evidence and it's basically impossible
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