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wrote...
4 months ago
Hello! I recently made an experiment at school to test the semi permeability of the cell membrane of yeast cells. I did as following:
1.Sodium bicarbonate solution was put into a conical flask
2. Drops of the indicator neutral red was added and the colour turned yellow as it was basic.
3. A mixture of yeastcells and sodium bicarbonate was added to the conical flask and colour turned pink.
4. An amount of Sodium hydroxide solution was added then but no colour change was observed.

Well I know that the colour changed to pink at first because the neutral red diffused into the yeast cells and changed color to pink due to the acidity of yeast cells.
I also know the sodium hydroxide is ionised in this condition and is thereby present as hydroxide ions and sodium ions. And these being polar(charged) and ions can't diffuse into the yeast cells and change the pH of it to produce a colour change. But why didn't they get into the cells with the help of channel or transport proteins that are made to transport ions? Why did they stay on the outside? I am also mostly curious about the hydroxide ions as it is they that can make a solution basic and change the colour.
Anyone could help!!!
Thank you so much!!!
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wrote...
Staff Member
4 months ago
Assuming that the yeast cells are alive, transport ions require ATP, so whether or not sufficient energy was being produced is something you need to consider in your results.

Wait for other replies, I'm sure others can chime in
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wrote...
Educator
4 months ago
Very interesting, so after adding the base (NaOH), you were expecting the pink solution to revert back to yellow. I'm assuming you think this because it tips the balance back to basic. I'm curious, is the neutral red indicator supposed to change color after a reaction has occurred? Most indicators have a point of no return, so maybe the ions are being transported, but you're not seeing that happen due to the indicators limitations.

Just a thought
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wrote...
4 months ago
Assuming that the yeast cells are alive, transport ions require ATP, so whether or not sufficient energy was being produced is something you need to consider in your results. Wait for other replies, I'm sure others can chime in
Thanks for your reply. That is exactly my question acually.
Post Merge: 4 months ago

Very interesting, so after adding the base (NaOH), you were expecting the pink solution to revert back to yellow. I'm assuming you think this because it tips the balance back to basic. I'm curious, is the neutral red indicator supposed to change color after a reaction has occurred? Most indicators have a point of no return, so maybe the ions are being transported, but you're not seeing that happen due to the indicators limitations. Just a thought
I tried it with ammonia and it changed colours so the indicator doesn't have limitations.
wrote...
Staff Member
4 months ago
Thanks for clearing that up.

Now, if transport ions embedded in the membrane require ATP (therefore, active transport), then the yeast cells need to be alive for them to generate ATP. If they don't need ATP, but rely on simple diffusion (passive transport), then the ion concentration outside must be at equilibrium with what it is inside the cell, accounting for your observations.
Thank you for using Biology-Forums.
wrote...
Educator
4 months ago
I tried it with ammonia and it changed colours so the indicator doesn't have limitations.

The fact that NH3 caused it to become yellow means there's evidence of transport into the cell. What you could do to determine if the yellow yeast cells are still alive by conducting a fermentation experiment or by observing the yeast cells bud after adding a glucose solution to the yellow yeast cells.

When you applied sodium bicarbonate, did you apply any heating to the solution containing the cells? The only way the cells can become permeable to sodium bicarbonate is through heating, allowing the sodium bicarbonate to interact with the neutral red.

Maybe your observations are correct. NaOH should keep the solution pink, creating no change.
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wrote...
4 months ago Edited: 4 months ago, Dima Harba
Thanks for clearing that up. Now, if transport ions embedded in the membrane require ATP (therefore, active transport), then the yeast cells need to be alive for them to generate ATP. If they don't need ATP, but rely on simple diffusion (passive transport), then the ion concentration outside must be at equilibrium with what it is inside the cell, accounting for your observations.
.. Aha.. Well, I'm guessing they aren't alive due to the basic pH of sodium bicarbonate solution so they won't be able to use ATP. But what about channel proteins? They don't require energy and they use facilitated diffusion? Is there something specific about hydroxide ions perhaps that make cells not transport them??
Post Merge: 4 months ago

I tried it with ammonia and it changed colours so the indicator doesn't have limitations.
The fact that NH3 caused it to become yellow means there's evidence of transport into the cell. What you could do to determine if the yellow yeast cells are still alive by conducting a fermentation experiment or by observing the yeast cells bud after adding a glucose solution to the yellow yeast cells. When you applied sodium bicarbonate, did you apply any heating to the solution containing the cells? The only way the cells can become permeable to sodium bicarbonate is through heating, allowing the sodium bicarbonate to interact with the neutral red. Maybe your observations are correct. NaOH should keep the solution pink, creating no change.
Thanks! I tried heating it and it changed colour because the permeability increased. But regarding this part of the experiment I wasn't required to heat or do anything other than add NaOH. Is there an explanation to why the ions (especially OH-) didn't move through the channel proteins which don't require any energy?
wrote...
Educator
4 months ago
Negatively charged ions aren't capable of moving through the membrane without help, especially a molecule like OH-. From experience, I've never heard of OH- molecules needing to pass through the membrane, that's why they weren't able to make a difference once you added hydroxide. Even facilitated diffusion via a channel not requiring ATP wouldn't be able to accomplish this.
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wrote...
4 months ago
Negatively charged ions aren't capable of moving through the membrane without help, especially a molecule like OH-. From experience, I've never heard of OH- molecules needing to pass through the membrane, that's why they weren't able to make a difference once you added hydroxide. Even facilitated diffusion via a channel not requiring ATP wouldn't be able to accomplish this.
. Oh.  It's settled then. Thanks so much for your help!
wrote...
Educator
4 months ago
Anything else, just holler!

See you around
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wrote...
3 months ago Edited: 3 months ago, bio_man
Hello! I've been reading this post again and figured that I may have misunderstood you when you said ion concentration must be at equilibrium. Anyhow, I now understand what you mean but I have a simple question. Is the ion concentration determined by the amount or type of ions present in and outside the cell? Like is it the concentration of ALL ions inside the cell despite their type( for example sodium, hydrogen, or even potassium) that determine the ion concentration or like sodium ion concentration inside can be compared with ONLY sodium ion concentration on the outside?
Thanks in advance!
wrote...
Educator
3 months ago
It's determined per EACH ion, not as a whole. For example, if the cell is permeable to sodium, what matters is if there's a gradient of sodium ions across the membrane, others don't matter.
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wrote...
3 months ago
It's determined per EACH ion, not as a whole. For example, if the cell is permeable to sodium, what matters is if there's a gradient of sodium ions across the membrane, others don't matter.

Thanks a lot !
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