Does starch have an osmotic effect and how does it affect osmosis?

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mrlaurenceo
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How does starch affect water concentration and osmosis?

My understanding was that because starch is insoluble in water it does not have an osmotic effect so it doesn't affect water concentration so there is no net movement of water from a pure water to water with starch but I've now realised that this is incorrect and that water would indeed move across the membrane. Do plants not store food molecules as starch precisely because it has no osmotic effect?
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username2896864
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(Original post by mrlaurenceo)
How does starch affect water concentration and osmosis?

My understanding was that because starch is insoluble in water it does not have an osmotic effect so it doesn't affect water concentration so there is no net movement of water from a pure water to water with starch but I've now realised that this is incorrect and that water would indeed move across the membrane. Do plants not store food molecules as starch precisely because it has no osmotic effect?
The insolubility of starch in water has nothing to do with it. Its presence means that there is a solute potential which decreases the water potential of the water and starch solution so water moves from the pure water (higher water potential) to the starch solution (lower water potential) by osmosis. Plants store their food in the form of starch simply because its insoluble and can't be transported around the plant unless it is converted to glucose.
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mrlaurenceo
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(Original post by Batgirl98)
The insolubility of starch in water has nothing to do with it. Its presence means that there is a solute potential which decreases the water potential of the water and starch solution so water moves from the pure water (higher water potential) to the starch solution (lower water potential) by osmosis. Plants store their food in the form of starch simply because its insoluble and can't be transported around the plant unless it is converted to glucose.
Thanks - this is very helpful!
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Amefish
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(Original post by Batgirl98)
The insolubility of starch in water has nothing to do with it. Its presence means that there is a solute potential which decreases the water potential of the water and starch solution so water moves from the pure water (higher water potential) to the starch solution (lower water potential) by osmosis. Plants store their food in the form of starch simply because its insoluble and can't be transported around the plant unless it is converted to glucose.
I'm not sure about this. Correct me if I'm wrong but we were told that starch is insoluble and therefore doesn't affect the water potential (or the solute potential, because starch is not a solute). Being insoluble means that there won't be too much water moving into the cells, so they won't burst.
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username2896864
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Ok fair point, starch is not a solute strictly speaking and is insoluble in water. However, it does decrease the water potential of the 'solution' because it is not pure water while it contains the starch. Water enters the plant cell by osmosis and the cell swells and becomes turgid, meaning it won't burst. If water were to move out of the cell then it would become flaccid.
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Amefish
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(Original post by Batgirl98)
Ok fair point, starch is not a solute strictly speaking and is insoluble in water. However, it does decrease the water potential of the 'solution' because it is not pure water while it contains the starch. Water enters the plant cell by osmosis and the cell swells and becomes turgid, meaning it won't burst. If water were to move out of the cell then it would become flaccid.
It is still pure water because there is no dissolved starch. The starch effectively floats (or sinks) and doesn't impact the water potential. Only solutions impact the water potential.

Water can move in and out of cells via osmosis, causing cells to become turgid or flaccid. However, if the water potential outside of the cell is higher than it is inside the cell, it can cause the cell to burst. Starch wouldn't cause this to happen though because it doesn't reduce the water potential.
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username2896864
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(Original post by Amefish)
It is still pure water because there is no dissolved starch. The starch effectively floats (or sinks) and doesn't impact the water potential. Only solutions impact the water potential.

Water can move in and out of cells via osmosis, causing cells to become turgid or flaccid. However, if the water potential outside of the cell is higher than it is inside the cell, it can cause the cell to burst. Starch wouldn't cause this to happen though because it doesn't reduce the water potential.
Animal cells will burst in that case, but plant cells wouldn't because they have a cellulose cell wall for that very purpose. I agree the starch wouldn't make a difference, thanks for explaining that
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Gerry-Atricks
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I agree with Amefish ,insoluble so no effect on osmotic potential, otherwise plants would have a hell of a time trying to store starch as it would just keep interfering with metabolic reactions.
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Amefish
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(Original post by Batgirl98)
Animal cells will burst in that case, but plant cells wouldn't because they have a cellulose cell wall for that very purpose. I agree the starch wouldn't make a difference, thanks for explaining that
Plant cells have a cell wall but it can still burst, it just requires more pressure than animal cells. Also, organelles can burst due to osmosis, which is why we use an isotonic solution when doing cell fractionation.

But it's ok, I wish I could explain it better but I don't do chemistry. If you're doing AQA, there is a section in the CGP revision guide about how water causes substances to dissolve and you will understand why starch doesn't impact the water potential
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Gerry-Atricks
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(Original post by Amefish)
Plant cells have a cell wall but it can still burst, it just requires more pressure than animal cells. Also, organelles can burst due to osmosis, which is why we use an isotonic solution when doing cell fractionation.

But it's ok, I wish I could explain it better but I don't do chemistry. If you're doing AQA, there is a section in the CGP revision guide about how water causes substances to dissolve and you will understand why starch doesn't impact the water potential
But the pressure that would burst the plant cells wouldnt be osmotic would it, max pressure from turgor pressure would cancel out osmotic potential and so no net water movement and cell is fully turgid, it would have to be an external, say hydrostatic pressure to burst it, or am i wrong?

I know organelles can burst by osmosis but didnt think whole plant cells could ( reasons above)
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username2896864
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(Original post by Amefish)
Plant cells have a cell wall but it can still burst, it just requires more pressure than animal cells. Also, organelles can burst due to osmosis, which is why we use an isotonic solution when doing cell fractionation.

But it's ok, I wish I could explain it better but I don't do chemistry. If you're doing AQA, there is a section in the CGP revision guide about how water causes substances to dissolve and you will understand why starch doesn't impact the water potential
Thanks, I will look those up. Btw, I saw you're doing A2 Biology and English Language too - I've just finished AS so looking forward to starting them
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username2769500
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(Original post by mrlaurenceo)
how does starch affect water concentration and osmosis?

My understanding was that because starch is insoluble in water it does not have an osmotic effect so it doesn't affect water concentration so there is no net movement of water from a pure water to water with starch but i've now realised that this is incorrect and that water would indeed move across the membrane. Do plants not store food molecules as starch precisely because it has no osmotic effect?
starch has no osmotic effect
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username2769500
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(Original post by glad-he-ate-her)
But the pressure that would burst the plant cells wouldnt be osmotic would it, max pressure from turgor pressure would cancel out osmotic potential and so no net water movement and cell is fully turgid, it would have to be an external, say hydrostatic pressure to burst it, or am i wrong?

I know organelles can burst by osmosis but didnt think whole plant cells could ( reasons above)
Plant cells can only explode because cellulose cell walls are very tough it's more like a metal gas cylinder in the way it would act when it has too much pressure.
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