[George Mcarthy]

Hello,

I have been doing some revision on plants to prepare me as best as I can for my June exams (AQA Unit 2 AS Level), however I am struggling a bit.

It's mainly how the stalk of the plant works, i understand how it grows towards light using auxins, but when the stalk is cut on its own and cut into longnitudinal strips, why do they bend automatically.

Also when they are placed into different concentrations of sugar, why do they tend to bend differently depending on the concentration.

I understand that water will move in or out by osmosis, but why does this make it bend in the opposite direction.

I hope someone can help.

Thank you.

[Darklight*]

You have hypo-tonic solution, hyper-tonic solution and isotonic solution.

Hypotonic:
Okay so you have your cell and you have water surrounding your cell. If the water is LESS negative (-20kPa) and the inside of the cell is MORE negative (-30kPa), water will then move INTO the cell by osmosis. The reason for this is that water moves from a higher water potential to a lower water potential - this is what osmosis means.
So when the water enters the cell, there is a thing inside the cell called 'protoplast' and the gain of water causes the protoplast to be pushed against the cell wall causing an expansion of the cell till it bursts and becomes turgid - this is one reason for your plant to bend due to different concentrations.

Hypertonic:
This is now the opposite of this ^
So basically, your water on the outside of the cell is MORE negative (-30kPa) and the inside of your plant cell is LESS negative (-20kPa). Now remember, water moves from less negative water potential to a higher water potential. So in this case, the water inside the cell will move towards the outside of the cell. This will cause the protoplast to be pulled away from the cell wall (this is called incipient plasmolysis). If more water leaves the cell, the cell protoplast will completely pull away from the cell wall (this is now said to be plasmolysed). This will cause you plant cell to shrivel up and shrink. - Another reason for why your plant bends due to different concentrations.

Isotonic:
amount of water that leaves your plant cell = amount of water that enters your plant cell. This basically means your cell will not burst or shrink and will keep it in a good condition.
The plant cell is said to be in 'equilibrium' - this is the reason why in some solutions and concentrations, the plant does not change its structure and remains straight.

I have my biology AS isa on osmosis and plant stalks on monday so i think this relates to your work?

I hope this helps! Good luck!

[thegodofgod]

(Original post by Darklight*)
You have hypo-tonic solution, hyper-tonic solution and isotonic solution.
x

Rememeber that plant cells do not 'burst' as they have cellulose cell walls, which are for protection and support for when too much water moves into the cell by osmosis. Hence, they just become 'turgid'.

Animal cells, on the other hand, can burst, as they do not have cell walls for protection or support. (This is why we shouldn't consume too much water over a short period of time, as the cells will burst due to the solution outside of cells being very hypotonic.)

[Darklight*]

(Original post by thegodofgod)
Rememeber that plant cells do not 'burst' as they have cellulose cell walls, which are for protection and support for when too much water moves into the cell by osmosis. Hence, they just become 'turgid'.

Animal cells, on the other hand, can burst, as they do not have cell walls for protection or support. (This is why we shouldn't consume too much water over a short period of time, as the cells will burst due to the solution outside of cells being very hypotonic.)

Oh thanks for that!
my teacher explained it in the sense that they burst, but they use the term turgid in my text book. Thought if more water enters the cell becomes turgid and then if water carries on moving into the cell it bursts. lol thanks, wont be making the same mistake again.

[thegodofgod]

(Original post by Darklight*)
Oh thanks for that!
my teacher explained it in the sense that they burst, but they use the term turgid in my text book. Thought if more water enters the cell becomes turgid and then if water carries on moving into the cell it bursts. lol thanks, wont be making the same mistake again.

Plant cells don't ever burst - after a certain point, the cellulose cell wall becomes impermeable to water, so no more water can move in by osmosis.

This is a good diagram:

[George Mcarthy]

(Original post by Darklight*)
You have hypo-tonic solution, hyper-tonic solution and isotonic solution.

Hypotonic:
Okay so you have your cell and you have water surrounding your cell. If the water is LESS negative (-20kPa) and the inside of the cell is MORE negative (-30kPa), water will then move INTO the cell by osmosis. The reason for this is that water moves from a higher water potential to a lower water potential - this is what osmosis means.
So when the water enters the cell, there is a thing inside the cell called 'protoplast' and the gain of water causes the protoplast to be pushed against the cell wall causing an expansion of the cell till it bursts and becomes turgid - this is one reason for your plant to bend due to different concentrations.

Hypertonic:
This is now the opposite of this ^
So basically, your water on the outside of the cell is MORE negative (-30kPa) and the inside of your plant cell is LESS negative (-20kPa). Now remember, water moves from less negative water potential to a higher water potential. So in this case, the water inside the cell will move towards the outside of the cell. This will cause the protoplast to be pulled away from the cell wall (this is called incipient plasmolysis). If more water leaves the cell, the cell protoplast will completely pull away from the cell wall (this is now said to be plasmolysed). This will cause you plant cell to shrivel up and shrink. - Another reason for why your plant bends due to different concentrations.

Isotonic:
amount of water that leaves your plant cell = amount of water that enters your plant cell. This basically means your cell will not burst or shrink and will keep it in a good condition.
The plant cell is said to be in 'equilibrium' - this is the reason why in some solutions and concentrations, the plant does not change its structure and remains straight.

I have my biology AS isa on osmosis and plant stalks on monday so i think this relates to your work?

I hope this helps! Good luck!

Thank you very much it does really help me a lot i understand plant cells and osmosis and everything, it was just really confusing me why the stalk would bend one way and then the other, but is this because the stalk is only permeable on one side of the plant? so water can only move in from one side.

But yeah that's a great help, thank you so much and i hope you do well on Monday.

[GeorgieLewis]

sorry if this has already been answered but which way does the stalk bend if its hypertonic or hypotonic? is it like hypertonic it bends up and hypo it bends down or what? still a bit confused and my ISA is in the morning :/

a quick reply would be really helpful

[Gangster786]

thanks

[George Mcarthy]

sorry i haven't been online for a while:\ but i wasn't too sure myself, i think that only one side of the stalk is permeable to water, so it depends on that. But i'm really not sure. sorry