I'm trying to work out a good explanation of the mechanism behind osmosis for my plant physiology class. The students are upper-level undergrads. There seem to be a lot of conflicting explantions, and most explanations fail to satisfy completely, for one reason or another. I was wondering if anyone on the forum had any ideas, or good sites to go to.
Osmosis
Try this: osmosis is one more example of nature seeking equilibrium. Osmosis occurs when there are more solute molecules on one side of a semipermeable membrane than the other. Since nature prefers equilibrium, solute molecules move from the more crowded side to the less crowded side and finally equilibrium is reached when there are the same number of solute molecules on each side (the concentrations are the same).
A crude analogy might be a two car train: at first the conductor only opens the first car. Of course it fills to capacity quickly and is crowded; when the connecting door is opened, passengers enter the second car to relieve the crowding. Eventually there are about the same number of passengers in each car.
A crude analogy might be a two car train: at first the conductor only opens the first car. Of course it fills to capacity quickly and is crowded; when the connecting door is opened, passengers enter the second car to relieve the crowding. Eventually there are about the same number of passengers in each car.
When you're talking about osmosis across a membrane, you have to think about osmotic pressure.
If you have a container of water and divide it in half with a semi-permeable membrane the water levels will equalize. That makes sense.
However if you add a substance to only one side of the tank, the substance doesn't migrate through the barrier. Water migrates through the barrier into the contaminated side and cause the level in the contaminated side to increase higher than the clean side. That difference in water levels is equal to the osmotic pressure.
To use a semi-permeable membrane as a filter, you have to force the dirty water through it at a pressure higher than the osmotic pressure to get any flow through it. That's called a reverse-osmosis filter. RO units are very common in industries.
If you have a container of water and divide it in half with a semi-permeable membrane the water levels will equalize. That makes sense.
However if you add a substance to only one side of the tank, the substance doesn't migrate through the barrier. Water migrates through the barrier into the contaminated side and cause the level in the contaminated side to increase higher than the clean side. That difference in water levels is equal to the osmotic pressure.
To use a semi-permeable membrane as a filter, you have to force the dirty water through it at a pressure higher than the osmotic pressure to get any flow through it. That's called a reverse-osmosis filter. RO units are very common in industries.
Tell the reason why dry grapes are absorbing water when placed in side water.
Thanks for the replies. I understand the principles of osmosis, but I wondered what the actual chemical process was that results in the change in potential.
I'm also curious as to whether water can move in response to changes in osmotic potential when an osmotic membrane is not present.
I'm also curious as to whether water can move in response to changes in osmotic potential when an osmotic membrane is not present.
| Gagnar The Unruly wrote: |
| I understand the principles of osmosis, but I wondered what the actual chemical process was that results in the change in potential. |
I'll tell you what I was told at GCSE level (I haven't taken Biology since then, I'm more of a Physics person
), but as it's only GCSE it may not be true.
Particles in the liquid move 'randomly'. If there is a higher concentration of particles on one side of a semi-permeable membrane in the liquid than on the other, it follows then that there is a greater chance that particles on the higher concentrated side will pass through than there is for particles on the lower concentrated side. This results in equilibrium after a time.
Someone else will need to confirm that though.
Osmosis requires a semipermeable membrane- this blocks some particles from crossing while allowing others to cross. Without the membrane solvent and solute particles would still move to make the concentrations equal; this is called diffusion.
An example: place salt in the bottom of a glass and gently pour water over the salt. If you hold the glass up to the light you will see "squiggles" of higher density salt water dissolving into the plain water. Eventually all of the salt will be dissolved.
An example: place salt in the bottom of a glass and gently pour water over the salt. If you hold the glass up to the light you will see "squiggles" of higher density salt water dissolving into the plain water. Eventually all of the salt will be dissolved.
| ninjakannon wrote: | ||
I'll tell you what I was told at GCSE level (I haven't taken Biology since then, I'm more of a Physics person ), but as it's only GCSE it may not be true.
Particles in the liquid move 'randomly'. If there is a higher concentration of particles on one side of a semi-permeable membrane in the liquid than on the other, it follows then that there is a greater chance that particles on the higher concentrated side will pass through than there is for particles on the lower concentrated side. This results in equilibrium after a time. Someone else will need to confirm that though. |
One of the arguments goes like this:
Water forms a lattice around solute molecules. The water in those lattices is no longer 'free' to bounce around randomly. This results in a lower effective concentration of water on the high-solute side of the membrane, so osmosis is a diffusion process.
The drawbacks to that argument are that the rate of osmosis suggests a bulk-flow type process, not diffusion. Also, it seems odd that, if that was what was really going on, particle size and charge would not matter (as it doesn't). I'm also stumped by the fact that hydronium and hydroxide ions don't apparently affect osmolarity.
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