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quantumwren quantumwren
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2 months ago
Hi! I am studying transpiration at the moment for GCSE Biology and I’m a bit confused about it.
It says in my book that the water vapour evaporated from the mesophyll cells into the air space. Is this an example of osmosis since the water is moving across the cell membrane or is it only diffusion? Can osmosis apply to  water in a gaseous state and if so is the process enough to ‘drive’ the water from the cell and to make it change state. Or is it a gas in the mesophyll already? I would have assumed that there would have to be an external factor supplying the energy for the change of state before the water can diffuse. I don’t seem to be able to find any examples of osmosis from one state in a cell to another state outside a cell.
Also, it says that the transpiration stream ‘pulls’ water up the xylem due to the difference in water potentials. It was my understanding that the difference in water potential only ‘causes’ osmosis in the way that it provides uneven water concentration levels which eventually lead even out due to the net movement of water particles, so that overall it seems as if all the particles have travelled from one side to the other because they are somehow drawn to lower water potential but in fact it just appears that way because they have moved in every direction. Does that make sense? I understand Water potential to be a condition which allows for this effect to be apparent but It is not a force in itself. I think perhaps I am wrong though because I have heard that water molecules are ‘attracted’ to solute particles and thus slowed down when they are near them, but again I’m not sure if that is just a simplification made because that is what appears to be happening but is in fact powered by another process that is hidden!!
Sorry for such a long question, I tried to explain it as unambiguously as I could so it’s rather repetitive I’m afraid!
I would very much appreciate any help with my confusion. Thank you  Slight Smile
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Anonymous
wrote...
2 months ago
Quote
It says in my book that the water vapour evaporated from the mesophyll cells into the air space. Is this an example of osmosis since the water is moving across the cell membrane or is it only diffusion?

In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.

Hence, this is considered diffusion.

In terms of water potential and the pull of water up the xylem, when water escapes the leaves, it creates tension in the xylem. Think of a sucking liquid from a straw; a straw works because when you suck the air out of the straw, it creates a vacuum. This causes a decrease in air pressure on the inside of the straw. Since the atmospheric pressure is greater on the outside of the straw, liquid is forced into and up the straw and into your mouth. In other words, by sucking on the straw, you decrease the pressure on the water, and therefore lower its water potential. Because water flows from regions of high water potential to regions of low water potential, the water flows out of the cup and into your mouth. The same idea applies to water inside a tree.

At the roots, their is root pressure, this is caused by the active transport of mineral ions into the root cells which results in water following and diffusing into the root by osmosis down a water potential gradient. This provides a plentiful supply of water to ensure a continuous flow.

Does that help?
quantumwren Author
wrote...
2 months ago
Thank you for your reply, it was very helpful! I’m still a little confused about the water diffusing out of mesophyll cells into the air spaces. Is the water already vapour in the cells and diffuses as a gas or is it liquid and changes state as it passes through the membrane?
Anonymous
wrote...
2 months ago
It's definitely a gas when it diffuses. When morning dew appears, that's the H2O gas reacting with the colder morning temperature forming liquid water on the surface. But it is not to be confused with liquid water escaping the leaves.
quantumwren Author
wrote...
2 months ago
I meant the stage where it leaves the mesophyll cells inside the leaf to go into the air space, also inside the leaf. So entirely in its internal environment.
I think you might be talking about the later stage when the gas then diffuses from the air spaces into the leave’s external environment. Is it also a gas inside the mesophyll cells?
Anonymous
wrote...
1 months ago
I meant the stage where it leaves the mesophyll cells inside the leaf to go into the air space, also inside the leaf. So entirely in its internal environment.

Given the limited amount of liquid released from those cells, water molecules could easily change state from liquid to gas, so it's not possible to tell. Ask yourself, what differentiates a single water molecule that is in a liquid state to one that is in gas state (it's a trick question). Nothing, since a you need many water molecules to form hydrogen bonds which dictate the state the water is in. The take home message is that it doesn't matter at that point.
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