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?
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