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Diffusion of water from its pure state or dilute solution into a solution or stronger solution when the two are separated by a semipermeable membrane is termed as osmosis.

(ii) The movement of water from its higher chemical potential (found in pure state or dilute solution) to its lower chemical potential (found in solution or stronger solution) without allowing the diffusion of solute by means of a semipermeable membrane is called osmosis. The chemical potential of water is also called water potential.

(iii) Osmosis is movement of solvent or water molecules from the region of their higher diffusion pressure or free energy to the region of their lower diffusion pressure or free energy across a semipermeable membrane.

n biology, there are three different types of solutions that cells can be in: isotonic, hypotonic, and hypertonic. Different types of solutions have different impacts on cells due to osmosis.

Isotonic
An isotonic solution has the same concentration of solutes both inside and outside the cell. For example, a cell with the same concentration of salt inside it as in the surrounding water/fluid would be said to be in an isotonic solution. Under these conditions, there is no net movement of solvent; in this case, the amount of water entering and exiting the cell’s membrane is equal.

Hypotonic
In a  HYPERLINK ;https://biologydictionary.net/hypotonic-solution/; o ;hypotonic solution; ;https://biologydictionary.net/osmosis/_blank; hypotonic solution, there is a higher concentration of solutes inside the cell than outside the cell. When this occurs, more solvent will enter the cell than leave it to balance out the concentration of solute.

Hypertonic
A  HYPERLINK ;https://biologydictionary.net/hypertonic-solution/; o ;hypertonic solution; ;https://biologydictionary.net/osmosis/_blank; hypertonic solution is the opposite of a hypotonic solution; there is more solute outside the cell than inside it. In this type of solution, more solvent will exit the cell than enter it in order to lower the concentration of solute outside the cell.

Hypotonic Solutions
When the overall concentration of solutes is lower on the outside of a cell than in the cytosol, we say that the cell is in a hypotonic solution (‘hypo-‘ means low). In hypotonic solutions, water flows into the cell by osmosis to try to equalize the concentration of solutes on both sides of the membrane. This means that in hypotonic solutions, our cells swell up. They can even burst!
But wait, didn’t we say earlier that water flows across membranes down its concentration gradient? However, the diagram shows the water in the hypotonic solution moving from where there is a low concentration of solutes to where there is a high concentration. Wouldn’t that be ‘up’ the concentration gradient? Well, yes, it would be up the solute’s concentration gradient, but still down the water’s concentration gradient. Just remember that where there’s a higher concentration of solutes, there is a lower concentration of water, and vice versa. So everything checks out: the water is still flowing down its concentration gradient.

It’s important to note that plant, algal, fungal, and bacterial cells have tough cell walls surrounding their plasma membranes. This means that in a hypotonic solution, the cells swell up but don’t burst. Instead, the pressure inside the cell increases. That’s one reason why plant stems can stand upright. Animal cells don’t have cell walls, so they have to regulate their volumes in other ways, such as controlling ion transport.

Hypertonic Solutions
‘Hyper-‘ means high, so a hypertonic solution is one in which the overall concentration of solutes is higher than it is in the cytosol. In a hypertonic solution, water flows out of the cell to try to even out the concentration of solutes on both sides of the membrane. This makes cells shrink or shrivel up.

Isotonic Solutions
So, if our cells want to stay the same volume without swelling or shrinking, they need to be in isotonic solutions, where the concentration of solutes is the same as the concentration in the cytosol. Here, the osmotic flow of water into and out of the cell is the same, so the cells don’t grow or shrink.

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