Osmosis

Solvent: a substance that dissolves another to form a solution: Water is a solvent for sugar.

Solute: The substance that is dissolved in a solution. For solutions of fluids, the solvent is present in greater amount than the solute.

For example: In salt water the solvent is the water and the solute is the salt.

Osmosis is a special example of diffusion. It is the diffusion of solvent molecules through a semipermeable membrane from a more dilute solution to a more concentrated solution. This process is passive, which means no energy from the outside is needed.

A semipermeable membrane is a barrier that allows the passage of some substances but not others. Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes (dissolved substances).

Some major examples of osmosis:

Absorption of water by plant roots.
Absorption of water by the stomach, small intestine and colon.

Visit How Osmosis Works

Types of solutions

Isotonic Solutions: ("ISO" means same)

If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side.

Hypotonic Solutions: ("HYPO" means less)

In this case there are less solute (salt) molecules outside the cell, water will move into the cell. The cell will gain water and grow larger.

In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid. (If you have ever seen a droopy plant become revived when you water it, you have seen this process).

In animal cells, the cell could burst. However organelles called CONTRACTILE VACUOLES pump water out of the cell to prevent this.

Hypertonic Solutions: ("HYPER" means more)

In this case, there are more solute (salt) molecules outside the cell, which causes the water to move to the outside. The cell will slowly shrivel up.

In plant cells, the central vacuole loses water and the cells shrink, causing the plant to droop and wilt.

In animal cells, the cells also shrink.

In both cases, the cell may die.

This is why drinking salt watercauses dehydration.