How does osmosis occur?

Hint: Osmosis is the diffusion of water molecules from the high concentration to the low concentration across the semipermeable membrane without the expense of energy.

Complete answer:
Osmosis is the spontaneous movement of solvent particles across the semipermeable membrane. The semipermeable membrane is the membrane across which small molecules of the solvent like water can pass through easily without any energy input (passively) but the large molecules of solute cannot cross the membrane.

Osmosis occurs when there are two different concentrations across the membrane. The solution which has a higher concentration has low free energy which makes it more thermodynamically stable. Due to the different concentrations, there is a free energy difference between the two sides and the solutions will try to move to the side with low free energy. This means the solvent molecules will move from an area of low concentrated solution to the high concentrated solution. This movement of the solution is called osmosis.

 There are two types of osmosis:
1)Endosmosis– When the solvent molecules move inside the cell and the cell becomes turgid (plasmolysis occurs). This is known as endosmosis.
2)Exosmosis– When the solvent molecules move outside the cell and the cell becomes flaccid (plasmolysis occurs). This is known as exosmosis.

Note: Osmosis helps in the transport of the nutrients and release of metabolic products across the membrane. They also maintain the turgidity of the cell and protect the plant cell against drought injury.