Define and explain the importance of the following phenomena in living cells:
$(a)$ Diffusion
$(b)$ Osmosis.

(N/A) Diffusion: This is a phenomenon of the spontaneous movement of molecules from a region of higher concentration to a region of lower concentration. This process is faster in the gaseous phase than in liquids and solids. This process leads to a uniform concentration within a cell.
Importance: Substances (molecules,ions) such as $CO_2, O_2, H_2O$,etc.,can move across the plasma membrane by the process of diffusion.
$(b)$ Osmosis: It involves the movement of water molecules (solvent particles) from a region of their higher concentration to a region of lower concentration through a semipermeable membrane.
Importance: It is a purely mechanical diffusion process by which cells absorb water without spending any metabolic energy.