Q.Write two examples of impermeable membrane.

Membrane Permeability (Permeable, Semipermeable, Impermeable)

Every cell has to solve the same problem: how do you let useful things in and keep harmful things out, while still exchanging water and nutrients with the surroundings? The answer lies in how choosy a membrane or wall is about what passes through it. That "choosiness" is called permeability, and biologists sort membranes into three types based on it.

The three categories

• Permeable: allows both the solvent (water) and dissolved solutes to pass freely. An ordinary cell wall of cellulose is essentially freely permeable — water and small solutes move through it without much resistance.
• Selectively (semi-) permeable: allows the solvent to pass but restricts solutes, or lets only certain molecules through. The plasma membrane is the classic example — it permits water and specific ions/molecules while blocking others. This selectivity is what makes controlled transport, osmosis and turgor possible.
• Impermeable: blocks the passage of water and solutes almost entirely. Walls that have become cutinised, suberised or lignified (as in cork, the Casparian strip, or heavily lignified sclerenchyma) act as impermeable barriers.

Why this matters for transport …