Answer:
Biological membranes are selectively permeable because they permit the transport of small molecules,gases and non polar,hydrophobic compounds across themselves.
Explanation:
Water is passed through the plasma membrane by facilitated diffusion with the help of a protein known as aquaporins which act as channel proteins.
Various ions such as Na+,K+,Ca2+ are transport across the plasma membrane through various ion channel with the additional requirement of energy in form of ATP.
Example Na+K+ATPase,H+ATPase,Ca2+ATPase etc.
Passive transport helps in the transport of gases,water molecules along their concentration gradient that is from high concentration region to the region of low concentration until the concentration of solute molecules in both the region becomes equal.
Example Diffusion,Osmosis.
