Osmosis is the diffusion of water through a semipermeable membrane. Cell membranes are semipermeable membranes that can exhibit different degrees of permeability to water. How can biological systems change cell membrane permeability to water?

Biological systems can alter cell membrane permeability to water by modifying membrane lipid composition, adjusting cholesterol content, and regulating aquaporins.

Explanation:

Osmosis is the diffusion of water through a semipermeable membrane from an area of higher water concentration to an area of lower water concentration, moving down the water's concentration gradient. In biological systems, this process is vital for maintaining cellular function and homeostasis. Biological systems can change the cell membrane permeability to water by altering the composition of fatty acids in the membrane, changing the amount of cholesterol within the membrane, or modifying the expression and function of aquaporins (protein channels that facilitate the movement of water).

For example, increasing the number of unsaturated fatty acids in the cell membrane can increase its fluidity and permeability to water. Conversely, adding more cholesterol can decrease membrane permeability by stabilizing the membrane structure. The regulation of aquaporins is especially important in cells that handle large volumes of water, such as in kidney tubules, where the presence or absence of these channels can significantly alter the amount of water reabsorbed into the bloodstream.