AP BIOLOGY

Detailed explanation-1: -Water transport across cell membranes occurs by diffusion and osmosis. The effective osmolality of a biological fluid is determined by the total solute concentrations and the solutes’ permeabilities, relative to water.

Detailed explanation-2: -Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins.

Detailed explanation-3: -Water can pass through biological membranes via two pathways: simple diffusion through the lipid bilayer, or water-selective facilitated diffusion through aquaporins (AQPs).

Detailed explanation-4: -Carrier proteins bind specific solutes and transfer them across the lipid bilayer by undergoing conformational changes that expose the solute-binding site sequentially on one side of the membrane and then on the other.

Detailed explanation-5: -Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney (where they minimize the amount of water lost as urine).