AP BIOLOGY

Detailed explanation-1: -The cell plasma membrane is largely composed of phospholipid bilayers, and water can cross the cell membranes via two basic pathways: simple diffusion through the lipid bilayer or transit through water-selective channels, i.e., facilitated diffusion (1).

Detailed explanation-2: -Recall that the interior of the phospholipid bilayer is made up of the hydrophobic tails. So, it’s not easy for water molecules to cross, and it is a somewhat slower process. Large, nonpolar molecules (e.g. carbon rings): These rings can pass through but it is also a slow process.

Detailed explanation-3: -Phospholipids do not interact with water because water is polar and lipids are nonpolar. The polar heads interact with water; the nonpolar tails do not.

Detailed explanation-4: -Water moves across cell membranes by diffusion, in a process known as osmosis. Osmosis refers specifically to the movement of water across a semipermeable membrane, with the solvent (water, for example) moving from an area of low solute (dissolved material) concentration to an area of high solute concentration.