AP BIOLOGY

Detailed explanation-1: -Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot.

Detailed explanation-2: -The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion. During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane.

Detailed explanation-3: -Small, moderately polar molecules are able to passively diffuse across the cell membrane. To transport larger, more polar compounds such as most sugars, amino acids, peptides, and nucleosides, membrane transporters are utilized.

Detailed explanation-4: -In simple diffusion, small noncharged molecules or lipid soluble molecules pass between the phospholipids to enter or leave the cell, moving from areas of high concentration to areas of low concentration (they move down their concentration gradient).

Detailed explanation-5: -The cell membrane is made of a bilayer of phospholipids, with an inner and outer layer of charged, hydrophilic “heads” and a middle layer of fatty acid chains, which are hydrophobic, or uncharged.