AP BIOLOGY

Detailed explanation-1: -The plasma membranes of many cells also contain water channel proteins (aquaporins), through which water molecules are able to cross the membrane much more rapidly than they can diffuse through the phospholipid bilayer.

Detailed explanation-2: -Proteins that extend all the way across the membrane are called transmembrane proteins. The portions of an integral membrane protein found inside the membrane are hydrophobic, while those that are exposed to the cytoplasm or extracellular fluid tend to be hydrophilic.

Detailed explanation-3: -Integral membrane proteins are embedded in the membrane, usually via -helical regions of 20 to 25 hydrophobic amino acids.

Detailed explanation-4: -Small lipids and steroids are hydrophobic and can readily cross the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the cell membrane-at least not without help-because they are water-loving like the exterior of the membrane, and are therefore excluded from the interior of the membrane.