Answer
b.
Work Step by Step
The membrane consists of a bilayer of phospholipids; these molecules have a phosphate head that is hydrophilic and a fatty acid tail that is hydrophobic. The bilayer is made up of two layers of phospholipids; the heads of the phospholipids of the inner layer face the inside of the cell whereas the heads of the phospholipids of the outer layer face the outside environment of the cell. The tails are buried on the inside of the two layers.
Because a lipid bilayer is impermeable to ions, suspended within the lipid bilayer are different proteins that allow passage of ions into and out of the cell. The bilayer also contains pumps that transport ions and other molecules against their concentration gradients. Enzymes are proteins that catalyze reactions; these proteins are also embedded in the lipid bilayer. Other membrane proteins include glycoproteins that serve to identify neighboring cells and junction proteins that connect cells to one another.
