The plasma membrane protects the integrity of the interior of the cell by allowing certain substances in, while keeping other substances out.
The eukaryotic cell membrane consists of two lipid layers. Lipid molecules are amphiphilic: they have a hydrophilic head such as phosphoglycerol and a hydrophobic tail of a long hydrocarbon chain. In the layers of the cell membrane, lipid molecules expose the hydrophilic heads to the external environment and into the cell, and the hydrophobic tails are exposed to the space between the two layers. The bilipid layer separates the internal contents of the cell from the external environment and does not allow most molecules to enter the cell. The bilipid layer contains proteins that can be receptors or transport molecules that cannot enter the bilipid layer on their own.
Passive transport through the membrane does not require energy consumption in the form of ATP, the substance is transferred along the concentration gradient. Active transport requires energy, the substance is transferred against the concentration gradient. Small molecules can freely diffuse through the bilipid layer if they are not charged. These molecules, for example, include gases: carbon dioxide, oxygen. Molecules that cannot flow through the membrane on their own or do it slowly are transported by facilitated diffusion and active transport. During facilitated diffusion, membrane proteins transport, for example, glucose and ions. During active transport, one molecule (uniport), or two molecules (coupled transport) can be transported. During coupled transport, one molecule is transported due to the concentration gradient of the other. If both molecules are transported in the same direction, it is a symport, and if in different directions, it is an antiport.
Endocytosis is the transport of molecules into the cell through the formation of vesicles (transport of cholesterol to build cell membrane). Exocytosis is the transport of substances out of the cell through the formation of vesicles (this is how stratum granulosum cells transport glycosylceramides, the component of the stratum corneum).