The cell membrane is composed of a phospholipid bi-layer. Each phospholipid has a hydrophilic region (head) and a hydrophobic region (tail). This leads to the phospholipids arranging themselves into two layers with the tails forming a hydrophobic region in the middle.
Within the phospholipid structure, many proteins are embedded. Peripheral proteins do not penetrate the lipid region, whilst integral proteins do. Many of the proteins are transmembrane, meaning they span the entire width of the membrane. The types of proteins involved in active transport include channel proteins, carrier proteins, or transport proteins. There are also proteins responsible for cell-cell joining, cell recognition, as well as receptor proteins and enzymes.
The plasma membrane features a fluid structure in which phospholipids and proteins are able to move laterally. The cytoskeleton also provides scaffolding and anchorage for membrane proteins.
Passive transport (no energy required), which involved movement of substances or molecules down their concentration gradient is possible across the cell membrane due to its structure. Small molecules such as diatomic oxygen, carbon dioxide and water can diffuse directly between the phospholipid structure. This is important as these substances are constantly used and exported from most cells.
Other molecules may be unable to diffuse in this manner due to their size or polarity and instead require channel or carrier proteins found in the cell membrane to be able to diffuse. This process is known as facilitated diffusion. The channel proteins allow specifically shaped molecules to travel through a corridor, whilst carrier proteins involve binding to a specific, complementary molecule and a slight shape change of the protein in order to translocate the substance.
Active transport, which involves the expenditure of energy to transport solutes against their concentration gradient, is also possible due to specific proteins found within the cell membrane. Transport proteins, such as the sodium-potassium pump, involve binding to specific solutes and shape change to translocate. These transport proteins are often similar to proteins used in facilitated diffusion however they require ATP to function.
Cytosis is another method of active transport. Endocytocis involves engulfing solids (in a process known as phagocytosis) or liquids (pinocytosis) to form vesicles or food vacuoles containing the engulfed substances. Exocytosis works in a similar way, but involves vesicles from within the cell fusing with the plasma membrane and expelling its contents into the extracellular environment. This process is possible particularly because of the fluid nature of the cell membrane.