Cell Membrane
The cell membrane, also known as the plasma membrane, is a double layer of lipids and proteins that surrounds a cell. It separates the cytoplasm (the contents of the cell) from the external environment. It is a feature of all cells, both prokaryotic and eukaryotic.
a 3D diagram of the cell membrane
The cell membrane gives the cell its structure and regulates the materials that enter and leave the cell. It is a selectively permeable barrier, meaning it allows some substances to cross, but not others. Like a drawbridge intended to protect a castle and keep out enemies, the cell membrane only allows certain molecules to enter or exit.
Small molecules, such as oxygen, which cells need in order to carry out metabolic functions such as cellular respiration, and carbon dioxide, a byproduct of these functions, can easily enter and exit through the membrane. Water can also freely cross the membrane, although it does so at a slower rate.
However, highly charged molecules, like ions, cannot directly pass through, nor can large macromolecules like carbohydrates or amino acids. Instead, these molecules must pass through proteins that are embedded in the membrane. In this way, the cell can control the rate of diffusion of these substances.
Another way the cell membrane can bring molecules into the cytoplasm is through endocytosis. The reverse process, where the cell delivers contents outside the membrane barrier, is called exocytosis.
Endocytosis includes phagocytosis (“cell eating”) and pinocytosis (“cell drinking”). During these processes, the cell membrane forms a depression, surrounding the particle that it is engulfing. It then “pinches off” to form a small sphere of membrane called a vesicle that contains the molecule and transports it to wherever it will be used in the cell.
Large molecules can be taken into the cell through the process of endocytosis.
Cells can also deliver substances across the cell membrane to the external environment through exocytosis, which is the opposite of endocytosis. During exocytosis, vesicles form in the cytoplasm and move to the surface of the cell membrane. Here, they merge with the membrane and release their contents to the outside of the cell. Exocytosis removes the cell’s waste products, which are the parts of molecules that are not used by the cell, including old organelles.
The cell membrane also plays an important role in cell signaling and communication. The membrane contains several embedded proteins that can bind molecules found outside of the cell and pass on messages to the inside of the cell. Importantly, these receptor proteins on the cell membrane can bind to substances produced by other areas of the body, such as hormones. When a molecule binds to its target receptor on the membrane, it initiates a signal transduction pathway inside the cell that transmits the signal to the appropriate molecules. As a result of these often complex signaling pathways, the cell can perform the action specified by the signaling molecule, such as making or stopping the production of a certain protein.