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The cell membrane, also known as the plasma membrane, is a double layer of lipids and proteins that surrounds a cell and separates the cytoplasm (the contents of the cell) from its surrounding environment. It is selectively permeable, which means that it only lets certain molecules enter and exit. It can also control the amount of some substances that go into or out of the cell. All cells have a cell membrane.
Function of the Cell Membrane
The cell membrane gives the cell its structure and regulates the materials that enter and leave the cell. Like a drawbridge intended to protect a castle and keep out enemies, the cell membrane only allows certain molecules to enter or exit. 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 inside it is through endocytosis. This includes phagocytosis (“cell eating”) and pinocytosis (“cell drinking”). During these processes, the cell membrane forms a depression and surrounds 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. Vesicles are also created from the cell membrane when endocytosis is not occurring, and are used to transport molecules to different areas within the cell. Cells can also get rid of molecules through exocytosis, which is the opposite of endocytosis. During exocytosis, vesicles come to the surface of the cell membrane, merge with it, and release their contents to the outside of the cell. Exocytosis removes the cell’s waste products– parts of molecules that are not used by the cell.
The cell membrane also plays a role in cell signalling and communication. Receptor proteins on the cell membrane can bind to molecules of 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. Then, the cell can perform the action specified by the signal molecule, such as making or stopping production of a certain protein.
Structure of the Cell Membrane
Phospholipids are a main component of the cell membrane. These are lipid molecules made up of a phosphate group head and two fatty acid tails. The properties of phospholipid molecules allow them to spontaneously form a double-layered membrane. When in water or an aqueous solution, which includes the inside of the body, the hydrophilic heads of phospholipids will orient themselves to be on the outside, while the hydrophobic tails will be on the inside. The technical term for this double layer of phospholipids that forms the cell membrane is a phospholipid bilayer. Eukaryotic cells, which make up the bodies of all organisms except for bacteria and archaea, also have a nucleus that is surrounded by a phospholipid bilayer membrane.
In addition, the cell membrane contains glycolipids and sterols. One important sterol is cholesterol, which regulates the fluidity of the cell membrane in animal cells. When there is less cholesterol, membranes become more fluid, but also more permeable to molecules. The amount of cholesterol in the membrane helps maintain its permeability so that the right amount of molecules can enter the cell at a time, not too many or too few.
The cell membrane also contains many different proteins. Proteins make up about half of the cell membrane. Many of these proteins are transmembrane proteins, which are embedded in the membrane but stick out on both sides. Some of these proteins are receptors which bind to signal molecules, while others are ion channels which are the only means of allowing ions into or out of the cell. Scientists use the fluid mosaic model to describe the structure of the cell membrane. The cell membrane has a fluid consistency due to being made up in large part of phospholipids, and because of this, proteins move freely across its surface. The multitude of different proteins and lipids in the cell membrane give it the look of a mosaic.