Cell membrane, which was discovered during the 2nd half of the 19th century, can be defined simply as a biological membrane that separates the interior of all cells from the outside environment. It acts as both the defense line and communication line of the cell.
Structure and function of the cell membrane
The fluid mosaic model of Singer and Nicolson is accepted internationally as the structure of the cell membrane. Before Singer and Nicolson put forward this model, two other models showing the structure of the cell membrane was proposed in the past. However, those models were rejected because they didn’t give an answer to all the questions that the scientists had about cell membrane.
Scientists’ experiments shows that the cell membrane is about 7mm thick and this model states that the cell surface membrane is made up a phospholipid bilayer with proteins randomly embedded into the bilayer.
The cell membrane’s specialty is that it composes of a variety of different proteins on its surface that are used for various functions. The phospholipid bilayer is made in such a way that the hydrophilic phosphate heads of the phospholipids face into the aqueous environment both inside and outside of the cell. The hydrophobic tails face inwards and create a hydrophobic interior. The phospholipids are fluids and move about rapidly by diffusion in their own layers. The proteins stay in the membrane because they have regions of hydrophobic amino acids which interact with the fatty acid tails to exclude water. Some proteins occur on the surface of the phospholipids while other extends into the bilayer. The ones extending in are either called integral or intrinsic proteins while the ones on the surface are called by the names peripherals or extrinsic proteins. The stability of the membrane is enabled by the mutual attraction between the hydrophobic tails and by the interaction between the hydrophilic heads with the proteins. This all embedded in the phospholipid bilayer gives the basic structure of membranes, while restricting the entry and exit of polar molecules and ions.
In the cell membrane there are also flippases and scramble’s that concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moieties moving through the membrane.
The cell membrane has the ability to form diverse types of “supra-membrane” structures such as caveola, postsynaptic density, podosome, invadopodium, focal adhesion, and various types of cell junctions.
Function of cell membrane
Cell membrane provides the mechanical structure of the cell Maintains the physical integrity of the cell by enclosing cell’s organelles inside Forms a barrier between the inside of the cell and the environment outside the cell – enclosing cytoplasm and any organelles within the cell, and enabling different chemical environments to exist on each side of the cell membrane. Separates the inside organelles from the exterior environment and regulate the entrance and exit of substances by being selectively permeable. Transport materials in and out of the cell by Active transport: movement of molecules across cell membrane from low concentration to high concentration using energy Diffusion: random movement of molecules from high concentration to low concentration until an equilibrium is achieved Facilitated diffusion: movement of specific molecules across cell membrane assisted by pore proteins or carrier proteins. Endocytosis: uptake of large particles or fluid through the surface membrane of the cell Exocytosis: cell moves the contents of secretory vesicles out of the cell via the cell membrane. Transferring information with other cells and the external environment Site where metabolic activities take place Provides protection to the cell Protects the cell from some harmful chemicals in external environment. Protects the cell from loss of useful biological macromolecules held within the cell by its plasma membrane.
Different molecules found in cell structure and their specific function
Proteins: allow specific polar molecules and ions to pass through the membrane Some act as enzymes, recognition sites and electron carriers Some serve as receptors for hormones or neurotransmitters Some play a role in cell adhesion by sticking to membrane proteins on the other cells Some act as antigens for cell recognition Channel protein: involved in the selective transport of polar molecules across the membrane Carrier protein transport molecules by facilitated diffusions and ions by active transport Glycocalyx: provides chemical protection and plays an important role in cell recognition and adhesion Glycoproteins: act as identity markers or ‘name tags’ to recognize other cells so that any foreign antigen can be detected and attacked by the immune system Glycolipids: involved in cell-cell recognition during tissue formation and act as receptor sites for chemical signals Cholesterol: acts like a plug, reducing the entry or escape of polar molecules through the membrane. It also interacts with the phospholipids to make the membrane less fluid and more stable