Some protein molecules exist outside the lipid layer called peripheral protein molecule and some are partially or entirely pass across the lipid layer, called integral protein molecules. Ĭhemically, the second major component of plasma membranes is proteins. They make the plasma membrane look like a mosaic. The phospholipid layer also contains protein and cholesterol.
Top head of phospholipids is hydrophilic while tail end is hydrophobic. Each phospholipid molecule contains three-carbon glycerol backbone with two fatty acid molecules which are attached to carbons 1 and 2 and a phosphate-containing group that is attached to the third carbon.Įach phospholipid molecule has a water-soluble polar head and two fat-soluble non-polar tails. In myelin, the proportion of proteins and lipid are 18% and 76% respectively while the inner membrane of mitochondrial contains 76% protein and 24% lipid.Īccording to this theory, the main component of the cell membrane is a bimolecular lipid layer which actually consists of two rows of amphiphilic phospholipids molecules. Generally, the percentages of proteins, carbohydrates, and lipids in the plasma membrane vary with cell type. which gives the membrane a fluid character. According to this model, the plasma membrane looks like a mosaic which contains some components like phospholipids, cholesterol, proteins, and carbohydrates, etc. This model is also known as Singer – Nicolson’s fluid mosaic model. This model was observed by English Scientists S. There are also some discontinuous peripheral globular proteins arranged just outside and along the surface of the phospholipid bilayer. They remain partially embedded in and partially protruding from the phospholipid bilayer. The lipid-globular protein mosaic model suggests, as the name implies, that instead of a continuous layer of protein on the surface of the membrane there is discontinuous mosaic globular protein. In this case, phosphate group is positively charged while lipid group is negatively charged.Īlso read: Cell Wall: Structure and Functions The Fluid Mosaic Model of Plasma Membrane Its top end contains phosphate group and the tail end bears lipid group. The lipid layers consist of most of the phospholipids. In this case, thickness of each protein layer is 20 Å. Two densely protein layers are also found in plasma membrane. Triple-layered structure of plasma membrane was discovered by J.D. If you observe under a high magnification electron microscope, you will find double layer of lipid molecules 35 Å thick. The plasma membrane is composed of triple-layered structure. Such thickened areas of the plasma membrane are known as desmosomes.Īccording to Dannielli and Davson (1935), the plasma membrane is about 75-80 Å in thick.
From these areas, many fine filaments are seen, known as tonofilaments radiate towards the interior of the cell. In between two adjacent cells, the plasma membranes become thicker in certain regions. If you observe it under an electron microscope, finger-like brush borders are available. The plasma membrane is invisible and sometimes it contains brush border or sac-like structure, known as pinocytic vacuoles. Plasma membrane lies between the cell wall and cytoplasm in the bacteria and plant cells and it is the outer limiting membrane of most animal cells. According to some Scientists, cell membrane originated from the endoplasmic reticulum. It distinct the content of cell from the outside environment.Ĭarl Nägeli and Carl Cramer first discovered the term cell membrane in 1855 while Janet Quentin Plowe given the term plasmalemma in 1931. All types of cells are bounded by a thin, porous selectively permeable membrane which is known as the plasma membrane, cell membrane, plasmalemma or cytoplasmic membrane.