The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type, but for a typical human cell, protein accounts for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent of the composition by mass, with the remaining 10 percent of the composition by mass being carbohydrates. Cholesterol, another lipid composed of four fused carbon rings, is found alongside the phospholipids in the core of the membrane. A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group. The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and some of the proteins.
Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. The membrane does look a bit like a sandwich ( Figure). For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane. Plasma membranes range from 5 to 10 nm in thickness. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components-including phospholipids, cholesterol, proteins, and carbohydrates-that gives the membrane a fluid character. The model has evolved somewhat over time, but it still best accounts for the structure and functions of the plasma membrane as we now understand them. The explanation proposed by Singer and Nicolson is called the fluid mosaic model. A new model that better explains both the microscopic observations and the function of that plasma membrane was proposed by S.J. In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the core of the plasma membrane consisted of a double, rather than a single, layer.
They theorized that the structure of the plasma membrane resembles a sandwich, with protein being analogous to the bread, and lipids being analogous to the filling. The first widely accepted model of the plasma membrane’s structure was proposed in 1935 by Hugh Davson and James Danielli it was based on the “railroad track” appearance of the plasma membrane in early electron micrographs. The principal components identified at that time were lipids and proteins. The existence of the plasma membrane was identified in the 1890s, and its chemical components were identified in 1915.
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