OpenStax Biology 2e
Scientists identified the plasma membrane in the 1890s, and its chemical components in 1915. The principal components they identified were lipids and proteins. In 1935, Hugh Davson and James Danielli proposed the plasma membrane’s structure. This was the first model that others in the scientific community widely accepted. It was based on the plasma membrane’s “railroad track” appearance in early electron micrographs. Davson and Danielli theorized that the plasma membrane’s structure resembles a sandwich. They made the analogy of proteins to bread, and lipids to the filling. In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the plasma membrane’s core consisted of a double, rather than a single, layer. In 1972, S.J. Singer and Garth L. Nicolson proposed a new model that provides microscopic observations and better explains plasma membrane function.– What are membrane proteins that adhere only temporarily to the biological membrane with which they are associated?
The explanation, the fluid mosaic model, has evolved somewhat over time, but it still best accounts for plasma membrane structure and function as we now understand them. The fluid mosaic model describes the plasma membrane structure as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. 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. The membrane does look a bit like a sandwich.– What is a type of membrane protein that is permanently attached to the biological membrane?
A plasma membrane’s principal components are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and proteins. A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group. Cholesterol, another lipid comprised of four fused carbon rings, is situated alongside the phospholipids in the membrane’s core. The protein, lipid, and carbohydrate proportions 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, and carbohydrates comprise the remaining 10 percent. However, protein and lipid concentration varies with different cell membranes. For example, myelin, an outgrowth of specialized cells’ membrane that insulates the peripheral nerves’ axons, contains only 18 percent protein and 76 percent lipid. The mitochondrial inner membrane contains 76 percent protein and only 24 percent lipid. The plasma membrane of human red blood cells is 30 percent lipid. Carbohydrates are present only on the plasma membrane’s exterior surface and are attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids.Cell membranes represent important cellular barriers and first-contact structures of normal and cancer cells. Extracellular signals from ions, hormones, cytokines, enzymes, growth and motility factors, receptors, extracellular matrix (ECM), other stromal elements, and subcellular membrane vesicles must first interact with the cell membrane to initiate signaling processes. Therefore, cell or plasma membranes are cellular filters that can selectively transmit signals and substances from outside cells and from adjacent cells into a cell’s interior. Conversely, they can also release signals and molecules to other cells and the micro- and macro-environment in a complex process that has been termed “social cell biology”. In addition, cells are compartmentalized into organelles by various intracellular membrane structures that are responsible for biosynthesis, energy production, replication, transportation, recycling, destruction, secretion, and other cellular activities.Cell membranes are intimately involved in the biochemical events that define cancers, and in particular, they are intensely involved in cancer metastasis. In addition, the establishment of metastases also requires a complex interplay between malignant cells, normal cells, stroma, and ECM in their new microenvironments, and these interactions are primarily mediated through cell membranes.
Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e