The cell membrane (Plasma Membrane): Structure, Function.

The cell membrane, also called the plasma membrane, is found in all cells and separates the interior of the cell from the outside environment. The cell membrane consists of a lipid bilayer that is semi-permeable. The cell membrane regulates the transport of materials entering and exiting the cell.

What is Cell Membrane?

The cell membrane is also referred to as the plasma membrane, cytoplasmic membrane, and has historically been referred to as plasmalemma. It is the semipermeable thin double-layer structure of lipids and proteins that surrounds almost every living cell.

Cell membrane provides the structure of the cell and it also separates the cytoplasm from the out environment. The cell membrane regulates the selective transport of the material entering and exiting the cell.

The term cell membrane was coined by C. Nageli and C. Cramer in 1855 and the term plasmalemma has been given by J.Q. Plowe in 1931.

All biological membranes including plasma membrane and internal membranes of the eukaryotic cell (i.e., membranes bounding endoplasmic reticulum or ER, nucleus, mitochondria, chloroplast, Golgi apparatus, lysosomes, peroxisomes, etc.) are similar in structure (i.e., fluid-mosaic) and selective permeability but differing in other functions.

Cell membrane constituent as the essential physiological barrier at the surface to the cells. Also, a cell has a different environment from the milieu which surrounds them. This difference is maintained by the cell membrane which is responsible for ion and fluid transport. The absorption of cell molecules and the uptake of macromolecules and particular material by endocytosis.

The cell membranes also interact with the surrounding extracellular matrix molecules and with the underlying intracellular cytoskeletal framework and its associated proteins, which regulate the cell shape and respond to external and internal stimuli.

composition of cell membrane

Cell membranes are composed primarily of fatty-acid-based lipids and proteins. Membrane lipids are the principal of two types, phospholipids and sterols (generally cholesterol). Both types share the defining characteristic of lipids they dissolve readily in organic solvents but in addition, they both have a region that is attracted to and soluble in water.

This “amphiphilic” property (having a dual attraction; i.e., containing both a lipid-soluble and a water-soluble region) is basic to the role of lipids as building blocks of cellular membranes. Membrane proteins are also of two general types. One type, called the extrinsic proteins, is loosely attached by ionic bonds or calcium bridges to the electrically charged phosphoryl surface of the bilayer.

They can also attach to the second type of protein, called intrinsic proteins. The intrinsic proteins, as their name implies, are firmly embedded within the phospholipid bilayer. In general, membranes actively involved in metabolism contain a higher proportion of protein.

Structure of Cell Membrane

Like all other cellular membranes, the plasma membrane (cell membrane) consists of both lipids and proteins. The fundamental structure of the membrane is the phospholipid bilayer, which forms a stable barrier between two aqueous compartments. In the case of the cell membrane, these compartments are the inside and the outside of the cell. Proteins embedded within the phospholipid bilayer carry out the specific functions of the cell membrane, including selective transport of molecules and cell-cell recognition.

Fluid mosaic model:

Scientists identified the cell 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 cell membrane’s (plasm membrane) 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.

The cell membrane, also called the plasma membrane, is found in all cells and separates the interior of the cell from the outside environment.

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 cell membrane function.

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 give the membrane a fluid character.

cell 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 cell membrane. The membrane does look a bit like a sandwich.

A cell 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 cell 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 cell membrane of human red blood cells is 30 percent lipid. Carbohydrates are present only on the cell membrane’s exterior surface and are attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids.

The function of Cell Membrane

Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly.

Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict the diffusion of highly charged molecules, such as ions, and large molecules, such as sugars and amino acids. The passage of these molecules relies on specific transport proteins embedded in the membrane.

Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to catalyze passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy.

The ability to maintain concentration gradients and sometimes move materials against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products.

Other transmembrane proteins have communication-related jobs. These proteins bind signals, such as hormones or immune mediators, to their extracellular portions. Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules. Like transport proteins, receptor proteins are specific and selective for the molecules they bind.

Peripheral membrane proteins are associated with the membrane but are not inserted into the bilayer. Rather, they are usually bound to other proteins in the membrane. Some peripheral proteins form a filamentous network just under the membrane that provides attachment sites for transmembrane proteins. Other peripheral proteins are secreted by the cell and form an extracellular matrix that functions in cell recognition.

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