The biological membrane (or cell membrane and plasma membrane) is a selectively permeable membrane that allows only certain substances to pass through it and acts as a barrier between the inner and outer surfaces of the cell. Cell membrane comprises lipids and proteins along with other living molecules, which participate in the normal functioning of cells, such as cell signaling, ion channel conductance, and cell adhesion. The inner cytoskeleton of the cell connects to its outer cell wall via the cell membrane. The cell membrane defines the cells’ external boundaries and regulates molecule traffic across the boundary. In eukaryotic cells, the cell membrane divides the internal space into discrete compartments to segregate processes and components. It regulates the sequences of complex biochemical reactions, conserves biological energy, and participates in cell-to-cell communication.
Models Depicting Structure of Cell Membrane
1. Danielli and Davson Model: Danielli and Davson (the early 1930s-40s) have given the Lamellar theory in which they studied the arrangement of the triglyceride lipid bilayer on the water surface. This model States that the plasma membrane has bimolecular phospholipids made up of two protein layers present as folded ß chains. Elect electrostatic bonds attach These protein molecules to the lipid at polar hydrophilic ends.
2. Unit Membrane Model: According to this model, a cell membrane is a continuous structure having cytoplasm on one side and extracellular fluid on the other. Under the electron microscope, it appears as a thin, triple-layered structure with 7.5- 10 nm thickness. The membrane has two parallel dense strata, each with 2.5nm thickness; these strata are separated by a light inter-zone of nearly the same thickness. Isolated vesicles are formed in the cell by folding the plasma membrane into the cytoplasm; these vesicles store extracellular material through endocytosis.
3. Robertson’s Model: Through an electron microscope. Robertson revealed the tri-laminar structure of biological membranes. He observed two parallel dark hydrophilic layers (20-25Å width) and a middle light hydrophobic layer (25-35Å width) comprising the biological membrane. Organelles like nuclei and mitochondria. Endoplasmic reticulum etc., also have the same tri-laminar membrane. Robertson stated that a biological membrane comprises bimolecular lipid layers sandwiched between outer and protein layers.
4. The Fluid mosaic model: It was first proposed by S.J. Singer and Garth L. Nicolson in 1972 to explain the structure of the plasma membrane. 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 fluid mosaic model describes the structure of the plasma membrane as a mosaic of components, including phospholipids, cholesterol, proteins, and carbohydrates, that give 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 proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type. For example, myelin contains 18% protein and 76% lipid. The mitochondrial inner membrane contains 76% protein and 24% lipid. The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules. These molecules’ hydrophilic or water-loving areas are in contact with the aqueous fluid inside and outside the cell. Hydrophobic, or water-hating molecules, tend to be non-polar. A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon.
This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. They interact with other non-polar molecules in chemical reactions but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on the exterior and interior of the cell. Thus, the membrane surfaces facing the cell’s interior and exterior are hydrophilic. In contrast, the middle of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent lipid bilayer cell membrane that separates fluid within and outside the cell.