In 1972 scientists S. J. Singer and G. L. Nicholson described the most widely accepted model of cell membrane structure called the fluid-mosaic model. In this model, protein molecules in the lipid layer are compared to icebergs floating in the ocean. This is called the iceberg model. Cell membrane structure is discussed according to fluid-mosaic model
- Phospholipid: There are five types of phospholipids in the cell membrane. Lecithin, cephalin, glycolipid, glycophosphotide and phosphotidic acid. Phospholipids are arranged in two layers. Each layer is composed of numerous lipid molecules. Each lipid molecule has two parts. head and tail The large outer part is called the head. It is spherical or oval and composed of phosphates. It is polar and hydrophilic. The two fine filament-like parts attached to the head are called tails. It is composed of fatty acids. The tail is non-polar and hydrophobic. Between the head and the tail is the gyserol. Lipid molecules are always moving, shaking and bouncing around each other. This type of movement is called flip flop movement. As the flip flop motion changes the space between the two layers, the membrane feels like a fluid.
- Protein: There are three types of proteins in the plasma membrane. These are-
(i) Peripheral protein: The protein located at the edge of the lipid layer is called peripheral protein. It looks round. It is enzyme in nature. It remains attached to the cytoskeleton. Adenylate cyclase produces cyclin AMP from ATP by the action of the membrane bound enzyme Adenylate cyclase.
(ii) Integral protein: The protein that is embedded inside the lipid layer is called integral protein. It looks oval. Acts as a carrier and transports essential substances. It maintains cell-to-cell adhesion.
(iii) Inter membrane protein: The protein that extends from one end of the lipid layer to the other end is called intermembrane protein. It is the largest protein. It takes energy from ATP to change its conformation. As a result, a hole is created through it. Essential substances move through these pores. It acts as a receptor for hormones, neurotransmitters, mediated endocytosis, insulin etc. Various molecules, ions and electrons move through channels, pumps and electron transport chains.
- Glycocalyx: Carbohydrate chains on the outside of the cell membrane. Carbohydrate chains are attached to lipids to form glycolipids and proteins to form glycoproteins. Glycoproteins and glycolipids together are called glycocalyx or cell coat. It acts as a cell recognizer. It helps in the movement of essential substances.
- Cholesterol: Cholesterol is a type of steroid. It has an OH group at the head and is hydrophilic. The other part is water repellent. Cholesterol is sandwiched between phospholipid molecules. It is irregularly arranged. Cholesterol is higher in animal cells and lower in plant cells.
Protein molecules are scattered between phospholipids in the cell membrane. Because of this, the protein molecules look like a mosaic when viewed from the surface of the cell membrane. To explain this situation in one word, the cell membrane model has been named fluid mosaic model.