Cell membrane is a protective covering that acts as a barrier between the inner and outer environment of a cell (in animals). In plant cells, the membrane encapsulates the protoplasm. This organelle is also referred to as plasma membrane. Images obtained through electron micrography reveal the bilayer structure of cell membranes. The characteristic feature of this organelle is that it allows only certain substances to pass through. Most of the research carried out for the purpose of studying cell membrane structure makes use of red blood cells (RBCs), as the absence of internal membranes and nuclei in RBCs results into the isolation process being carried out quite easily.
Information pertaining to the function of the cell membrane and its structure is presented in the following paragraphs. This description about cell membrane structure and functions should help in understanding the working in a better manner.
The cell membrane is made up of two layers that are composed of phospholipids. The bilayer is formed by the arrangement of phospholipids in a manner that their head regions (which are hydrophilic) face external environment as well as the internal cytosolic environment. The (hydrophobic) tails of these phospholipids face each other. Forces underlying the formation of this bilayer are electrostatic, van der Waals, non-covalent interactions, and hydrogen bonds. This peculiar arrangement of hydrophilic and hydrophobic layers doesn't allow nucleic acids, amino acids, proteins, carbohydrates, and ions to pass through the bilayer. Following are the various parts of the cell membrane.
- Integral Membrane Proteins:
These are structures present on the inside, outside, and also throughout the cell membrane. Fluorescence and electron microscopy can be used in viewing these proteins. These proteins are present on the entire/whole surface of the cell membrane. Examples of these structures include the cadherins, integrins, clathrin-coated pits, desmosomes, caveoles, etc.
- Peripheral Membrane Proteins:
These proteins are attached/bound to the surface of the membrane by means of hydrogen bonds and electrostatic interactions. The hydrogen bonds of these peripheral proteins are formed of hydrophilic phospholipid heads which form the bilayer.
- Skeleton of Cell Membrane:
Surface of the cell membrane on the side of cytoplasm is lined by the cytoskeleton. The framework or cytoskeleton proves to be useful in the processes of organelles like cilia. Cytoskeleton also helps in anchoring the membrane proteins to the cell membrane.
- Composition of Cell Membrane:
Proteins and lipids are important components which form the cell membrane. Different mechanisms carry out the function of incorporation and removal of materials into and out of the membrane. The process of the fusion of cell membrane with intracellular vesicles results into excretion of contents present in vesicles.
Demarcating the boundaries of a cell is the primary function of plasma membrane. The contents of a cell are supported by this membrane. Not just supporting the matter present in cells, but also the function of maintaining contact with other cells is carried out by the cell membrane. The plant cell membranes enjoy extra protection in the form of cell walls; however, in animals, cell membrane is the only covering/encapsulation. Proteins which compose (or get embedded in) the membrane carry out the diffusion of elements in a selective manner.
The plasma membrane is an important part of a cell, as it provides it with protection and also helps in maintaining a proper shape. The cell membrane structure and functions presented in the article should help in knowing more about this organelle.