Plasma Membrane or Cell Membrane | Definition, Structure, Functions
Plasma membrane is an absolute requirement for all living organisms as it is responsible for the relationship of a cell with the outside world. The detaled structure of the membrane was studied only after the invention of electron microscope in the 1950s. Meanwhile, chemical studies on the cll membrane enabled the scientists to deduce the possible structure of the plasma membrane. The chemical studies done especially on the human red blood cells (RBCS), enabled the scientists to deduce the possible structure of plasma membrane.
On the basis of these studies, scientists found that the cell membrane is composed of lipids that are arranged in a bilayer. These lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic (non-polar) tails towards the inner sides. The polar ends (head) interact with water and are called hydrophilic. This ensures that the non-polar tail of saturated hydrocarbons or hydrophobic tail is protected from the aqueous environment.
Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte (RBC) has approximately 52 percent protein and 40 percent lipids.
Depending on the ease of extraction, membrane proteins can be classified as integral or peripheral. The peripheral proteins lie on the surface of the membrane while the integral proteins are partially or totaly buried in the membrane. The integral proteins which run throughout lipid bilayer are known as tunnel proteins (Trans membrane proteins). These proteins cannot be removed easily and their removal requires crude methods of treatment like detergents. Thus, the membrane has been described as Protein icebergs floating in sea of phospholipids. An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as Fluid Mosaic Model.
According to this, the quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.
1. The fluid nature of the membrane is important from the point of view of functions like cll growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.
2. Plasma membrane allows the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side of it. The passage of substances across cell membranes occurs by various methods such as passive transport, active transport.
- Passive transport : Many molecules can move across the membrane without any requirement of energy and this is called the passive transport. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient i.e., from higher concentration to the lower.
Water may also move across the plasma membrane from higher to lower concentration. The movement of water by diffusion through membrane is called osmosis. As the polar molecules cannot pass through the non-polar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
- Active transport: It is an uphill movement of materials across the membrane where the solute particles move against their concentration gradient i.e., from their lower to higher concentration. Such a transport requires energy, which is obtained from ATP. Thus, it is an energy dependent process.
For example, Na+/K+ pump in animal cells.