THE FUNCTIONS OF CELL MEMBRANE
The cell membrane is about seven mm wide and is a functional organelle which serves as a boundary between the cell and its environment, thereby separating the contents of cells from their external environment, controlling exchange of materials such as nutrients and waste products between the two. Membranes act as receptor sites for recognizing hormones, neurotransmitters and other chemicals, either from the external environment or from other parts of the organism. One of the major properties of the cell membrane is that it is partially permeable, which is its ability to permit different substances to pass across it at different rates. Thus, it allows the passage of substances like glucose, amino acids, fatty acids, glycerol and ions along with water.
The cell membrane is almost entirely made up of proteins and phospholipids. The hydrophobic tails of phospholipids project inwards forming phospholipid bilayers which provide the basic structure of cell membranes in addition to restricting entry and exit of polar molecules and ions. The phospholipids are fluid and move about rapidly by diffusion in their own layer, thus affecting the fluidity and permeability of the membrane. The protein molecules form an irregular arrangement and float about in the fluid phospholipid bilayer forming a fluid mosaic pattern. Proteins which occur on the surface of the phospholipid layer are called extrinsic/peripheral proteins; those which extend into it are called intrinsic/integral proteins and some extend completely across the layer are known as transmembrane proteins. The proteins provide structural support for the membrane and assist the active transport of materials across the membrane. Since they are specific, they act as recognition sites for enzymes, hormones and antibodies. In general, the more protein particles present, the more metabolically active the membrane is. In addition to this, the membrane also constitutes cholesterol which makes it less fluid and increases its stability and flexibility. Some proteins and lipids have short branching carbohydrate chains like antennae, forming glycoproteins and glycolipids respectively. Glycoproteins act as antigens and recognition sites; glycolipids may act as receptor sites for chemical signals apart from being involved in cell-cell recognition.
