Biomembranes

Introduction

     Every cell, whether it is prokaryotic or eukaryotic, is enclosed in a thin covering called cell membrane. It is also called as plasma membrane or plasma lemma. The cell membrane can only be seen through the electron microscope since its thickness is well below the resolving power of human eye.

Unit Membrane Concept

     The molecular structure of cell membrane is still not clearly known. Several models have been proposed from time to time to explain its structure based on its physico-chemical properties.

Fluid Mosaic Model

     In the year 1972 Singer and Nicolson proposed a model for explaining the membrane structure, taking into account all the known facts. According to this model, cell membrane consists of a highly viscous fluid matrix of a bilayer of phospholipids having globular proteins associated with them. This model came to be known as fluid mosaic model.

Transport Across Membranes

     Biomembranes form physical boundaries between organelles and cytoplasm as well as cell and its surroundings medium. Materials are passed across the biomembrane by various processes. The membrane allows only selected particles based on their size, shape, texture etc, to move in and out. Hence, it is described as selectively permeable. The movement of materials across the membrane involves both physical and biological processes.

Physical Processes

     The physical processes involving movement of materials in and out of a cell are diffusion and osmosis. Both these movements involve movement along the concentration gradient. Hence, there is no expenditure of energy.

Biological Processes

     The movement of certain substances will have to occur against a concentration gradient. As such the movement requires some biological process involving expenditure of energy.

Summary

     The plasma membrane forms a protective covering around the cell to maintain its size and shape. In addition, it is also responsible for the interactions that occur between the cell and its environment.