The unit membrane, in biology, refers to the concept that all cellular membranes share a fundamental structure consisting of a phospholipid bilayer. This bilayer is composed of two layers of phospholipid molecules, with their hydrophilic (water-attracting) heads facing outwards and their hydrophobic (water-repelling) tails facing inwards.
The Unit Membrane Model
The unit membrane model, proposed by Davson and Danielli in 1935, initially suggested that the phospholipid bilayer was coated on both sides by layers of globular proteins. This model aimed to explain the observed permeability properties of cell membranes. It suggested that the proteins aided in transport across the membrane.
Key Features of the Unit Membrane:
- Phospholipid Bilayer: This is the core structural component. Phospholipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions.
- Arrangement: The hydrophobic tails of phospholipids face each other in the interior of the membrane, creating a barrier to water-soluble substances. The hydrophilic heads face outwards, interacting with the aqueous environments inside and outside the cell.
- Original Protein Placement: The initial model posited protein layers on both surfaces of the bilayer.
Evolution of the Model
The original Davson-Danielli model was later refined. While the fundamental concept of the phospholipid bilayer remained central, the placement and nature of proteins were revised. The fluid mosaic model, proposed by Singer and Nicolson in 1972, superseded the original unit membrane model.
Fluid Mosaic Model
The fluid mosaic model describes the cell membrane as a fluid structure with a "mosaic" of various proteins embedded within or attached to the phospholipid bilayer. This model highlights the following:
- Fluidity: The lipid molecules and some proteins are able to move laterally within the membrane.
- Mosaic: The membrane contains a diverse array of proteins, including transmembrane proteins (proteins that span the entire bilayer), peripheral proteins (proteins that are attached to the surface of the membrane), and glycoproteins (proteins with carbohydrate chains attached).
Importance of the Bilayer
Despite the evolution of membrane models, the phospholipid bilayer remains a central feature of all biological membranes. It provides the structural foundation and acts as a selective barrier, controlling the passage of substances into and out of the cell.
Summary
The unit membrane concept, while initially oversimplified, highlighted the crucial role of the phospholipid bilayer in cell membrane structure. Although the original model has been superseded by the fluid mosaic model, the principle of a fundamental lipid bilayer remains a cornerstone of our understanding of biological membranes.
