Composition of the Plasma Membrane
The plasma membrane, also known as the cell membrane, is primarily composed of the following components:
- Phospholipids : These form the fundamental structure of the membrane, arranged in a bilayer. Each phospholipid molecule consists of a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate-containing head (hydrophilic)
. This bilayer creates a semi-permeable barrier between the cell's interior and its external environment.
- Proteins : Embedded within or attached to the phospholipid bilayer, proteins can be integral (spanning the membrane) or peripheral (attached to the surface). They are involved in various functions such as transport, signaling, and structural support
- Cholesterol : Found interspersed among the phospholipids (especially in animal cells), cholesterol helps modulate the fluidity and stability of the membrane
- Carbohydrates : These are attached to proteins (forming glycoproteins) or lipids (forming glycolipids) on the extracellular surface of the membrane. Carbohydrates play key roles in cell recognition, signaling, and adhesion
Typical Composition by Mass (Human Cell Example)
Component| Approximate Percentage by Mass
---|---
Proteins| 50%
Lipids| 40%
Carbohydrates| 10%
_Note: The exact proportions can vary between different cell types and organisms
._
Summary Table
Component| Structure/Function
---|---
Phospholipids| Bilayer forms basic membrane structure, barrier function
Proteins| Transport, signaling, structural support
Cholesterol| Modulates fluidity and stability
Carbohydrates| Cell recognition, signaling, adhesion
The plasma membrane is thus best described by the fluid mosaic model, in which a dynamic mix of lipids, proteins, and carbohydrates gives the membrane both structure and flexibility
