Plasma Membrane: Structure and Fluid Mosaic Model

Plasma Membrane Overview

• Provides a boundary controlling transport in and out of the cell.
• Primarily composed of phospholipids.

Phospholipids

• Are amphipathic: possess both hydrophilic (polar phosphate head) and hydrophobic (nonpolar fatty acid tail) regions.
• Spontaneously form a bilayer in an aqueous environment.
• Hydrophilic heads face the aqueous environment, hydrophobic tails interact internally.

Membrane Proteins

• Can be hydrophilic or hydrophobic.
• Peripheral Proteins: Loosely bound to the membrane surface, hydrophilic.
• Integral Proteins: Span the membrane, with hydrophilic ends and hydrophobic segments penetrating the bilayer.
• Functions: Transport, cell-to-cell recognition, enzymatic activity, signal transduction, intercellular joining, and attachment to extracellular matrix/cytoskeleton.

Fluid Mosaic Model

• Describes the cell membrane as a mosaic of protein molecules in a fluid bilayer of phospholipids.
• Structure is not static; held by weaker hydrophobic interactions, allowing lipids and some proteins to shift and flow.

Other Components of the Fluid Mosaic Model

• Cholesterol: A steroid that regulates bilayer fluidity under varying environmental conditions.
• Carbohydrates: Function as cell markers.
  • Glycoproteins: Carbohydrates attached to embedded proteins.
  • Glycolipids: Carbohydrates attached to lipids in the bilayer.