The Fluid Mosaic Model: Skills (SL IB Biology)

Membranes

• Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles
• Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport
• Membranes play a role in cell signalling by acting as an interface for communication between cells

Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane

Fluid mosaic model

• The fluid mosaic model of membranes was first outlined in 1972 by Singer and Nicolson and it explains how biological molecules are arranged to form cell membranes
• The fluid mosaic model also helps to explain:
  • Passive and active movement between cells and their surroundings
  • Cell-to-cell interactions
  • Cell signalling
• The fluid mosaic model describes cell membranes as ‘fluid’ because:
  • The phospholipids and proteins can move around within their own layers
• The fluid mosaic model describes cell membranes as ‘mosaics’ because:
  • The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
• The fluid mosaic model of membranes includes four main components:
  • Phospholipids
  • Cholesterol
  • Glycoproteins and glycolipids
  • Integral and peripheral proteins

The fluid mosaic model diagram

The distribution of the proteins within the membrane gives a mosaic appearance and the structure of the proteins determines their position in the membrane