The Fluid Mosaic Model (CIE A Level Biology)

• Membranes are vital structures found in all cells
• The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment
  • Intracellular membranes form compartments within the cell such as the nucleus, mitochondria and RER
• Membranes do not only separate different areas but also control the exchange of materials across them, as well as acting as an interface for communication
  • Membranes are selectively permeable
  • Substances can cross membranes by diffusion, osmosis and active transport

• Cellular membranes are formed from a bilayer of phospholipids which is roughly 7nm (7 × 10^-9 metres) wide and therefore just visible under an electron microscope at very high magnifications
• The fluid mosaic model of the membrane was first outlined in 1972 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

Phospholipids

• Phospholipids structurally contain two distinct regions: a polar head and two nonpolar tails
  • The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water
  • The lipid tail is non-polar (hydrophobic) and insoluble in water
• If phospholipids are spread over the surface of water they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water
  • This is called a phospholipid monolayer

Phospholipid Monolayer Diagram

A phospholipid monolayer

• If phospholipids are mixed/shaken with water they form spheres
  • With the hydrophilic phosphate heads facing out towards the water, and
  • The hydrophobic fatty acid tails facing in towards each other
  • This is called a micelle

Micelle Diagram

A micelle

• Alternatively, two-layered structures may form in sheets
• These are called phospholipid bilayers – this is the basic structure of the cell membrane

Phospholipid Bilayer Diagram

A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads facing outwards

• Phospholipid bilayers can form compartments – the bilayer forming the cell surface membrane establishing the boundary of each cell
• Internally, membrane-bound compartments formed from phospholipid bilayers provide the basic structure of organelles, allowing for specialisation of processes within the cell
• An example of a membrane-bound organelle is the lysosome (found in animal cells), each one of which contains many hydrolytic enzymes that can break down many different kinds of biomolecule
• These enzymes need to be kept compartmentalised otherwise they would break down most of the cellular components

Membranes in Membrane-Bound Organelles Diagram

Membranes formed from phospholipid bilayers help to compartmentalise different regions of the cell

Structure of membranes

• The phospholipid bilayers that make up cell membranes also contain proteins
  • The proteins can either be intrinsic (or integral) or extrinsic (peripheral)
  • Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions
  • Extrinsic proteins are found on the outer or inner surfaces of the membrane
• The fluid mosaic model describes cell membranes as ‘fluid’ because:
  • The phospholipids and proteins can move around via diffusion
  • The phospholipids mainly move sideways, within their own layers
  • The many different types of protein that are interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
• 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

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