The Fluid Mosaic Model (Cambridge (CIE) AS Biology): Revision Note

The fluid mosaic model of membranes

• 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

• 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

• Alternatively, two-layered structures may form in sheets

• These are called phospholipid bilayers – this is the basic structure of the cell membrane

• 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

  • This allows for specialisation of processes within the cell

• An example of a membrane-bound organelle is the lysosome (found in animal cells)

  • Each lysosome 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

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