The Structure of Cell Membranes (AQA 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, and 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 material across them, as well as acting as an interface for communication
  • Membranes are partially permeable
  • Substances can cross membranes by diffusion, osmosis and active transport

• Cellular membranes are formed from a bilayer of phospholipids which is roughly 7nm 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

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

A micelle

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

A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads 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 containing many hydrolytic enzymes that can break down many different kinds of biomolecule
• These enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components

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