Lipid membranes

• Biological membranes

  • Separate the cell contents from its surroundings (or divide cell into compartments, i.e. organelles)
  • Maintain different (bio)chemical environments between the inside and outside of the cell (or organelles)
  • Selectively permeable
• "The ability of the cell to discriminate in its chemical exchanges with the environment is fundamental to life" - selectively permeable
• Membrane structure models

  • 1915 - Red blood cell (erythrocyte) membranes were first isolated and analyzed, found to contain lipids and proteins
  • 1925 - Membranes described as a bilayer of phospholipids
  • 1935 - 'Sandwich model' - phospholipid bilayer between two layers of globular proteins
  • 1972 - 'Fluid Mosaic model' - proteins inserted into the membrane sheltering the hydrophobic regions from water
• Fluid Mosaic model


  • Membrane proteins are 'bumps' in the two layers (evidence from freeze-fracture)
• Membrane fluidity

  • Movement of phospholipids - phospholipids move laterally very fast, but flip-flopping is quite rare
  • Movement of proteins - Membrane proteins also move laterally
• Cholesterol


  Helps stabilize the membrane - high temps reduce movement (less fluid), lower temps hinder packing (maintain fluidity), acts as a 'temperature buffer'
• Fatty acid saturation


  Kinks keep the phospholipids from packing together, enhancing membrane fluidity
• 'Hop diffusion' of proteins


  • Proteins are 'fenced-in' by the cell's actin cytoskeleton but may 'hop' into another area
  • Proteins diffuse 100x slower in natural membranes compared to artificial membranes
• Mosaic nature of membranes

  • Membranes contain a complex mixture (a 'mosaic') of proteins that can be classified into 3 main groups: Integral proteins, Peripheral proteins, Lipid-anchored proteins
• Membrane proteins


  Protein structure (hydrophilic and hydrophobic domains) determines how a protein associates with a membrane
• Membrane carbohydrates


  • Short, branched oligosaccharide chains of <15 sugar units
  • Highly diverse (between species, individuals, cells)
  • Act as identity 'tags', e.g. blood groups are due to variation in carbohydrates on the surface of red blood cells
  • Most are covalently bound to proteins (glycoproteins)
  • Some covalently bound to lipids (glycolipids)
• Membrane asymmetry

  • Membranes have distinct inside / outside faces that differ in: Lipid composition, Peripheral proteins attached, Carbohydrates attached
  • Integral proteins are asymmetrical with a defined orientation
• Membrane functions

  • Compartmentalisation - Cells, organelles (mitochondrion, nucleus, chloroplast, etc.)
  • Spatial organisation of biochemical reactions
  • To provide a selectively permeable barrier
  • Cell-to-cell recognition/communication
• Selectively permeable barrier


  • The rate a molecule diffuses across a lipid bilayer depends on: Its size, But mostly on its relative solubility in oil
  • Small / hydrophobic molecules cross the membrane easily
  • The movement of ions and polar molecules, including H2O, is impeded by the hydrophobic interior of the bilayer
• Membrane transport proteins

  • Proteins with a hydrophilic channel allowing ions or molecules to diffuse passively from one side to the other
  • Proteins that bind and transport molecules in an energy requiring process
  • Transport proteins are usually quite specific for the substance transported
• Cell-cell recognition

  • Some glycoproteins (proteins covalently bound to carbohydrates) act as identification tags that are recognised by receptor proteins in other cells
  • The ability to distinguish one type of cell from another is crucial to, e.g.: During embryogenesis for the sorting of cells into tissues and organs, As the basis for recognition and rejection of foreign cells by the immune system
• Functions of membrane proteins

  • Enzymatic activity - sometimes organised in 'teams' to carry out sequential steps in a metabolic pathway
  • Signal transduction - Receptors for chemical messengers (signal molecules)
  • Intercellular joining - 'gap junctions' or 'tight junctions'
  • Attachment to the cytoskeleton and extracellular matrix (ECM) - coordination of extracellular processes with intracellular processes, maintains cell shape, stabilises location of the protein
• Issues with the sandwich model:

  • Membranes differ in composition and structure
  • Membranes proteins not very water soluble
• Membrane proteins move laterally

  Frye and Edidin studied fusion markers in mice and human cells labelled with different markers
• Types of membrane proteins:

  • Integral (transmembrane)
  • Peripheral (loosely bound via other proteins)
  • Lipid-anchored proteins (covalently attached to lipids)