• 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?endoplasmic reticulum
• Membranes do not only separate different areas but also control the?exchange of substances?from one side of a membrane to the other, as well as acting as an?interface for communication
  • Membranes are?partially permeable
    • Substances can cross membranes by?diffusion?and?active transport
  • Membranes contain receptor proteins, e.g. for binding to hormones, and?antigens

Phospholipids

• Cellular membranes are formed from a double layer, or?bilayer, of?phospholipids
• Phospholipids consist of
  • A molecule of?glycerol
  • A?phosphate group, which forms the?phosphate head
  • Two fatty acid tails, making up the?lipid tail
• Phospholipids contain two distinct regions: a?polar?head and two non-polar tails
  • The?phosphate head?of a phospholipid is?polar,?meaning that it can interact with polar water molecules; the head is therefore described as being?hydrophilic
    • Hydro = water
    • Philic = loving
  • The lipid tail is?non-polar,?meaning that it cannot interact with polar molecules; the tail is therefore described as hydrophobic
    • Hydro = water
    • Phobic = hating

Phospholipids can be visually represented in different ways. In the image above the left-hand diagram shows a simple representation of the phosphate head and lipid tails, while the central diagram shows the chemical structure of each region; the separate glycerol and phosphate group can be seen, as well as the ester bonds that join the components together. The right-hand diagram shows a diagrammatic representation of the chemical structure.

• 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
• Alternatively, two-layered structures may form in sheets; these are called?phospholipid bilayers
  • Phospholipid bilayers form the basic structure of the cell membrane

Phospholipids can form monolayers and bilayers

Structure of membranes

• The phospholipid bilayers that make up cell membranes also contain?non-lipid components
  • Proteins?are involved with?cell transport?and?communication
    • The proteins can either be?intrinsic?or?extrinsic
      • Intrinsic proteins can also be referred to as integral
      • Extrinsic proteins can also be referred to as peripheral
    • Intrinsic proteins are?embedded?in the membrane with their precise arrangement determined by their hydrophilic?and?hydrophobic?regions
    • Extrinsic proteins are found on the?outer or inner surface?of the membrane
  • Cholesterol?can be found between the phospholipids, where it?regulates membrane fluidity
    • Cholesterol?increases the fluidity of the membrane at low temperatures,?stopping it from becoming too rigid
      • This occurs because cholesterol?stops the phospholipid tails packing too closely together
    • Interaction between cholesterol and phospholipid tails also?stabilises the cell membrane at higher temperatures?by stopping the membrane from becoming too fluid
      • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
    • Cholesterol?increases the mechanical strength and stability of membranes;?without it membranes would break down and cells would burst
  • Glycolipids?and?glycoproteins?are present on the surface of the cell, where they aid?cell-to-cell communication
    • Glycoproteins are?proteins with carbohydrate?attached, while glycolipids are?lipids with carbohydrate?attached
    • The glycolipids and glycoproteins?bind with substances at the cell’s surface, e.g. hormones
    • Some glycolipids and glycoproteins act as cell markers or?antigens?for?cell-to-cell recognition
      • E.g. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains

• The phospholipid bilayer with its additional components is often described as a?'fluid mosaic'
  • The scattered pattern produced by the components within the phospholipid bilayer looks somewhat like a?mosaic?when viewed from above
  • The mosaic of?phospholipids and proteins?can?move around within the bilayer by diffusion, hence the mosaic is said to be 'fluid'
    • The phospholipids mainly move sideways, within their own layer
    • The many different types of proteins interspersed throughout the bilayer move about within it, although some may be fixed in position
  • Note that the?fluid mosaic model?is one?model?of membrane structure; other models have been considered and rejected as knowledge of membrane structure has advanced
• The membrane is?partially permeable
  • Small, non-polar molecules?can pass through the gaps between the phospholipids
  • Large, polar molecules?must pass through specialised membrane proteins called?channel proteins?and?carrier proteins

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

• Scientists use?models?to represent real world ideas, organisms, processes, and systems that cannot be easily investigated
• Over time, as?technological developments?have been made, the models used to?represent the structure of cell membranes?have changed
• The?fluid mosaic model?of cell membrane structure was first proposed in 1972 by?Singer and Nicolson
• The model has?evolved over time,?and is thought to best account for the structure and functions of cell membranes as scientists?currently?understand them
  • Models can change?on the basis of new discoveries; if scientists found a new feature of cell membranes that didn't fit with the fluid mosaic model, the model would be?altered, or a?new model?introduced

• Evolving cell membrane models have included the following
  • The?Gorter and Grendel?model of the 1920s showed that the?phospholipids?in the membrane of cells were arranged into a?bilayer
    • Evidence?for this model
      • The number of phospholipids extracted from red blood cell membranes was double the area of the plasma membrane if it was arranged as a monolayer
    • Problems?with this model
      • Their model did not explain the location of proteins or how molecules that were insoluble in lipids moved into and out of the cell
  • Davson and Danielli's?model of the membrane from the 1930s suggested that the?proteins?were arranged in?layers?above?and?below?the?phospholipid bilayer
    • Evidence?for this model
      • Membranes were effective at controlling the movement of substances in and out of cells
      • Electron micrographs showed the membrane had two dark lines with a lighter band between
        • In electron micrographs, proteins appear darker than phospholipids
    • Problems?with this model
      • Freeze etched?electron micrographs of the centre of the membrane showed globular structures?scattered throughout
      • Improvements in technology used to analyse the proteins in the membranes showed that?proteins?were?globular,?varied in size?and had parts that were?hydrophobic
  • Singer and Nicolson?proposed the?fluid mosaic model?in the 1970s; the model?stated that membranes were?fluid?and that the globular?proteins?were both peripheral and?integral
    • Evidence?for this model
      • Analysis of?freeze-etched electron micrographs?showed proteins?extending?into the?centre of membranes
      • Biochemical analysis?of the plasma membrane components showed that membrane proteins are?free to move?within the bilayer

Models of cell membrane structure have evolved over time