models of cell membranes

Cards (23)

  • the gorter and grendel model of the 1920s showed that the phospholipids in the membrane of cells were arranged into a bilayer
  • gorter and grendel
    evidence:
    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
  • gorter and grendel
    problems
    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
  • davson and danielli
    evidence:
    • 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.
  • davson and danielli
    problems
    • 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
  • singer and nicolson
    evidence:
    • 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
  • 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, they contain receptor proteins e.g for binding to hormones and antigens
  • phospholipids consist of a molecule of glycerol, a phosphate group which forms the phosphate head and two fatty acid tails making up the lipid tail
  • phospholipids have a polar hydrophilic head and a non polar hydrophobic tail
  • proteins in cell membranes are involved with cell transport and communication
  • cholesterol in the cell membrane increases fluidity of the membrane at low temperatures preventing it from becoming too rigid
    it 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 mechanical strength and stability of membranes; without it membranes would break down and cells would burst
  • glycolipids and glycoproteins
    on the surface of the cell
    they aid cell-to-cell communication
  • glycoproteins: proteins with carbohydrate attached
    glycolipids: lipids with carbohydrate attached
    they both 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
  • fluid mosaic phospholipid bilayer
    mosaic: scattered pattern produced by proteins and phospholipids
    fluid: molecules can move around within the membrane by diffusion
  • 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 and carrier proteins
  • 3 functions of the phospholipid bilayer
    • provides fluidity to membrane
    • creates enclosed space separating internal cell environment from external environment
    • controls exchange of substances across a membrane