Cell membrane

• Cell (plasma) membrane


  • Found between the phospholipids making it more rigid and stable
  • Found on either outer surface of the bilayer
  • Those with sugars attached (glycoproteins) form the glycocalyx layer of the membrane which has a role in cell-to-cell recognition or hormone receptor sites
• Components of cell membrane

  • Cholesterol
  • Extrinsic proteins
  • Phospholipid bilayer
• Phospholipid bilayer

  • The hydrophilic phosphate heads of the phospholipids form the outer and inner surface of the cell membrane
  • The hydrophobic fatty acid tails of the phospholipids point towards each other in the centre of the bilayer
• Intrinsic proteins

  • Channel proteins - pores lined with polar (hydrophilic) groups that allow charged ions through, e.g. Na+
  • Carrier proteins - allow larger polar molecules through, such as water-soluble sugars and amino acids. Binding of the molecules changes the shape of the protein moving the substance into or out of the cell
• Polarity of proteins


  Determines if they sit on the membrane (extrinsic) or through it (intrinsic)
• Water potential (ψ)


  • The tendency of water molecules to move
  • The solute potential is the osmotic strength of the solution
  • As the concentration of the solution increases, the water potential becomes more negative
• In plant cells: ψ = ψp + ψs

  • Turgid (firm) cells - in a hypotonic (less concentrated solution), cells take up water by osmosis. The pressure potential of the cell increases as the cytoplasm pushes on the cell wall
  • Incipient plasmolysis - a cell in this state has lost enough water for the cell membrane to start being drawn away from the cell wall. This lowers the pressure potential to 0
  • Plasmolysed - cells in hypertonic (more concentrated) solutions become flaccid (floppy)
• In animal cells


  • It is important animal cells are in an isotonic solution (same concentration of dissolved solutes inside and outside cell) as they lack a cell wall
  • Cells can burst in hypotonic and shrink in hypertonic solutions due to osmosis
• Tonicity

  • Hypertonic - higher concentration of solute and therefore lower water potential
  • Hypotonic - lower concentration of solute and therefore higher water potential
  • Isotonic - same concentration of solute and the same water potential, so no net movement of water between the two solutions
• Fluid mosaic model


  • Fluid - because the phospholipid molecules within a layer can move relative to each other
  • Mosaic - because the proteins within the phospholipid layer are of different sizes and shapes and form different patterns
• Cell membranes

  • Are selectively permeable, only allowing certain molecules through
  • Permeability can be increased by temperature (increases above 40ºC) and organic solvents (dissolve phospholipids)
  • Lipid soluble substances (vit A) and small molecules (O2 and CO2) can dissolve and move directly through the phospholipid bilayer
  • Water soluble substances (glucose, ions, all polar molecules) cannot pass through the hydrophobic fatty acid tails and so must use intrinsic proteins to pass though
• Diffusion


  1. Simple diffusion - movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient, a passive process requiring no energy from ATP

  2. Facilitated diffusion - diffusion of polar molecules or ions that cannot pass directly though the phospholipid bilayer, using protein channels or carriers

  3. Co-transport - a type of facilitated diffusion where two different substances use the same carrier protein at the same time
• Diffusion rate

  • Increased by higher concentration gradient, thinner membrane/shorter diffusion distance, larger surface area, smaller molecules, being non-polar or fat soluble, and increased temperature
• Respiratory inhibitor added

  No effect on rate of simple diffusion, but limits rate of facilitated diffusion due to limiting number of channels available
• Active transport

  Moves molecules against a concentration gradient, requires energy in the form of ATP from respiration to activate carrier proteins
• Respiratory inhibitor added

  Prevents active transport as there will be no ATP available
• Bulk transport

  1. Endocytosis - solids (phagocytosis) or liquids (pinocytosis) enter the cell by the plasma membrane folding inwards and engulfing the material

  2. Exocytosis - vesicle formed from the golgi fuses with the plasma membrane, emptying its contents out of the cell
• ATP required

  To move the vesicles in bulk transport, so it is an active process