Cell membrane and transport 1.3

• Cell (plasma) membrane


  • Found between the phospholipids making it more rigid and stable
  • 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

  
• Extrinsic proteins


  • 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

  
• Intrinsic proteins


  • These proteins span the whole phospholipid bilayer and can form:
  • 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
• Cholesterol


  Found in the phospholipid bilayer
• 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
• Osmosis


  The diffusion of water from a region of high water potential to low water potential across a selectively permeable membrane
• In plant cells


  1. ψ = ψp + ψs

  2. 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

  3. 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

  4. 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


  • Refers to concentration of solute
  • Hypertonic – means higher concentration of solute and therefore lower water potential
  • Hypotonic – means lower concentration of solute and therefore higher water potential
  • Isotonic – means the 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
• Permeability


  • Cell membranes are selectively permeable. They only allow certain molecules through
  • Permeability can be increased by:
  • Temperature – increases above 40ºC increase vibrations of phospholipids, moving them further apart
  • 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 – the movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient. It is a passive process and so requires no energy from ATP

  2. Diffusion rate is increased by: higher concentration gradient, thinner membrane/shorter diffusion distance, larger surface area, smaller molecules, being non-polar or fat soluble, increased temperature
• Facilitated diffusion


  1. This is the process of diffusion but for polar molecules or ions that cannot pass directly though the phospholipid bilayer. Protein channels or carriers are used

  2. A continuing increase in the concentration will eventually lead to a maximum rate being reached due to the limiting effect of the number of channels available. This is a passive process; therefore, the respiratory inhibitor has no effect
• Co-transport


  This is a type of facilitated diffusion where two different substances use the same carrier protein at the same time. An example would be a molecule of glucose and 2 sodium ions attaching to a carrier protein on the outer side of the membrane. This changes the shape of the protein sufficiently to flip them to the inside of the membrane. They can then diffuse separately through the cell
• Active transport


  1. This moves molecules against a concentration gradient, i.e. from where they are in lower concentration to where they are already at a higher concentration. This process requires energy in the form of ATP from respiration. The ATP activates carrier proteins to move molecules across the cell membrane

  2. As this relies on ATP, the addition of a respiratory inhibitor or lack of oxygen will also prevent transport as there will be no ATP available
• Bulk transport

  Endocytosis (two main types):

  Phagocytosis – solids enter the cell

  Pinocytosis – liquids enter the cell

  1. Plasma membrane folds inwards
  2. Plasma membrane engulfs the material
  3. Vesicle formed from plasma membrane enters the cell
• Exocytosis:

  1. Vesicle formed from the golgi moves towards the plasma membrane
  2. Vesicle fuses with plasma membrane
  3. Vesicle contents empty out of cell
  4. ATP is required to move the vesicles so this is an active process