3.2.3. Transport across cell membranes

• All cells and organelles are surrounded by a partially permeable membrane; protein molecules between the phospholipid molecules
• Cell membrane

  Controls the movement of substances in and out of the cell/organelle.

  • Contains receptors for other molecules (hormones)
  • Enables adjacent cells to stick together
• Cell membrane Properties

  Phospholipid

  • Barrier to most water soluble substances; cannot enter or escape
  • Non-polar tails prevent polar molecules or ions from crossing

  Fluid

  • Flexible; self-sealing

  Chemically modified to act as signalling molecules

  • Moving within belayer to activate other molecules
  • Be hydrolysed, releasing smaller water-soluble molecules that bins to specific receptors in the cytoplasm
• Protein Types

  Intrinsic - Embedded within

  Extrinsic - On the surface

  Transmembrane - Spans the entire membrane
• Protein Properties

  Aid movement across the membrane

  • Provides mechanical support; acts in conjunction with glycolipids as receptors.
• Cholesterol

  Makes membrane more rigid; reduces lateral movement of phospholipids

  • Prevents leakage of water and dissolved ions (hydrophobic tail; hydrophilic head)
  • Fits between phospholipid molecules; oriented same way
  • Prevents from packing too closely in cold; from becoming too fluid in heat
  • Increases impermeability of membranes to ions
  • Increases mechanical strength and stability of membranes
• Glycolipids

  Lipids with carbohydrate chains attached

  • Cell surface receptors; signalling and recognition
• Glycoproteins

  Extrinsic proteins with carbohydrate chains attached

  • Cell surface receptors; signalling and recognition (antigens)
  • Binds cells together to form tissues
• Diffusion

  Net passive movement of molecules from an area of high to low concentration without the use of energy

  • Passive movement
• Facilitated Diffusion

  The passive movement of substances from an area of high to lower concentration through transport proteins without the use of energy
• Osmosis

  The passive diffusion of water molecules from a region of higher to lower water potential through a selectively permeable membrane without the use of energy
• Active Transport

  Active movement of substances from an area of low to higher concentration (up a concentration gradient) through carrier proteins with the use of energy in the form of ATP
• Co-transport

  Use of ions to move substances into and out of cells
• Phospholipid bilayers can form compartments

  • Establishes boundary of each cell
  • Allows for specialisation of cell processes by membrane-bound compartments
• Fluid Mosaic Model

  Fluid:

  • Phospholipids and proteins can move around via diffusion

  Mosaic:

  • Scattered pattern produced by the proteins looks somewhat like a mosaic when viewed from above
• Describe the stricture and function of the cell-surface membrane

  ‘Fluid mosaic’ phospholipid bilayer with extrinsic and intrinsic proteins embedded

  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate transport of substances
  • Involved in cell signalling / cell recognition