Membrane transport

• Fatty acid tails


  • Unsaturated - double C=C bond, increases membrane fluidity
  • Saturated - decreases membrane fluidity
  • Composition can be adjusted to regulate membrane fluidity
• Fluid mosaic model


  Model of cell membrane structure
• Cell membrane


  • Amphipathic - has both hydrophobic and hydrophilic parts
  • Separates compounds inside from outside cell
  • Semi-permeable and selective
  • Phospholipid bilayer with membrane proteins
  • Glycolipids and glycoproteins act as antigens
• Cholesterol


  • Makes membrane less permeable to small water soluble molecules
  • Helps secure peripheral proteins
  • Increased temperature decreases membrane fluidity
  • Prevents stiffening at low temps, increases membrane fluidity
• Functions of membrane proteins

  • Junctions - connect 2 cells
  • Enzymes - metabolic reactions
  • Transport - substances in/out
  • Anchorage - attachment points for cytoskeleton + extracellular matrix
  • Transduction - receptors for peptide hormones
• Glycoproteins and lipids


  • Receptors for hormones
  • Immune response - markers or antigens
  • Cell to cell adhesion - form extracellular matrix, structured support, cell-cell communication, cell regulation
  • CAMS (cell adhesion molecules) - tight, gap, adherens, desmosomes
• How substances enter cells

  • Passive - osmosis, diffusion, facilitated diffusion
  • Active - active transport, endocytosis, exocytosis
• Membrane permeability


  • Permeable to non-polar, lipid soluble molecules (oxygen, carbon dioxide, steroids)
  • Mostly permeable to small uncharged polar molecules (water, urea, ethanol)
  • Mostly impermeable to ions (glucose, sucrose)
• Cell adhesion molecules


  • Tight junctions - form a seal between cells
  • Gap junctions - channels between cells that allow molecules to pass through, cell communication
  • Adherens junctions - protein complexes connect cells
  • Desmosomes - protein complexes form strong connections, structural integrity
• Simple diffusion


  1. Passive transport of particles from high to low concentration

  2. No energy from the cell is used

  3. Movement results in equilibrium
• Water movement

  • Net movement from low to high solute concentration
  • Cell membranes contain aquaporins that facilitate faster water transport
  • Aquaporin levels can be regulated to control cell osmotic conditions
  • Water is polar but small, solutes are charged so can't pass through
• Facilitated diffusion


  Passive movement of molecules across the cell membrane via the aid of a membrane protein
• Channel proteins


  • Integral lipoproteins with hydrophilic core, allow ions to cross membrane
  • Ion-sensitive, may be gated to regulate ion passage
  • Only move molecules along concentration gradient
• Carrier proteins


  • Integral glycoproteins which bind a solute and undergo conformational change to translocate it
  • Only bind specific molecules
  • When moving against gradient, require ATP hydrolysis (protein pumps)
  • Have smaller transport rate than channel proteins
• Active transport


  1. Energy moves molecules against concentration gradient

  2. Direct hydrolysis of ATP (primary)

  3. Cotransport - indirect coupling with another molecule moving along its gradient (secondary)
• Membrane permeability


  • Facilitated diffusion and active transport allow selective permeability
  • Simple diffusion permeability is not selective, depends on size and hydrophobicity
• Membrane fluidity and vesicle formation

  • Held together by weak hydrophobic associations between phospholipid tails
  • Spontaneous breaking and reforming of bilayer allows phospholipids to rearrange
  • Materials can enter/leave cell without crossing membrane
  • Requires ATP hydrolysis but is not active transport
• Endocytosis


  1. Large substances/bulk small ones enter cell without crossing membrane

  2. Invagination of membrane forms flask-like depression enveloping extracellular material

  3. Invagination sealed off to form intracellular vesicle

  4. Phagocytosis - solid substances ingested, usually transported to lysosome

  5. Pinocytosis - liquids/dissolved substances ingested

  6. Can be receptor mediated (clathrin coated pits) to control what enters
• Exocytosis


  1. Large/bulk substances exit cell without crossing membrane

  2. Vesicles (usually from Golgi) fuse with plasma membrane, expelling contents

  3. Adds vesicular phospholipids to cell membrane, replacing those lost when vesicles formed
• Gated ion channels in neurons

  • Integral membrane proteins with hydrophilic inner pore for ion passage
  • Ions exit/enter according to concentration gradient (facilitated diffusion)
  • May be ion selective and gated
• Types of gated ion channels

  • Voltage gated - cycle between open and closed conformations based on transmembrane voltage
  • Ligand gated - change conformation in response to ligand binding
• Sodium dependent glucose cotransporters

  • Carrier proteins that undergo conformational change to translocate materials across bilayer
  • Differ from ion channels in transport method
  • Can move two substances at same time in same or opposite directions (cotransport, antiport)
• Cotransport


  1. Link movement of ion along conc gradient to solute against gradient

  2. Secondary active transport, uses electrochemical gradient as energy source instead of ATP hydrolysis

  3. Absorption of glucose in kidneys and small intestine cotransported with sodium ions
• Antiport


  1. Move two molecules in opposite directions across membrane

  2. Sodium potassium pump used by neurons to establish electrochemical gradient (resting potential)