GV

4 & 5 Human Physiology

4.1 Diffusion

  • Simple diffusion: Random movement of molecules due to thermal motion
    • Net diffusion: Always occurs down a concentration gradient
    • Diffusion does not require energy nor binding to a transporter protein
    • Flux: The amount of a substance crossing a membrane per unit time
    • Net flux: The difference between two one-way fluxes
  • The magnitude of diffusion flux across a membrane is determined by
    • Concentration gradient
    • Bigger difference in concentraion increases flux
    • Temperature
    • Higher temperature increases flux
    • Mass of molecule
    • Larger molecules have a harder time diffusing
    • Surface area of membrane
    • Larger surface area makes more opportunity for substances to permeate
    • Diffusion medium
    • Easier travel through water than oil
    • Diffusion distance
    • Smaller distance traveled means more flux
    • Membrane permeability
  • Lipid bilayer: The limiting factor for diffusion across cell membranes
    • Simple diffusion: The type of diffusion by while non-polar molecules readily move
  • Polar or charged molecules cannot readily cross a cell membrane via simple diffusion

Channel Diffusion

  • Ions can only move down a concentration gradient if channels are present in the plasma membrane
  • Ion Channels
    • Ion channels allow downhill movement of ions
    • Ion channels do not require binding
    • Ion channels can either be open or closed to allow movement
    • Ion channels are selective for their specific ions (sodium vs potassium channels)
  • Movement of ions across membrane influenced by electrical gradient as well as concentration gradient
  • Inside of cell has negative membrane potential
  • Intracellular fluid attracts cations and repels anions
  • Electrochemical gradient: The net flux in and out of a cell is ultimately determined by this gradient.
  • Regulation of ion channels (aka gates)
    • Ligand-gated: Cell membrane channels that open on one side of the cell membrane to open or close to passage
    • Voltage-gated: Channels that rely on electrical signals to move open or closed
    • Mechanically-gated channels: Channels of a cell membrane that must move themselves open or closed using energy
    • Leak: Non-gated cell membrane channels

Osmosis

  • Osmosis: Net diffusion of water from a region of higher water concentration to a region of lower concentration
    • Water concentration is inverselyrelated to solute concentration
    • Osmolarity: total moles of solute divided by Liters of solution
  • Aquaporins: Channels that allow the rapid passage of water through the cell membrane, necessary because water is polar
  • Permeable: If a membrane is fully _ to water and solute, both will eventually reach equilibrium
  • Semi-permeable: If membrane is _ (only permeable to water), water will move to area of higher solute concentration

Mediated Transport

  • Mediated transport: The process by which the solute moves by binding to a transmembrane protein known as a transporter

  • Mediated transport flux is affected by number of transporters, affinity of transporters, and the rate of transporter conformation change

  • Facilitated diffusion, active transport: Two broad categories of mediated transport

  • Facilitated diffusion: A type of mediated transport that involves downhill movement, no energy required

    • In facilitated diffusion, Net flux continues until equilibrium reached

  • Active transport: A type of mediated transport that involves uphill movement and requires energy (ATP)

    • Primary: This Active Transport type occurs when energy comes directly from ATP hydrolysis
    • A sodium/potassium ATPase pump is an example of Primary Active Transport
    • Secondary: This Active Transport type occurs when energy comes from an existing ion gradient
    • Cotransport: A type of secondary active transport; Transported substance can move in same direction as Na+ and it kinda hitches a ride
    • Countertransport: A type of secondary active transport; Transported substance can move in opposite direction as Na+ where it kinda sneaks against the sodium flow

Endo & Exocytosis

  • Some substances enter/exit cell without passing through cell membrane or membrane-transporter, which is endo or exocytosis
  • Endocytosis: Cell membrane folds in from the outside to create a baby pocket, losing some cell membrane and requiring ATP, and is generally used for bulk movement.
    • Endocytosis moves stuff into the cell
    • Pinocytosis, phagocytosis, and receptor-mediated endocytosis are all types of endocytosis
  • Exocytosis: Vesicle fuses to te cell membrane to release stuff outside of the cell; replaces the membrane it lost during endocytosis; most secreted substances come from the Golgi apparatus
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