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