4 & 5 Human Physiology

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

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: 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: 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

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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