Physio I Block 1


  1. Main focus.. how stuff gets in and out of cells
  2. Cell membrane – phospholipid bilayer very impermeable
    1. What gets thru: gases
    2. Everything else repelled, except some lipid-soluble molecules
      1. i. The smaller they are, more likely move by diffusion
        1. Ethanol, methanol, urea (even though urea is relatively insoluble)
  3. Everything else repelled – ions and glucose repelled
  4. Passive vs. Active
    1. Passive Mechanism – don’t require energy – move along concentration gradients.
      1. i. Aquaporins – when open, water goes through
      2. ii. Solvent vs. solute (7:30min)
      3. iii. Diffusion vs. Osmosis –
        1. if membrane permeable to both solute and solvent, then both solute and solvent will move to and fro until same concentration on both sides.
        2. Diffusion = movement of solute from high to low concentration.
        3. Osmosis = movement of solvent
          1. Measured by thistle tube (looks like plunger with semipermeable membrane with more concentration of solute in the plunger). Forward osmosis is when water goes from the pail up the plunger. Reverse osmosis when force the water out of the pail.
          2. Reverse Osmosis happens in capillaries – water gets pushed out of the capillary while RBC stay behind, then later on, past the capillaries, then water gets pushed back in.
          3. Reverse Osmosis also happens in kidneys – glomerulus filters plasma out of blood.
    2. iv. Fick’s Law – Flux of Solute proportional to Diffusion constant and Area and Difference in concentration on both sides, and reversely proportional to thickness of membrane. J = AD(C1-C2)/T
    3. v. Time to Equilibrate is proportional to K(thickness)2
      1. If 2x thick, then take 4x longer to equilibrate.
    4. vi. Tonicity – osmolarity of a solution compared to the osmolarity of a cell.
      1. 60-40-20 Rule
        1. 60% body is water
        2. 40% is intracellular
          1. i. 28L intracellular
          2. 20% is extracellular
            1. i. 10L interstitial fluid
            2. ii. 4L plasma
      2. Rules to do these problems:
        1. diarrhea, vomiting, IV all arrive via plasma department. (25:40min)
        2. b. barrier between plasma and interstitial fluid is permeable to both solute and solvent
        3. between ICF and ECF is only permeable to solvent. No movement of solute.
    5. vii. Facilitated Diffusion – cell provides no energy, but has carrier molecule.
      1. Saturable.
  5. Active Mechanism – need ATP.
    1. i. Leak channels – i.e. potassium and sodium ion leak channels.
    2. ii. V-gated channel
    3. iii. Ligand-gated channel
      1. called “receptors” rather than “channels”
      2. Named for artificial analogs that bind to them i.e. kainite, AMPA, nicotinic, cholinergic, etc.
      3. Inhibitory channels open Chloride channels – negative ions rush into cell down concentration gradient, causing membrane potential to be more negative.
      4. 2 Types:
        1. Ionotropic – post synaptic mechanism is an ion channel – fast and short duration. Ligand binds, ion channel opens.
        2. Metabotropic – receptor coupled to G-protein – slow and longer duration. Ligand binds, cell signaling happens.
        3. Equations for Membrane Equilibrium
          1. Nernst Equation – figure out the magnitude of the membrane potential with just that specific ion, then later see which direction (and whether the inside of the cell will become more + or -) the ions will flow.
          2. Goldman Equation – Big, lumped, average Nernst Equation – takes into account of all the ions, and the permeability of the cell to all the ions.
          3. 3:2 Pumps – 3 Na out, 2 K in. If 4:2 pump, too much sodium will go out, and cell will become more negatively charge (membrane potential more negative).
          4. Cl channel – is not pumped. It flows freely in and out of the cell.


Ca uptake into presynaptic nerve bind to synaptotagmin to cause neurotransmitter vesicle fusion at active site → Presynaptic nerve releases GABA, Glycine (inhibitory), or Glutamate (stimulatory) → these bind to cation-selective receptors, which cause either both Na and K (stimulatory), or Cl (inhibitory)to flow into the post-synaptic nerve. These then cause EPSP or IPSP → once enough EPSP add together to reach threshhold, they form an action potential, triggering voltage gated Na or K channels to open. These travel unidirectionally because the closing of the inactivation gate (ball and chain) cause refractory period.

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