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Just 2 lectures to go! Today : Diffusion & Nerves Tuesday : Nerves & Review . Announcements. HW #10 assigned. Covers diffusion & Nerves Due Wed 12/11, 5 pm, Rm 365 LLP . Marco: Office Hrs 10am- 12pm, Tuesday. . Final exam: Dec 18 th , 7-10pm Here .

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  1. Just 2 lectures to go! Today : Diffusion & Nerves Tuesday : Nerves & Review Announcements HW #10assigned. Covers diffusion & Nerves Due Wed 12/11, 5 pm, Rm 365 LLP. Marco: Office Hrs 10am- 12pm, Tuesday. Final exam: Dec 18th, 7-10pm Here. Marco’s Office hr: Dec 17, 7-9pm Formally covers everything, but aside from some fundamentals, particularly covers Lec 13 (Oct 8th) – end HW # 6-10. i.e., after Mid-term exam, Oct 10th) Final written Report Due 7pm Dec 18th, at start of exam.

  2. Today: briefly more diffusion Why x2 = #Dt (from Equipartition Function) D= diffusion constant # = 2, 4 ,6 depending on 1-2- 3-dimensions short distances, diffusion of small molecules very good. Biological examples 1. Bacterial vs. Eukaryotic Cells (Bacteria small enough to use solely diffusion: eukaryotes requires cytoplasmic motors) 2. Oxygen transport: cells need to be to close enough for oxygen to diffuse from blood. Oxygen in blood in Lungs 3. TodayStopping time of Bacteria.

  3. How Bacteria moveInertia doesn’t matter for microscopic worldLife at low Reynold’s number • Why study? • Simple Example of F= ma • Doesn’t need much biology • Results are broadly applicable to microscopic level. Bacteria Compared to you? Walking, swimming? Walking: 4 miles/hr = 6 ft/sec = body length/sec Swimming: 50 m in 0.5-1 minute (me, Olympian) = ½-1 body length/sec Bacteria: really good swimmers

  4. If turn off “propeller,” how far Bacteria coast? (linear in v at low Reynold’snumber) F = ma m dv/dt= Fdrag-friction = -gv g = drag coefficient What is drag cofficient? What does it depend on? Goopiness of fluid: h = viscosity b) Dimension of object: bigger object, harder to push through Fdrag = 6phrv :assume a sphere, with radius r, moving at velocity v. g = 6phr

  5. Solve equation of motion F = ma m dv/dt = -gv What’s mass, drag of bacteria?

  6. Plugging in the #s to get characteristic time, t = m/g • m = 4 x 10-15 kg • = 20 nN-s/m • t = m/g = 0.2 msec So bacteria stops in 200 nsec! Very fast. So once forces are turned off (or on), bacteria forgets about history very quickly. History does not matter for bacteria (or small things). Once force is over, no forward motion! Inertia is irrelevant for small things.

  7. Size of drag force on bacteria How does this compare to it’s own weight? w = mg = (4 x 10-15 kg) (9.8 m/s2) = 0.04 pN Bacteria swims as if dragging 10x their own weight!

  8. Compare you and bacteria swimming. You: good swimmer. Stop stroking, coast about 1 body length. Bacteria: no coasting. Going through concrete. Inertia is much more important to a bigger organism.

  9. Today: Ion Channels Ion Channels are membrane-bound proteins Involved in communication 3 types, voltage, ligand and mechanically-sensitive Nerves—flow of ions is electricity for transferring messages and activating muscles. They rely on “batteries”—constant source of voltage Voltage generated through K+/Na+ exchange. On/Off is digital, not analog–have transistors in you. Analogs in Fruit flies have relevance for humans.

  10. Nerve Mutation in Potassium Ion Gene

  11. Ion channels are used to communicate to a cell. Are turned on/off by 3 types of signals. Every cell in every organism has ion channels. 1. 2. Photons C.N.S. 3. how many different ion channels? Ans: 25,000 genes: 5000 genes. One or more polypeptide/ion channel– could get less, or more, ion channels.

  12. www.nikonsmallworld.com/gallery/year/2005/36 http://en.wikipedia.org/wiki/Electrical_synapse (Focus on Chemical Synapse) Ion Channels Electrical 3.5 nm synapses Fastest. Bidirectional, no gain (post< pre). Chemical 20-40 nm synapses Unidirectional (gain). 0 mV (Outside) 0 mV (Inside) open (depolarized) -60 to -100 mV (Inside) closed (polarized) In general, every cell is like battery.

  13. Major source of drug targets. Valium binds to serotonin (ligand) receptor called GABA receptor– relaxes nerves.

  14. Nerves How (electrical) signal is transported along a nerve

  15. Action Potential– Nerves Firing http://www.biologymad.com/NervousSystem/nerveimpulses.htm

  16. To close ion channels, to stop wave: 4. Na+ spontaneously close. It’s a wave instead of a spreading of signal because Na channels spontaneously close. 5. K+ open  brings membrane potential back down negative.

  17. Gradient set-up by Na/K Transporters [will go over]

  18. Membrane permeant to only one ion What is voltage (electrical potential) in each case Membrane permeant to Na+ Membrane permeant to K+ V>0 or <0? Just a tiny amount of charge causes potential: much less than 15 mM or 150 mM. What causes charge to stop flowing? A sufficiently large force (electrical potential) preventing more ions from going. Given that V ~ -60mV and Na/K are two major ions, which is your membranes permeant to? K+

  19. e e - What is Boltzmann’s Factor?: Z-1exp(-Ei/kT) exp(Eout-Ein/kT) Probability of being inside/outside? Let f = voltage Energy outside? = q fout = 0 = q fin Energy inside?

  20. q = ? for Na. +59 mV if permeable only to Na+ If permeant to only K+, resting potential = -59 mV Resting potential = -60 -100 mV

  21. Class evaluation • What was the most interesting thing you learned in class today? • 2. What are you confused about? • 3. Related to today’s subject, what would you like to know more about? • 4. Any helpful comments. Answer, and turn in at the end of class.

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