Physiology 3120 Lecture Notes - Electrochemical Gradient, Integral Membrane Protein, Pressure Gradient

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26 Nov 2011
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Human Physiology
Wednesday, September 16, 2009
Membrane Transport Mechanisms
Diffusion through the cell membrane
Substance must be nonpolar/lipid soluble (oxygen, carbon dioxides, large alcohols)
Must be able to get through the inner portion of the bilayer
Driving force is the concentration gradient
Fick’s First Law of Diffusion predicts rate:
See equation in the notes
Negative sign because the concentration gradient produces a negative slope, so we need
the negative sign to make the rate of diffusion positive
Rate is proportional to...
Area for diffusion
Concentration gradient
Inverse of the molecular radius
Inverse of the viscosity of the medium
Diffusion through protein channels/pores
Water uses special pores called aquaporins
Polar molecules diffuse through protein channels
Integral transmembrane proteins
Factors affecting rate of movement:
1. Size of the molecule (<0.8nm)*
2. Charge (protein select for one charge over the other)*
3. Electrochemical gradient (move down both concentration & electrical gradient)
4. Pressure gradient (increases the kinetic energy of molecules)
5. Hydration energy*
Hille’s theory of closest fit (hydration shell must be stripped before ions can pass,
which can only be done by the proper channel
Some factors act as ‘filters’*
Molecules passing through the channels don’t physically interact with them
Facilitated diffusion
Large molecules (i.e. glucose) use facilitated diffusion
Attaches to integral membrane protein
Conformational change in protein, which moves the molecule across the membrane
Form of carrier-mediated transport, but driving force is still the concentration gradient
3 characteristics
1. Chemical specificity
2. Competitive inhibition (method of many drugs)
3. Show saturation kinetics (channels don’t show this at physiological levels)
Rate limited by (I) number of carriers, and (II) speed of conformational change
Rate will eventually plateau, or become ‘saturated’
Active transport
Form of carrier mediated transport
1. Chemical specificity
2. Competitively inhibited
3. Shows saturation kinetics
Moves substances AGAINST concentration gradient, which requires energy (i.e. ATP)
Na/K pump/ATPase
Pumps 3 Na out & 2 K in for every ATP
See notes for sequence of events
Metabolic inhibitors (i.e. ouabain & digoxin/digitalis can inhibit the pump)
Helps maintain concentration gradients of Na & K
Causes slight increased negativity inside the cell (more positive is being removed than
Keeps the cell from swelling & bursting due to osmosis