Week 4 MPW Lecture 8 Flashcards
What are the uses of concentration gradients?
Include:
a) To transmit nerve signals (action potential)
i) flow of Na+ and K+
b) To generate ATP from a H+ gradient
c) To transport a variety of molecules across membranes against a concentration gradient by using the membrane potential to hitch a ride
- glucose transporter transports Na+ down a concentration gradient, and uses this to transport glucose against a concentration gradient (ie to concentrate it inside the cell)
d) In reverse, to pump H+ into vesicles (eg endosomes) using ATP and therefore acidify them
How is the energy produced by ion gradients across be calculated?
Calculayed by 2 equations:
a) Equation 1
i) (delta)G=zFE
ii) z = charge on ion
E = membrane potential (in V)
F is a constant, the Faraday constant, = 96485 J mol-1 V-1
iii) Relates the membrane potential (in Volts) to the free energy difference acrossthe membrane
b) Equation 2 (Nernst equation):
i) E= (RT/zF)xln([OUT/IN])
ii) which relates the membrane potential to the concentrations outside and inside
iii) In this equation, [out] is shorthand for “the concentration of ion on the outside of the membrane”; R is a constant (often called the gas constant) = 8.31 J K-1 mol-1; and T is the absolute temperature (ie the temperature in degrees K). ln is the natural logarithm (log to base e)
What is the free energy change for moving a Ca2+ ion across a membrane with a membrane potential of -50 mV?
(delta)G = zFE, where here z=+2, so crude answer is -9648.5, and actual answer is -9.6 kJ mol-1 (2sf)
A student calculated that the free energy for transporting a Mg2+ ion across the mitochondrial membrane is 8753.5 kJ. Give three reasons why they should be suspicious of the result and do it again.
(a) It is much too large a number to be possible (b) It is in the wrong units (c) It almost certainly has far too many significant figures. If you thought the sign was wrong then it might be, but it depends which direction you are going.
