25 - Monday, October 25, 2010 Lecture 25 Announcements For...

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Monday, October 25, 2010 Lecture 25 Announcements For this week's lectures ( Quiz on 11/3 ), assignments are as in LG for 10/27 and 10/29 EXCEPT move one homework problem to next week: Ch 15 #4; AND no PyMOL for the quiz next week and for the lectures this week. Friday's lecture: Bioenergetics: Concentration gradients influence the Gibbs Function! Electrical potential difference influences the Gibbs Function, and must be considered for ion transport across membranes Table of standard reduction potentials TODAY: p. 178 Redox reactions involve electrons moving from one molecule to another. This tendency of electrons to flow from one molecule to another plays a key role in driving the chemical rxns of almost all living systems. In biology, the action starts with light. Light raises an electron in chlorophyll to a new state, making the chlorophyll a strong reductant . Then, reactions occur that create NADPH, ATP, and a proton concentration gradient across a chloroplast membrane (we'll consider these in detail later in the course). We want to put this “raising electron level” into the language of driving reactions, i.e. the language of G. In comparison with redox chemistry, consideration of electron flow in simple electrical circuits is rather straightforward. We can treat volts (the driving force), ohms, current, and capacitance without explicit discussion of chemistry. In biochemistry, the tendency for electrons to flow is much more complicated. We must consider the chemistry of every step, since electrons usually flow by steps of contact between molecules (inside proteins, electron flow is the subject of active research!). Each step always is accompanied by a loss of electrons from one molecule, and a gain of electrons by another molecule. Thus, each step in this transfer of electrons depends on chemical properties of the molecule that loses the electron (this molecule becomes oxidized) and the chemical properties of the molecule that gains the electron (this molecule becomes reduced). The transfer of electrons depends separately, in an additive way, on the chemical properties of each molecule or ion participating in the transfer of electrons.
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p. 179 For any chemical rxn we can write G = G o ' + RTln(products)/(reactants) , and this holds true for redox rxns. From this equation, you can see where the concentrations come into the G calculation for redox rxns. The special treatment of redox rxns is the relationship between G and the measured voltage: G = -n F E , where F is Faraday’s constant, and n is the number of electrons in the rxn as written. Redox reactions have the property that since voltages and charges are involved,
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25 - Monday, October 25, 2010 Lecture 25 Announcements For...

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