Unit 10
1
U
NIT
10
P
ART
A: B
IOENERGETICS
P
ART
B: C
HEMISTRY OF
C
ARBOHYDRATES
P
ART
C: G
LYCOLYSIS
P
ART
A: B
IOENERGETICS
Assignment:
Nelson & Cox, pp. 481 – 503, 507  517.
All living cells must obtain energy from their surroundings and expend it as
efficiently as possible. Plants gather most of their energy from sunlight;
animals use the energy stored in plants or other foods that they consume.
The processing of this energy is central to the understanding of
biochemistry. Bioenergetics, the quantitative analysis of how organisms
gain and utilize energy, is a special part of the general science of energy
transformation which is called thermodynamics.
1.
Use Fig. 3 (p. 483) to distinguish between catabolic and anabolic pathways.
2.
Of the equations shown in the textbook, the Gibbs equation is the most
useful for biochemists (equation 133; p. 494).
∆
G =
∆
G’° + RT ln ([C][D] / [A][B])
(Note that the terms
∆
G’° and
∆
G°’ used elsewhere are the same.)
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Unit 10
2
The term,
Δ
G'° is different from the chemist's
Δ
G° in that the standard state
is at a pH of 7.0 (or other specified value) instead of a pH of zero. The
equation, a quantitative version of Le Châtelier's principle, shows the
relationship of the free energy of a reaction to the concentration of reactants
and products. The logarithmic term in the equation comes from an
integration of the ideal gas law used in calculating the free energy necessary
to compress a gas.
The units of
Δ
G are Kcal/mol (or KJoules/mol). Both units are commonly
used but the use of Joules is increasing. 1 cal.= 4.184 J.
(Note that the Calorie used for food is one Kcal!)
a.
Which term in the Gibbs equation can be used to predict whether a
reaction will proceed?
Note
:
It is
Δ
G,
not
Δ
G
'°
, which determines whether a reaction will
go spontaneously or not. Thus, why do the authors of most texts talk
about coupled reactions, thermodynamic feasibility,
etc
. in terms of
Δ
G
'°
? The reason is mostly a matter of convenience because these
values can easily be looked up in tables. Sometimes
Δ
G and
Δ
G'°
can, however, be very different! For example, one reaction in the
citric acid cycle has a
Δ
G'° of +7.1 Kcal/mol and a negative
Δ
G.
Although the
Δ
G'° would not suggest it, the reaction occurs
spontaneously in the citric acid cycle due to the concentration term in
the Gibbs equation.
b.
What does
Δ
G tell you about the rate of the reaction?
3.
Δ
G
'°
is determined by measuring the equilibrium constant.
∆
G’° =  RT ln Keq
Using the sample calculations on pp. 492 and 494  495 as a guide, do
problems 2 and 11 (pp. 518  519).
4.
It is possible for a reaction with a positive
Δ
G'° to be driven by a coupled
reaction which has a negative
Δ
G'°. Explain why this can occur. Using the
example on page 501 as a guide, do problem 10 (p. 519).
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 Fall '08
 BLANKENSHI
 Glycolysis, Redox, Adenosine triphosphate

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