Q:
In glycolysis there are two reactions that require one ATP each and two reactions that produce one ATP
each. This being the case, how can glycolysis of glucose to lactate lead to the net production of two ATP molecules
per glucose?
A:
In the case of the two reactions that produce ATP, each reaction is performed twice since each reaction
is produced on a three-carbon fragment of six-carbon glucose. Thus, although there are two reactions that
require one ATP each, there are actually 4 ATP-producing reactions, leading to a net production of two ATP
molecules per glucose molecule.
Q:
Although molecular oxygen does not participate directly in the citric acid cycle, the cycle operates only
when oxygen is present. Why?
A:
The citric acid cycle indeed does not have oxygen entering into any step. However, in order for the citric
acid cycle to function it is necessary to have FAD and NAD + available. Since the citric acid cycle converts FAD
to FADH2 and NAD+ to NADH, were there no way to regenerate FAD and NAD+ the citric acid cycle would
cease operation. Thus, the NADH and FADH2 are passed into the electron transport chain which regenerates
NADH and FADH2 by passing the electrons of these two species ultimately to molecular oxygen.
Q:
The oxidation of glyceraldehyde-3-phosphate to 3-phosphoglyceroyl phosphate, catalyzed by
glyceraldehyde phosphate dehydrogenase, proceeds with an unfavorable equilibrium constant (Keq = 0.08; G' =
+1.5 kcal/mol). Despite this unfavorable equilibrium, the flow through this point in the pathway proceeds smoothly.
How does the cell overcome this unfavorable equilibrium?
A:
No biochemical reaction occurs apart from other reactions. Any thermodynamically unfavorable
equilibrium may be driven by coupling an unfavorable reaction to more favorable reactions, and this is the case
in the above reaction. Thus, the cell overcomes this unfavorable equilibrium by coupling it to a favorable
equilibirum.
Q:
Under anaerobic conditions one glucose molecule is converted to two lactate molecules. Using ratios of
carbon to hydrogen to oxygen in glucose and lactate, demonstrate that there is no net oxidation of carbon in going
from glucose to lactate.
A:
Glucose:
C6H12O6
Ratio of C to H to O: 1 to 2 to 1
Lactic Acid:
C3H6O3
Ratio of C to H to O: 1 to 2 to 1
Thus, glycolysis (splitting of glucose) is not an oxidation/reduction reaction (gain or loss of oxygen), but a true
splitting reaction.
Q:
How can it be said that the discovery of petroleum was instrumental in leading to increased technological
developments in the area of fermentation of biomass?
A:
The discovery of petroleum drove the market for petroleum-derived products. Against the background of
the establishment of this extensive market, the latter drives alternative feedstocks for the original products.