The cell’s supply of ADP, P
, and NAD
is finite (limited). What happens to cellular respiration when all of
the cell’s NAD
has been converted to NADH?
If NAD is unavailable, the cell is unable to conduct any processes that involve the conversion of NAD
NADH. Because both glycolysis and the Krebs cycle produce NADH, both of these processes shut down
when there is no available NAD
If the Krebs cycle does not require oxygen, why does cellular respiration stop after glycolysis when no
oxygen is present?
When no oxygen is present, oxidative phosphorylation cannot occur. As a result, the NADH produced in
glycolysis and the Krebs cycle cannot be oxidized to NAD
. When no NAD
is available, pyruvate cannot
be converted to the acetyl CoA that is required for the Krebs cycle.
Many organisms can withstand periods of oxygen debt (anaerobic conditions). Yeast undergoing oxygen
debt converts pyruvic acid to ethanol and carbon dioxide. Animals undergoing oxygen debt convert pyruvic
acid to lactic acid. Pyruvic acid is fairly nontoxic in even high concentrations. Both ethanol and lactic acid
are toxic in even moderate concentrations. Explain why this conversion occurs in organisms.
As noted in question 4, when no NAD
is available, even glycolysis stops. No ATP will be produced and the
cell (or organism) will die. The conversion of pyruvic acid (pyruvate) to lactic acid (or ethanol) requires the
input of NADH and generates NAD
. This process, called fermentation, allows the cell to continue getting
at least 2 ATP per glucose.
How efficient is fermentation? How efficient is cellular respiration? Remember that efficiency is the amount
of useful energy (as ATP) gained during the process divided by the total amount of energy available in
glucose. Use 686 kcal as the total energy available in 1 mole of glucose and 8 kcal as the energy available in
1 mol of ATP.
Efficiency of fermentation
Efficiency of aerobic respiration
kcal/mole of ATP
2 ATP = 16 kcal
16 kcal / 2 moles of ATP
686 kcal / mole of glucose
8 kcal/mole of ATP
38 ATP (maximum) = 304
304 kcal / 38 moles of ATP
686 kcal / mole of glucose
Why can’t cells store large quantities of ATP? (
Consider both the chemical stability of the
molecule and the cell’s osmotic potential.)
ATP is highly reactive at normal body temperatures and therefore difficult for cells to store for any
period of time. (In the lab, ATP is usually stored at very low temperatures, for example, at -20°C.) In
addition, ATP is a relatively small molecule. As a result, if cells could store high concentrations of ATP,
their osmotic potential would change. This is also why cells don’t store glucose. The cells would
become hypertonic to the fluid around them and could pick up enough water to burst.
b. Given that cells can’t store ATP for long periods of time, how do they store energy?