The Energy Yield of Cellular Respiration
In a marathon in Mexico City, at 2,240 m above sea level, the runners finish exhausted and overheated. How much ATP does a cell get from a single glucose, why is fermentation so much less productive, and where does the energy that isn't converted into ATP go?
The Balance of Aerobic Respiration
You have already studied glycolysis, the Krebs cycle, and the respiratory chain. Now we add up what each stage produces per molecule of glucose in a eukaryotic cell.
| Stage | Net direct ATP | NADH | FADH₂ |
|---|---|---|---|
| Glycolysis (cytosol) | 2 | 2 | 0 |
| Oxidation of 2 pyruvates to acetyl-CoA (matrix) | 0 | 2 | 0 |
| Two turns of the Krebs cycle (matrix) | 2 | 6 | 2 |
NADH and FADH₂ donate their electrons to the transport chain; the resulting flow of protons drives ATP synthase and produces most of the ATP. That is why the final figure depends on how many ATP are counted for each NADH and each FADH₂.
How many net ATP molecules are obtained in glycolysis per glucose molecule, and how many in total (classic theoretical figure) when completely respired aerobically?
How many net ATP molecules are obtained in glycolysis per glucose molecule, and how many in total (classic theoretical figure) when completely respired aerobically?
In aerobic respiration, about $36$-$38\ \text{ATP}$ are obtained per glucose, of which only $2$ come from glycolysis. Calculate what percentage of the total yield is…
In aerobic respiration, about $36$-$38\ \text{ATP}$ are obtained per glucose, of which only $2$ come from glycolysis. Calculate what percentage of the total yield is contributed by the mitochondrial stages (taking $36\ \text{ATP}$ as the total) and comment on the result.
Respiration vs. Fermentation
Both respiration and fermentation begin with glycolysis: one glucose is converted into two pyruvates and 2 net ATP are gained. The difference lies in what happens next to the pyruvate.
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With oxygen (aerobic respiration): the pyruvate enters the mitochondrion and is fully oxidized to $\mathrm{CO_2}$ and $\mathrm{H_2O}$. It yields about 30-38 ATP in total.
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Without oxygen (fermentation): the pyruvate is converted into lactate (lactic acid fermentation, as in muscle or yogurt) or into ethanol and $\mathrm{CO_2}$ (alcoholic fermentation, as in pulque, bread, or beer). It yields only 2 ATP.
Fermentation yields little because the pyruvate is not fully oxidized: the lactate or ethanol still holds a lot of chemical energy. Its real function is to regenerate the $\mathrm{NAD^+}$ that glycolysis needs in order to continue. That conversion does not produce ATP.
This bar chart compares the ATP per glucose by pathway (classic model for respiration):
Order of yield per glucose: aerobic respiration (30-38) $\gg$ lactic acid fermentation (2) = alcoholic fermentation (2).
Why is fermentation much less energy-efficient than aerobic respiration, even though both begin with glycolysis? Explain what happens to pyruvate in each case.
Why is fermentation much less energy-efficient than aerobic respiration, even though both begin with glycolysis? Explain what happens to pyruvate in each case.
Rank from highest to lowest energy yield per glucose molecule: aerobic respiration, lactic acid fermentation, and alcoholic fermentation. Justify the ranking.
Rank from highest to lowest energy yield per glucose molecule: aerobic respiration, lactic acid fermentation, and alcoholic fermentation. Justify the ranking.
Calculate how many glucose molecules a cell needs to break down to obtain $180\ \text{ATP}$ aerobically ($36\ \text{ATP/glucosa}$) and how many by fermentation (…
Calculate how many glucose molecules a cell needs to break down to obtain $180\ \text{ATP}$ aerobically ($36\ \text{ATP/glucosa}$) and how many by fermentation ($2\ \text{ATP/glucosa}$). Comment on the implication of this difference for anaerobic organisms.
The graph compares the ATP molecules obtained per glucose molecule according to the metabolic pathway used. A yeast culture without oxygen degrades $100\ \text{mol}$ of…
The graph compares the ATP molecules obtained per glucose molecule according to the metabolic pathway used. A yeast culture without oxygen degrades $100\ \text{mol}$ of glucose by alcoholic fermentation. Calculate, from the graph, how many moles of ATP it obtains, how many it would obtain if it respired that same glucose aerobically, and how many times more efficient the aerobic pathway is.
Actual Yield and Its Losses
When you run, you sweat and your body heats up: part of the energy in glucose is not converted into ATP but into heat.
There are two main reasons why the actual yield (about 30 ATP per glucose) is lower than the theoretical one (36-38):
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Transport costs. Moving pyruvate, phosphate, and the electrons of cytosolic NADH into the mitochondrion uses up part of the proton gradient.
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Proton leaks. The inner membrane lets some $\mathrm{H^+}$ through without passing through ATP synthase; that energy is released as heat.
During a maximum speed $100\ \text{m}$ sprint, fast-twitch muscle fibers demand ATP at a rate of approximately $3.5\ \text{mmol}/(\text{kg}\times\text{s})$. In those…
During a maximum speed $100\ \text{m}$ sprint, fast-twitch muscle fibers demand ATP at a rate of approximately $3.5\ \text{mmol}/(\text{kg}\times\text{s})$. In those same fibers, mitochondrial oxidative phosphorylation has a maximum production rate of barely $0.4\ \text{mmol}/(\text{kg}\times\text{s})$, whereas homolactic fermentation can generate up to $4.0\ \text{mmol}/(\text{kg}\times\text{s})$.
- a) Based on these kinetic data, justify why it is energetically profitable for the muscle cell to massively activate lactic fermentation even though its stoichiometric yield per mole of glucose is about 16-18 times lower than that of aerobic respiration.
- b) Predict what immediate metabolic and intracellular consequences (in relation to pH, substrate availability, and the activity of enzymes such as phosphofructokinase-1) would occur if an attempt were made to sustain this fermentative rate for several minutes, explaining why muscle fatigue develops.
The classic theoretical balance of cellular respiration predicts the synthesis of $36$ to $38\ \text{ATP}$ per glucose molecule, but in vivo measurements in mammalian…
The classic theoretical balance of cellular respiration predicts the synthesis of $36$ to $38\ \text{ATP}$ per glucose molecule, but in vivo measurements in mammalian mitochondria show that the actual yield rarely exceeds $30$-$32\ \text{ATP}$.
- a) Justify at a molecular and thermodynamic level two physiological causes that explain this discrepancy, referring to the use and dissipation of the electrochemical proton gradient between the intermembrane space and the mitochondrial matrix.
- b) The brown adipose tissue of hibernating mammals and newborns expresses the uncoupling protein thermogenin (UCP-1) in its inner mitochondrial membrane. Apply Mitchell's chemiosmotic hypothesis to explain how UCP-1 alters the ratio of $\text{ATP/O}_2$ consumed, and deduce what adaptive advantage it provides against cold thermal stress.
How Much Energy Does an ATP Really Hold?
To know what percentage of the glucose's energy is captured, you need to know how much energy it costs to make each ATP. And that value is not the same in a test tube as inside a cell.
Chemists measure the energy of a reaction with the Gibbs free energy, $\Delta G$ (in kJ/mol). There are two values:
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$\Delta G^{\circ\prime}$ (biochemical standard): measured with reference concentrations of 1 M of reactants and products (plus pH 7.0 and 25 °C). For the hydrolysis of ATP it is about $-30.5$ kJ/mol.
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$\Delta G$ (actual): measured with the concentrations found in the living cell.
They are related as follows:
$$\Delta G=\Delta G^{\circ\prime}+RT\ln Q$$Here $Q$ is the ratio of the concentrations of products to reactants at that moment, $R$ is the gas constant, and $T$ is the temperature. At 25 °C, $RT\approx 2.5$ kJ/mol. You don't need to calculate logarithms to understand the idea: if $Q$ changes, $\Delta G$ changes.
The cell keeps ATP highly concentrated and ADP and $\mathrm{P_i}$ very dilute, that is, far from equilibrium. For that reason, making ATP costs more than the standard value and, conversely, hydrolyzing it releases more: instead of $30.5$ kJ/mol, about $50$ kJ/mol is released.
Under standard biochemical conditions ($\text{pH} = 7.0$, $25\ ^\circ\text{C}$), the complete combustion of $1\ \text{mol}$ of glucose yields…
Under standard biochemical conditions ($\text{pH} = 7.0$, $25\ ^\circ\text{C}$), the complete combustion of $1\ \text{mol}$ of glucose yields $\Delta G^{\circ\prime} = -2870\ \text{kJ/mol}$ and the hydrolysis of $1\ \text{mol}$ of ATP yields $\Delta G^{\circ\prime} = -30.5\ \text{kJ/mol}$. However, in the cytoplasm of a living hepatocyte, the actual concentrations of ATP, ADP, and $\text{P}_\text{i}$ mean that the phosphorylation of ADP requires around $+50.0\ \text{kJ/mol}$, with roughly $30\ \text{mol of ATP}$ net per mole of oxidized glucose obtained experimentally.
- a) Calculate and compare the theoretical percentage energy efficiency under standard conditions (assuming $36\ \text{ATP/glucose}$) versus the actual intracellular thermodynamic efficiency. Justify the fate of the fraction of free energy that is not stored in the phosphate bonds of ATP and its significance for homeothermic animals.
- b) 2.4-Dinitrophenol (DNP) is a lipophilic molecule capable of transporting protons across the inner mitochondrial membrane, dissipating the electrochemical gradient. Predict, with reasoning, how treatment with DNP will affect the net yield of ATP per glucose, the rate of $\text{O}_2$ consumption by the cell, and the body temperature of the organism.
Energy per Gram of Fuel
A carbohydrate such as glucose provides about 4 kcal/g and a fat about 9 kcal/g.
Why? Fatty acids have very reduced carbons (many C–H bonds and little oxygen), so they donate more electrons to the respiratory chain per gram. Also, glycogen is stored with a lot of water, while fat is stored almost pure. That is why long-term reserves, such as the fat that migratory birds accumulate before their journey, are lipids.
Back to the beginning: a marathon runner gets about 30-38 ATP per glucose by respiring, only 2 if fermenting, and almost half of the energy is dissipated as heat, which is why the runner finishes so overheated.