Energy yield
Picture Aoife bursting into an all-out sprint for the ball during a camogie match, and Conor catching his breath after finishing a parkrun. During Aoife's short sprint, her muscle cells lean heavily on glycolysis and lactic acid fermentation, which are fast but low-yield (over a whole match, aerobic respiration still supplies most of her ATP). In Conor's longer run, they rely mainly on aerobic respiration (glycolysis, the citric acid cycle (Krebs cycle) and the electron transport chain), which is slower but far more productive. Now it is time to do the sums: how much ATP is actually obtained from one glucose molecule, and why is respiring so much more worthwhile than fermenting? (Why do fast sprints still lean on fermentation? We come back to this at the end.)
The complete balance of aerobic respiration
Per glucose molecule, in a eukaryotic cell, we count each stage separately.
| Stage | Net direct ATP | NADH | FADH₂ |
|---|---|---|---|
| Glycolysis, cytosol | 2 | 2 | 0 |
| Oxidation of two pyruvates to acetyl CoA, matrix | 0 | 2 | 0 |
| Two turns of the citric acid cycle (Krebs cycle), matrix | 2 equivalents (GTP/ATP) | 6 | 2 |
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?
State how many ATP molecules are obtained, according to currently accepted values, for each NADH and for each $\text{FADH}2$ that deliver their electrons to the…
State how many ATP molecules are obtained, according to currently accepted values, for each NADH and for each $\text{FADH}_2$ that deliver their electrons to the respiratory chain.
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.
In alcoholic fermentation, $2\ \text{mol}$ of ATP are obtained per mole of glucose. Calculate how many moles of glucose would need to be fermented to equal the ATP…
In alcoholic fermentation, $2\ \text{mol}$ of ATP are obtained per mole of glucose. Calculate how many moles of glucose would need to be fermented to equal the ATP obtained by respiring a single mole of glucose, and reason what problem this poses for an anaerobic microorganism in a sugar-poor medium.
Respiration versus fermentation
Aerobic respiration oxidises glucose completely and yields approximately 30–32 ATP per molecule in the model above. In aerobic respiration the pyruvate enters the mitochondrion and is fully oxidised to CO₂ and water; in fermentation it stays in the cytosol and is only reduced to lactic acid (animals) or ethanol and CO₂ (yeast). Fermentation regenerates NAD⁺ to keep glycolysis going (the electron transport chain and ATP synthase are Higher Level material): overall, both lactic acid fermentation and alcoholic fermentation yield 2 net ATP per glucose, which come from glycolysis, not from the conversion of pyruvate into lactate or ethanol. That is far below aerobic respiration, and large, active organisms need a lot of ATP, so they depend on the aerobic yield.
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.
Why can organisms that respire aerobically maintain larger and more active bodies than those that only ferment? Relate your answer to the energy yield of each pathway.
Why can organisms that respire aerobically maintain larger and more active bodies than those that only ferment? Relate your answer to the energy yield of each pathway.
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.
Real yield and its losses
After a hard hurling training session in Kilkenny, you are sweating and giving off a large amount of body heat: how much of the energy in glucose does a cell really use to synthesise ATP, and why is the rest dissipated?
What does it mean that a metabolic pathway has an energy yield of $40\ \%$? Explain what happens to the rest of the energy contained in the food.
What does it mean that a metabolic pathway has an energy yield of $40\ \%$? Explain what happens to the rest of the energy contained in the food.
The energy of foods is usually expressed in kilocalories. Explain what a kilocalorie is and what relationship it has with the energy that the cell finally stores in the…
The energy of foods is usually expressed in kilocalories. Explain what a kilocalorie is and what relationship it has with the energy that the cell finally stores in the form of ATP.
During a marathon, a runner performs about $10000\ \text{kJ}$ of useful work. If their organism has an overall yield of $25\ \%$, calculate how much chemical energy from…
During a marathon, a runner performs about $10000\ \text{kJ}$ of useful work. If their organism has an overall yield of $25\ \%$, calculate how much chemical energy from food they have had to mobilise and reason why their body heats up so much during the race.
Energy per gram of fuel
The human body spends energy even in absolute rest (basal metabolism). Name two processes that consume ATP even when we are lying down and still, and explain why they…
The human body spends energy even in absolute rest (basal metabolism). Name two processes that consume ATP even when we are lying down and still, and explain why they are essential to stay alive.
A person ingests $100\ \text{g}$ of glucose in a meal (molar mass $180\ \text{g/mol}$, that is, about $0.56\ \text{mol}$). Calculate how many moles of ATP their organism…
A person ingests $100\ \text{g}$ of glucose in a meal (molar mass $180\ \text{g/mol}$, that is, about $0.56\ \text{mol}$). Calculate how many moles of ATP their organism can obtain aerobically ($36\ \text{ATP}$ per glucose) and how many grams of ATP they are equivalent to (molar mass of ATP, $507\ \text{g/mol}$). Comment on the figure obtained.
The combustion of $1\ \text{g}$ of glucose releases about $4\ \text{kcal}$ and that of $1\ \text{g}$ of fat about $9\ \text{kcal}$. Explain what that difference is due…
The combustion of $1\ \text{g}$ of glucose releases about $4\ \text{kcal}$ and that of $1\ \text{g}$ of fat about $9\ \text{kcal}$. Explain what that difference is due to and calculate how many grams of fat provide the same energy as $90\ \text{g}$ of glucose.