Title: InkBiology / Respiration and cellular energy
On every note of this deck, unless a note says otherwise:
Footer: Continue on InkBiology · © 2026 InkBiology. All rights reserved.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Anaerobic respiration is what happens when your muscles run out of energy.
Back: Nothing runs out of energy: glucose and glycogen are still there. What runs short is oxygen, and the immediate casualty is a coenzyme. Glycolysis needs oxidised NAD, and lactate production exists to regenerate it when the electron transport chain cannot.

Front: DEFINE
Define: Dehydrogenation.
Back: Removal of hydrogen from a molecule, catalysed by a dehydrogenase, with the hydrogen taken up by a coenzyme.

Front: DEFINE
Define: ATP.
Back: Adenosine triphosphate: a phosphorylated nucleotide made of adenine, ribose and three phosphate groups, used as the immediate source of energy in every cell.

Front: DEFINE
Define: FAD.
Back: A hydrogen-carrying coenzyme like NAD, but one that hands its electrons to the transport chain further along, so it yields less ATP per molecule.

Front: DEFINE
Define: Substrate-level phosphorylation.
Back: The direct transfer of a phosphate group from a substrate molecule onto ADP, as opposed to ATP made using a proton gradient.

Front: DEFINE
Define: Dehydrogenase.
Back: An enzyme that catalyses the removal of hydrogen from a substrate, passing it to a coenzyme such as NAD or FAD.

Front: CHECK YOURSELF
A 100 m sprinter and a marathon runner are both working hard. Explain why the sprinter's muscles produce lactate while the marathon runner's largely do not, and explain why the marathon runner's main respiratory substrate changes during the race.
Back: The sprinter's muscles demand ATP far faster than the heart and lungs can supply oxygen to them, so the electron transport chain cannot oxidise reduced NAD quickly enough. Without oxidised NAD being returned, glycolysis would stop, so pyruvate is reduced to lactate instead, which hands the hydrogen over and lets glycolysis continue. The yield is only 2 ATP per glucose, but it arrives quickly and without waiting for oxygen.
The marathon runner works at an intensity oxygen delivery can keep up with. Reduced NAD is oxidised at the inner membrane as fast as it is made, oxidised NAD is returned to the cytoplasm and the matrix, and pyruvate goes into the link reaction rather than being reduced. Little lactate accumulates, and what does is taken up and respired by cardiac and slow-twitch muscle.
The substrate changes because the stores run out at different rates. Muscle and liver glycogen last roughly an hour and a half of hard running; as carbohydrate is depleted, an increasing proportion of the ATP comes from fatty acids, hydrolysed from stored triglyceride and fed in as acetyl CoA. Lipid gives about 39 kJ per gram against 16 for carbohydrate, so the reserve is large, but it needs more oxygen per carbon dioxide released, which is why the runner's RQ falls from near 1.0 towards 0.8 or below as the race goes on, and why the pace that can be held drops with it.

Front: DEFINE
Define: Fermentation.
Back: The anaerobic conversion of pyruvate to lactate or to ethanol and carbon dioxide, regenerating oxidised NAD.

Front: DEFINE
Define: Acetyl coenzyme A.
Back: The two-carbon acetyl group attached to coenzyme A, which carries it into the Krebs cycle.

Front: DEFINE
Define: Coenzyme.
Back: A non-protein molecule that an enzyme needs in order to work. NAD accepts hydrogen from a substrate and carries it elsewhere, so it is a hydrogen carrier.

Front: DEFINE
Define: Phosphorylation.
Back: The addition of a phosphate group to a molecule. The phosphate released from ATP is often added to another molecule, making it more reactive.

Front: DEFINE
Define: Final electron acceptor.
Back: The molecule that takes electrons from the end of the chain. In aerobic respiration it is oxygen, which combines with protons to form water.

Front: DEFINE
Define: Glycolysis.
Back: The splitting of glucose into two molecules of pyruvate in the cytoplasm, producing a net gain of two ATP and two reduced NAD, without oxygen.

Front: SPOT THE ERROR
Say what is wrong with this claim:
ATP is the cell's energy store, so cells build up a supply and draw on it when they need it.
Back: Cells store energy as glycogen and triglyceride, not as ATP. An adult holds about 50 g of ATP at any moment and turns over close to their own body mass of it in a day, so the whole pool would be gone in about ninety seconds without resynthesis.

Front: DEFINE
Define: Hydrolysis of ATP.
Back: The breaking of the bond to the terminal phosphate by addition of water, catalysed by ATP hydrolase, giving ADP and inorganic phosphate and releasing about 30.5 kJ per mole.

Front: DEFINE
Define: Chemiosmosis.
Back: The synthesis of ATP driven by the movement of protons down an electrochemical gradient through ATP synthase.

Front: DEFINE
Define: Anaerobic respiration.
Back: The release of energy from an organic molecule without oxygen, in which glycolysis continues because reduced NAD is oxidised by a reaction other than the electron transport chain.

Front: DEFINE
Define: Decarboxylase.
Back: An enzyme that catalyses the removal of a carbon atom from a substrate, released as carbon dioxide.

Front: DEFINE
Define: Link reaction.
Back: The oxidative decarboxylation of pyruvate in the mitochondrial matrix, producing carbon dioxide, reduced NAD and a two-carbon acetyl group carried by coenzyme A.

Front: DEFINE
Define: Oxidation.
Back: In respiration, most easily read as the loss of hydrogen. A substrate that hands hydrogen to NAD has been oxidised; the NAD has been reduced.

Front: DEFINE
Define: Electron transport chain.
Back: A series of carrier proteins in the inner mitochondrial membrane that pass electrons from one to the next, releasing energy used to pump protons into the intermembrane space.

Front: DEFINE
Define: Krebs cycle.
Back: A closed series of reactions in the mitochondrial matrix in which an acetyl group is combined with a four-carbon acceptor and completely oxidised, releasing carbon dioxide and reducing coenzymes, and regenerating the acceptor.

Front: CHECK YOURSELF
A student writes: 'Glycolysis makes 2 ATP, so it is not worth much to the cell, and it needs the mitochondrion to be useful.' Evaluate that statement, using what glycolysis produces and where it happens.
Back: The 2 ATP figure is right as a net gain, and small: four are made by substrate-level phosphorylation and two were spent phosphorylating the glucose at the start.
But ATP is not the main product worth counting here. Glycolysis also produces two reduced NAD per glucose, and each of those carries hydrogen to the inner membrane where it will yield roughly two and a half more ATP. The pyruvate it leaves behind still holds most of the original energy, and the link reaction and Krebs cycle exist to get at it.
The claim about needing a mitochondrion is wrong twice over. Glycolysis takes place in the cytoplasm and uses no oxygen and no organelle, which is why a mature red blood cell, having neither nucleus nor mitochondria, is entirely dependent on it, and why yeast in a sealed fermenter goes on respiring.
A fairer summary: glycolysis on its own is a poor yield but a reliable one, available to every cell in every condition, and it is the only stage that keeps running when oxygen does not.

Front: DEFINE
Define: Decarboxylation.
Back: Removal of a carbon atom from a molecule, released as carbon dioxide.

Front: DEFINE
Define: Oxidative phosphorylation.
Back: The formation of ATP from ADP and inorganic phosphate using energy from the electron transport chain, with oxygen as the final electron acceptor.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Oxygen burns the glucose in respiration, in the same way it burns petrol in an engine.
Back: No glucose molecule ever meets an oxygen molecule inside you. The carbon has already left as carbon dioxide before oxygen appears, and oxygen's only job is to accept spent electrons at the end of the chain and combine them with protons to make water.

Front: CHECK YOURSELF
A drug makes the inner mitochondrial membrane freely permeable to protons. Explain what happens to the rate of electron transport, to ATP production and to the temperature of the tissue. Then explain why a person taking such a drug loses mass rapidly but is in serious danger.
Back: Protons can now return to the matrix anywhere, so no gradient can be maintained. ATP synthase has nothing driving protons through it and ATP production by oxidative phosphorylation falls to almost nothing.
Electron transport does not slow. It speeds up. Pumping protons into a space that no longer resists them is easy, so the carriers pass electrons faster than before, and oxygen is consumed faster than usual while producing no ATP.
The energy released at each transfer is no longer captured in a gradient, so it appears as heat, and the tissue warms. This is not hypothetical: brown adipose tissue does exactly this on purpose, using a channel protein to warm a newborn.
Mass is lost because respiratory substrates are being oxidised at a high rate to no useful end, so stored lipid and then protein are consumed quickly. The danger is that cells still need ATP and are now getting almost none of it from the mitochondrion, while body temperature climbs beyond what enzymes tolerate. Drugs of this kind were sold as slimming aids in the 1930s and killed people.