Cellular Respiration and Fermentation
General Biology · Section 3.2 · 18 study cards
Redox and electron carriers, the four stages with their locations and yields, chemiosmosis and ATP synthase, and fermentation.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Build one table and keep filling it in
The single most useful object in this unit is a four-row table with columns for stage, location, inputs, outputs and ATP made. Write it from memory before every study session. Nearly every exam question is a request for one cell of that table, or for the consequence of blocking one row of it.
Follow the electrons, not the ATP
The pathway is easier to reason about as a relay of electrons than as a list of reactions. Electrons begin in the covalent bonds of glucose, are stripped off in pairs along with hydrogens, are carried by NADH and FADH₂ to the inner membrane, fall through the chain releasing energy, and end on oxygen, which becomes water. ATP is a side effect of that fall, made by a turbine driven by the protons the fall pumps.
Work yield problems in a fixed sequence
- Count the reduced carriers first: 2 NADH from glycolysis, 2 from pyruvate oxidation, 6 from the citric acid cycle, and 2 FADH₂ from the cycle.
- Convert them at about 2.5 ATP per NADH and 1.5 per FADH₂, and state the conversion factors you are using.
- Add the substrate-level ATP: 2 from glycolysis and 2 from the citric acid cycle.
- Subtract for the shuttle if the question says cytosolic NADH enters by the glycerol phosphate route, which delivers electrons to FAD and costs about 1 ATP per NADH.
- Report a range rather than a single number, and give the reason for the range.
Answer poison and mutation questions by asking what backs up
If a question blocks a step, trace forwards and backwards from it. Blocking the chain leaves carriers reduced, so NAD⁺ runs out and everything upstream halts. An uncoupler that makes the membrane leaky to H⁺ lets electron transport continue at full speed, and even accelerate, while ATP synthesis collapses and the energy appears as heat. Those two cases have opposite effects on oxygen consumption, which is the detail exams test.
Definitions and theorems
Worked example
A yeast cell is supplied with glucose in which all six carbon atoms are labelled with ¹⁴C, and with ordinary oxygen. The cell respires the glucose completely. Account for every labelled carbon atom, then account for every electron removed from the glucose, and show that the electrons delivered to oxygen match the amount of oxygen consumed.
Track the carbons through glycolysis. One six-carbon glucose is split into two three-carbon pyruvates. No CO₂ is released here, so all six labelled carbons are still in the pathway.
Track them through pyruvate oxidation. Each pyruvate loses one carbon as CO₂, so 2 labelled CO₂ are released and two two-carbon acetyl groups continue, holding four labelled carbons.
Track them through the citric acid cycle. The cycle turns twice, releasing two CO₂ per turn, so 4 more labelled CO₂ are released. Total: 6 CO₂, matching the six carbons of the original glucose, with nothing left over.
Count the reduced carriers. Glycolysis gives 2 NADH, pyruvate oxidation gives 2 NADH, and the two turns of the cycle give 6 NADH and 2 FADH₂. That is 10 NADH and 2 FADH₂, twelve carriers in total.
Convert carriers to electrons. Each NADH and each FADH₂ carries two electrons, so 12 carriers deliver 24 electrons to the electron transport chain. This matches the 24 electrons available from the twelve C–H and other bonds oxidised in glucose.
Balance against oxygen. Each O₂ accepts four electrons, together with four protons, to form two molecules of water. Therefore 24 electrons require 24 divided by 4, which is 6 O₂, and produce 12 H₂O.
Check against the overall equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O. Six O₂ consumed and six CO₂ released agree with the carbon and electron accounting. The twelve waters formed at the chain are reduced to a net six in the summary equation because six water molecules are consumed earlier in the pathway.
Common mistakes
- Placing glycolysis in the mitochondrion. Glycolysis is cytosolic in every organism, which is precisely why red blood cells and anaerobic bacteria can run it. Only the last three stages need a mitochondrion in a eukaryote.
- Saying the electron transport chain makes ATP. The chain makes only a proton gradient. ATP synthase, a separate protein, makes the ATP. Questions about uncouplers and about cyanide have opposite answers and both depend on keeping these two things apart.
- Forgetting that glycolysis yields a net of 2 ATP. Four are made and two are invested. Writing 4 ATP for glycolysis in a yield calculation throws the whole total out and is the most common arithmetic error in this unit.
- Claiming fermentation produces ATP. Fermentation makes no ATP at all. Its sole function is to reoxidise NADH so that glycolysis, which does make ATP, can keep going. The 2 ATP per glucose under anaerobic conditions come from glycolysis alone.
- Quoting 36 to 38 ATP as fact. The modern figure is about 30 to 32, because the yield per carrier is fractional and cytosolic NADH costs energy to import. Either quote the range and say why it is a range, or state the ATP per NADH you are assuming.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.