The shape of the lesson
By the end of the lesson, learners can
- Four stages, two places: Click each structure of the mitochondrion in turn: outer membrane, inner membrane, cristae, matrix, ATP synthase, ribosomes and circular DNA.
- Glycolysis: split and oxidise: Press Next step and follow the carbon atoms of one glucose through the first four steps.
- The link reaction and the Krebs cycle: Each step removes carbon as carbon dioxide or hydrogen for NAD and FAD.
- Cytoplasm: glycolysis. Matrix: link reaction and Krebs cycle. Inner membrane: oxidative phosphorylation.
- Glycolysis: phosphorylation, lysis, oxidation to pyruvate; net 2 ATP, 2 reduced NAD.
- Link: decarboxylation + dehydrogenation; coenzyme A carries the acetyl group.
- Krebs: 4C + 2C → 6C, back to 4C; per turn 2 CO2, 3 reduced NAD, 1 reduced FAD, 1 ATP.
One picture every idea returns to
Aerobic respiration has four stages: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane. In glycolysis, glucose is phosphorylated using 2 ATP to fructose 1,6-bisphosphate (6C), which splits into two triose phosphates (3C); these are oxidised to pyruvate (3C), giving a net 2 ATP and 2 reduced NAD. If oxygen is available, pyruvate enters the mitochondrion. In the link reaction it is decarboxylated and dehydrogenated, and the 2C acetyl group is carried by coenzyme A. In the Krebs cycle, oxaloacetate (4C) accepts the acetyl group to form citrate (6C), which is converted back to oxaloacetate in small steps, releasing CO2 and reducing NAD and FAD.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Strip the glucose, keep the hydrogen.. Glucose is taken apart in stages: split in the cytoplasm, then fed into the mitochondrial matrix two carbons at a time. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Four stages, two places | Click each structure of the mitochondrion in turn: outer membrane, inner membrane, cristae, matrix, ATP synthase, ribosomes and circular DNA. Project tab 4 of the lab (Energy and respiration lab, mitochondrion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Glycolysis: split and oxidise | Press Next step and follow the carbon atoms of one glucose through the first four steps. Project tab 5 of the lab (Energy and respiration lab, glycolysis to Krebs) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: The link reaction and the Krebs cycle | Each step removes carbon as carbon dioxide or hydrogen for NAD and FAD. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The link reaction and the Krebs cycle, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Glycolysis does not happen in the mitochondrion. | Glucose never enters it: only pyruvate does, and only when oxygen is available. |
| Glycolysis makes 4 ATP but uses 2, so the net yield is 2 ATP per glucose. | Questions that ask for the net yield expect 2, not 4. |
| The Krebs cycle itself makes very little ATP. | Its main job is to supply reduced NAD and reduced FAD, whose hydrogen drives oxidative phosphorylation on the inner membrane. |
Mitochondria, glycolysis and the Krebs cycle: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
State where each of the four stages of aerobic respiration occurs in a eukaryotic cell.
Outline the stages of glycolysis.
Describe the link reaction, including the role of coenzyme A.
Outline the Krebs cycle, using the terms decarboxylation and dehydrogenation.
Calculate the total number of molecules of reduced NAD, reduced FAD and carbon dioxide produced from one molecule of glucose in glycolysis, the link reaction and the Krebs cycle.
Electron micrographs show that mitochondria in heart muscle cells have many more cristae than those in skin cells. Explain the significance of this observation.
Answer key and marking notes
- Glycolysis: cytoplasm.
- Link reaction: mitochondrial matrix.
- Krebs cycle: mitochondrial matrix.
- Oxidative phosphorylation: inner mitochondrial membrane (cristae).
- Glucose is phosphorylated using two ATP, forming fructose 1,6-bisphosphate (6C).
- Fructose 1,6-bisphosphate splits (lysis) into two triose phosphate molecules (3C).
- Each triose phosphate is oxidised (dehydrogenated) to pyruvate (3C); the hydrogen reduces NAD.
- Four ATP are made by substrate-linked phosphorylation, a net gain of two ATP and two reduced NAD per glucose.
- Pyruvate enters the mitochondrial matrix, where it is decarboxylated (CO2 removed).
- It is also dehydrogenated; the hydrogen reduces NAD.
- The remaining acetyl group (2C) combines with coenzyme A, forming acetyl coenzyme A, which carries it to the Krebs cycle.
- The acetyl group (2C) from acetyl coenzyme A combines with oxaloacetate (4C), the acceptor, to form citrate (6C).
- Citrate is converted back to oxaloacetate in a series of small steps.
- Decarboxylation removes carbon as CO2 (6C → 5C → 4C); dehydrogenation removes hydrogen, which reduces NAD and FAD.
- One ATP is formed per turn by substrate-linked phosphorylation; oxaloacetate is regenerated to accept another acetyl group.
- Reduced NAD: 2 (glycolysis) + 2 (link reaction) + 6 (two turns of the Krebs cycle) = 10.
- Reduced FAD: 2 (one per turn of the Krebs cycle).
- CO2: 2 (link reaction) + 4 (Krebs cycle) = 6, equal to the six carbon atoms in glucose.
- Cristae are folds of the inner membrane that carry the electron transport chain and ATP synthase.
- More cristae give a larger surface area for oxidative phosphorylation, so more ATP can be made.
- Heart muscle contracts continuously and needs a large, constant supply of ATP; skin cells are less active.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which compound in aerobic respiration contains six carbon atoms and is formed in the Krebs cycle?
- Acitrate
- Boxaloacetate
- Cacetyl coenzyme A
- Dpyruvate
During which step is NAD reduced in the cytoplasm of a respiring cell?
- Athe oxidation of triose phosphate to pyruvate
- Bthe phosphorylation of glucose to fructose 1,6-bisphosphate
- Cthe splitting of fructose 1,6-bisphosphate into two triose phosphates
- Dthe conversion of pyruvate to acetyl coenzyme A
Glycolysis begins by using two ATP to phosphorylate glucose. What is the main reason for this step?
- Ait makes the sugar more reactive, so it can be split into two triose phosphates
- Bit releases energy that is used to make ATP later in glycolysis
- Cit allows glucose to enter the mitochondrion for the Krebs cycle
- Dit provides hydrogen that reduces NAD
The diagram shows a mitochondrion cut lengthways. Which label shows where the enzymes of the Krebs cycle are found?
- AD
- BA
- CB
- DC
What is meant by decarboxylation?
- Athe removal of carbon dioxide from a molecule
- Bthe removal of hydrogen from a molecule
- Cthe addition of a phosphate group to a molecule
- Dthe transfer of an acetyl group from coenzyme A
How many molecules of reduced NAD are produced inside the mitochondrial matrix from one molecule of glucose respired aerobically?
- A8
- B10
- C4
- D6
Yeast respiring aerobically was given glucose in which every carbon atom was radioactive. In which stages is radioactive carbon dioxide released?
- Athe link reaction and the Krebs cycle
- Bglycolysis and the link reaction
- Cglycolysis only
- Doxidative phosphorylation only
A drug inhibits one Krebs cycle enzyme that converts a 4C compound into another 4C compound while reducing FAD. What is the most likely effect on a cell respiring aerobically?
- Aless oxaloacetate is regenerated, so fewer acetyl groups enter the cycle and less reduced NAD and FAD are made
- Bglycolysis stops at once, because pyruvate can no longer be made
- Cmore citrate is broken down, so more CO2 is released
- Donly reduced FAD production falls; reduced NAD production by the cycle is unchanged
Quiz key
Oxaloacetate (4C) accepts the acetyl group (2C) to form citrate (6C), the first compound of the Krebs cycle. Oxaloacetate itself has four carbons and is regenerated at the end of the cycle. Acetyl coenzyme A carries a 2C acetyl group. Pyruvate has three carbons and is made in glycolysis, in the cytoplasm.
Triose phosphate is dehydrogenated as it is converted to pyruvate, and the hydrogen reduces NAD. Phosphorylation adds phosphate from ATP and lysis splits the 6C sugar; neither removes hydrogen. Pyruvate is converted to acetyl coenzyme A in the link reaction, which does reduce NAD, but in the mitochondrial matrix, not the cytoplasm.
Adding two phosphate groups raises the energy of the sugar and makes it more reactive, so fructose 1,6-bisphosphate splits easily into two 3C molecules. The step uses energy rather than releasing it. Glucose never enters the mitochondrion; only pyruvate does. Phosphorylation adds phosphate, it does not remove hydrogen, so no NAD is reduced.
D is the matrix, which contains the enzymes of the link reaction and the Krebs cycle. A is the outer membrane, which contains transport proteins but no Krebs cycle enzymes. B is the intermembrane space, where protons accumulate during oxidative phosphorylation. C is a crista, a fold of the inner membrane carrying the electron transport chain and ATP synthase.
Decarboxylation removes carbon as carbon dioxide, catalysed by decarboxylase enzymes; it happens in the link reaction and twice in each turn of the Krebs cycle. Removing hydrogen is dehydrogenation, which reduces NAD or FAD. Adding phosphate is phosphorylation. Transferring the acetyl group to oxaloacetate is how the Krebs cycle starts, not a decarboxylation.
Two link reactions give 2 reduced NAD and two turns of the Krebs cycle give 6, so 8 are made in the matrix. 10 also includes the 2 from glycolysis, which are made in the cytoplasm. 4 is the number from one pyruvate only (1 from the link reaction and 3 from one turn), forgetting that each glucose gives two pyruvates. 6 counts the Krebs cycle alone and leaves out the link reaction.
CO2 is released only by decarboxylation: once per pyruvate in the link reaction and twice per turn of the Krebs cycle, so all six labelled carbons leave from these two stages. Glycolysis splits glucose into two 3C pyruvates without removing any carbon. Oxidative phosphorylation uses hydrogen and oxygen and releases water, not CO2.
The cycle is a loop: if one step is blocked, the intermediates after it, including oxaloacetate, are not regenerated, so acetyl groups cannot be accepted and the whole cycle slows, making less reduced NAD, reduced FAD and ATP. Glycolysis happens in the cytoplasm and can continue. Citrate is not broken down faster; it is made more slowly as oxaloacetate runs out. Because the whole cycle slows, NAD reduction falls as well as FAD reduction.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.