The shape of the lesson
By the end of the lesson, learners can
- Hydrogen to the inner membrane: Watch reduced NAD deliver its hydrogen with oxygen available: protons are yellow and electrons blue.
- Chemiosmosis: protons flow back: The pumping builds up stored energy, as a gradient.
- Why the chain needs oxygen: Oxygen is used at the very end of aerobic respiration, but everything before it depends on it.
- NAD and FAD carry hydrogen to the inner membrane.
- H splits: protons and energetic electrons; electrons go along the chain.
- Energy released pumps protons into the intermembrane space.
- Protons flow back through ATP synthase; oxygen accepts the electrons, forming water.
One picture every idea returns to
Reduced NAD and reduced FAD carry hydrogen from glycolysis, the link reaction and the Krebs cycle to carriers in the inner mitochondrial membrane. There each hydrogen atom splits into a proton and an energetic electron. The electrons pass along the electron transport chain, releasing energy that is used to transfer protons from the matrix into the intermembrane space. The protons return to the matrix by facilitated diffusion through ATP synthase, providing the energy to make ATP from ADP and phosphate: this is chemiosmosis. Oxygen is the final electron acceptor: it combines with electrons and protons to form water. Without oxygen the chain stops, and so do the Krebs cycle and link reaction.
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, Pump the protons, let them flow back.. Most of the ATP from a glucose molecule is made on the folded inner membrane of the mitochondrion. 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: Hydrogen to the inner membrane | Watch reduced NAD deliver its hydrogen with oxygen available: protons are yellow and electrons blue. Project tab 6 of the lab (Energy and respiration lab, oxidative phosphorylation) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Chemiosmosis: protons flow back | The pumping builds up stored energy, as a gradient. | Pairs |
| 21 to 30 min | Teach and check: Why the chain needs oxygen | Oxygen is used at the very end of aerobic respiration, but everything before it depends on it. | Anticipation / Reaction Guide |
| 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, 18 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 Why the chain needs oxygen, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| Electrons move along the chain; protons are pumped across the membrane. | Do not write that electrons pass through ATP synthase: it is protons that flow through it. |
| Oxygen accepts electrons (and protons) to form water. | It does not "make ATP" or "react with glucose" directly, and the CO 2 released in respiration does not come from this oxygen. |
Oxidative phosphorylation and chemiosmosis: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the role of NAD and FAD in aerobic respiration.
Explain how ATP is produced during oxidative phosphorylation.
Explain why the Krebs cycle stops when there is no oxygen, even though oxygen is not used in the Krebs cycle.
Explain how the structure of the inner mitochondrial membrane is related to its function in oxidative phosphorylation.
In an active mitochondrion the pH of the intermembrane space is 7.0 and the pH of the matrix is 8.0. How many times greater is the proton concentration in the intermembrane space? Explain the difference.
Isolated mitochondria were kept in a solution containing pyruvate and phosphate. Oxygen uptake was slow. When ADP was added, oxygen uptake rose sharply; when cyanide was then added, oxygen uptake stopped. Explain these results.
Answer key and marking notes
- NAD and FAD are coenzymes that accept hydrogen removed by dehydrogenase enzymes in glycolysis, the link reaction and the Krebs cycle, becoming reduced.
- Reduced NAD and FAD transfer the hydrogen to carriers in the inner mitochondrial membrane, and are oxidised so they can be used again.
- Hydrogen from reduced NAD and FAD is passed to the electron transport chain on the inner mitochondrial membrane; each hydrogen atom splits into a proton and an energetic electron.
- Electrons pass along the chain of carriers, releasing energy.
- This energy is used to pump protons from the matrix into the intermembrane space, forming a proton gradient.
- Protons diffuse back into the matrix through ATP synthase (facilitated diffusion), which uses the energy to make ATP from ADP and phosphate: chemiosmosis.
- Oxygen is the final electron acceptor, combining with electrons and protons to form water.
- Oxygen is the final electron acceptor; without it electrons cannot leave the electron transport chain.
- So reduced NAD and reduced FAD cannot pass on their hydrogen and are not oxidised.
- No NAD or FAD is available to accept hydrogen in the dehydrogenation steps, so the Krebs cycle (and link reaction) stop.
- It is folded into cristae, giving a large surface area for many electron transport chains and ATP synthase molecules.
- It is impermeable to protons, so a proton gradient can be maintained between the intermembrane space and the matrix.
- Protons can only return through ATP synthase, so their flow is coupled to ATP synthesis.
- pH is a logarithmic scale: one pH unit lower means a 10 times higher H+ concentration, so 10 times greater.
- Protons are pumped from the matrix into the intermembrane space using energy released by electrons in the electron transport chain.
- Without ADP, ATP synthase cannot work, so protons do not flow back; the gradient becomes steep and slows electron transport, so little oxygen is used.
- Adding ADP lets protons flow through ATP synthase, the gradient falls, electron transport speeds up and more oxygen is used as the final electron acceptor.
- Cyanide inhibits the last carrier in the chain, so electrons cannot be passed to oxygen and oxygen uptake stops.
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 substance is formed at the end of the electron transport chain in aerobic respiration?
- Awater
- Bcarbon dioxide
- Creduced NAD
- Dglucose
Which statement defines chemiosmosis?
- Athe synthesis of ATP using energy released as protons diffuse down their gradient through ATP synthase
- Bthe synthesis of ATP by transferring a phosphate group directly from a substrate to ADP
- Cthe movement of water across a partially permeable membrane down a water potential gradient
- Dthe pumping of protons across a membrane using energy from the hydrolysis of ATP
Each reduced FAD leads to the synthesis of less ATP than each reduced NAD. Which explanation is correct?
- Areduced FAD passes its hydrogen to a carrier further along the chain, so fewer protons are pumped
- Breduced FAD carries only one hydrogen atom, while reduced NAD carries four
- Creduced FAD is oxidised in the cytoplasm, so its hydrogen never reaches the chain
- Dreduced FAD gives its electrons directly to oxygen, so no energy is released
Which property of the inner mitochondrial membrane is essential for ATP synthesis by chemiosmosis?
- Ait is impermeable to protons except through ATP synthase
- Bit is freely permeable to protons over its whole surface
- Cit is smooth, so protons can flow along it quickly
- Dit contains the enzymes of the Krebs cycle
What happens to the energetic electrons as they pass along the electron transport chain?
- Athey lose energy in steps, and this energy is used to pump protons into the intermembrane space
- Bthey gain energy from ATP and are pumped into the intermembrane space
- Cthey combine with carbon dioxide to form glucose
- Dthey flow through ATP synthase and turn it to make ATP
The graph shows the pH in the intermembrane space of isolated respiring mitochondria. Oxygen was removed at time T. Which explanation of the change is correct?
- Aelectrons can no longer pass to oxygen, so proton pumping stops while protons keep diffusing back to the matrix
- Bprotons build up in the intermembrane space because they can no longer form water
- Cwater is no longer formed, so the hydroxide ion concentration rises in the intermembrane space
- Dcarbon dioxide stops being made, so less carbonic acid forms in the intermembrane space
Assume each reduced NAD gives about 2.5 ATP and each reduced FAD about 1.5 ATP in oxidative phosphorylation. Using 10 reduced NAD, 2 reduced FAD and 4 ATP from substrate-linked phosphorylation per glucose, estimate the total ATP from one glucose.
- A32
- B28
- C29
- D27
Mitochondria were soaked in a buffer at pH 8, so that the matrix reached pH 8, then moved quickly into a buffer at pH 4 containing ADP and phosphate, with no oxygen and no respiratory substrate. A burst of ATP was made. What does this show?
- Aa proton gradient across the inner membrane is enough to drive ATP synthesis through ATP synthase
- Boxygen is never needed for ATP synthesis in mitochondria
- CATP synthase is driven by a flow of electrons rather than protons
- Dsubstrate-linked phosphorylation in the matrix can occur without any substrate
Quiz key
Oxygen accepts electrons from the last carrier and combines with protons to form water. Carbon dioxide is released by decarboxylation in the link reaction and Krebs cycle, not by the chain. Reduced NAD is oxidised at the start of the chain, not formed at the end. Glucose is the substrate of respiration, made in photosynthesis, not a product of the chain.
Chemiosmosis couples the flow of protons back across a membrane, through ATP synthase, to the synthesis of ATP. Direct transfer of phosphate is substrate-linked phosphorylation. Movement of water is osmosis, which only shares part of the name. In respiration the protons are pumped using energy from electrons in the chain, not from ATP hydrolysis.
Reduced FAD enters the chain at a later carrier than reduced NAD, so its electrons pass fewer proton-pumping stages, a smaller gradient is built and less ATP is made. Both reduced coenzymes carry a pair of hydrogen atoms. FAD is reduced in the Krebs cycle in the matrix, and its hydrogen does reach the chain. Its electrons still pass along carriers before oxygen, releasing energy on the way.
Because protons cannot cross the phospholipid bilayer, the pumped protons build up in the intermembrane space and can only return through ATP synthase, which couples their flow to ATP synthesis. A freely permeable membrane would let the gradient leak away with no ATP made. The inner membrane is folded into cristae, not smooth, to give a large surface area. Krebs cycle enzymes are in the matrix.
Each carrier passes the electrons to the next at a lower energy level, releasing energy in small steps that is used to transfer protons across the inner membrane. The electrons are not energised by ATP, and they stay in the membrane rather than being pumped. Electrons end up on oxygen, forming water, not glucose. It is protons, not electrons, that flow through ATP synthase.
Without oxygen as the final electron acceptor the chain stops, so no more protons are pumped into the intermembrane space; those already there continue to flow back through ATP synthase, so the proton concentration falls and the pH rises towards that of the matrix. Protons would build up only if pumping continued, which would lower the pH. Water is formed in the matrix, not the intermembrane space, and its formation does not set the pH of that space. Decarboxylation happens in the matrix and its carbon dioxide is not what maintains the gradient.
10 × 2.5 = 25, 2 × 1.5 = 3, plus 4 = 32 ATP. 28 leaves out the 4 ATP made by substrate-linked phosphorylation. 29 leaves out the reduced FAD. 27 counts only the 8 reduced NAD made in the matrix (20 + 3 + 4) and forgets the 2 from glycolysis.
Protons from the pH 4 buffer reach the intermembrane space (the outer membrane is permeable) and flow down the artificial gradient into the pH 8 matrix through ATP synthase, making ATP with no electron transport at all, which supports chemiosmosis. It does not show oxygen is never needed: in a cell oxygen keeps the chain running so the gradient is maintained, and here the ATP was only a short burst. No electrons were moving, so it cannot be electron flow. Substrate-linked phosphorylation needs a phosphorylated substrate, and none was present.
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.