Unit 7: The Citric Acid Cycle and Oxidative Phosphorylation
Where the real ATP payoff comes from: a spinning loop of oxidation feeding a proton gradient that spins a molecular motor.
8 topics · 28 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Pyruvate Oxidation and Acetyl-CoA Formation
- How it worksHow does the pyruvate dehydrogenase complex strip off one carbon and attach what's left to coenzyme A?
- Going furtherWhat happens to a cell's aerobic energy output when pyruvate dehydrogenase gets inhibited?
- Common mistakeWhy isn't pyruvate oxidation actually a step inside the citric acid cycle itself?
Your turn1 of 2
Pyruvate emerges from glycolysis in the cytoplasm, but before it can enter the citric acid cycle, the pyruvate dehydrogenase complex strips off one carbon and attaches what remains to coenzyme A. A student assumes this must be a wasteful extra detour, since pyruvate could presumably just enter the cycle directly. What does this conversion actually accomplish?The Citric Acid Cycle: An Overview of the Loop
- How it worksHow does a two-carbon acetyl group entering the cycle end up fully oxidized into two molecules of carbon dioxide?
- Common mistakeWhy isn't the citric acid cycle where a cell makes most of its ATP directly?
- Going furtherWhat happens to the citric acid cycle's rate when oxaloacetate keeps getting pulled away to build other molecules?
Your turn1 of 2
A student notices the citric acid cycle produces only a small amount of ATP directly per turn, far less than oxidative phosphorylation eventually yields, and assumes the cycle therefore must not matter much for a cell's overall energy production. Why is this dismissal wrong?Tracking Carbons and Electrons Through the Citric Acid Cycle
- How it worksHow do the citric acid cycle's oxidation steps hand electrons off to NAD+ and FAD at three separate points?
- How it worksWalk me through following one entering acetyl group's two carbons all the way to their release as carbon dioxide.
- Worked exampleFind the total NADH, FADH2, and ATP produced directly from one single turn of the citric acid cycle.
- Going furtherWhat happens to the cycle's total output when a cell runs it twice per glucose instead of once?
Your turn1 of 2
One turn of the citric acid cycle produces 3 NADH. Find the total NADH produced when the cycle runs 28 times.The Electron Transport Chain
- How it worksHow does each electron transport chain complex use the energy of passing electrons to pump protons across the membrane?
- How it worksWalk me through electrons finally landing on oxygen at the end of the chain to form water.
- Common mistakeWhy isn't the electron transport chain itself the step that directly builds most of a cell's ATP?
- Going furtherWhat happens to electron flow through the chain when oxygen, its final acceptor, isn't available?
Your turn1 of 2
Electrons from NADH pass through four separate protein complexes in sequence before finally reaching oxygen, rather than transferring to oxygen directly in one step. A student assumes this multi-step relay must just add unnecessary delay. What does passing electrons through each complex actually accomplish?Chemiosmosis and the Proton Gradient
- How it worksHow does the electron transport chain build up a steep concentration and charge gradient of protons in the intermembrane space?
- How it worksWalk me through how that stored proton gradient actually gets spent to make ATP.
- Common mistakeWhy isn't the proton gradient itself a form of chemical bond energy?
- Ties togetherHow does chemiosmosis's proton gradient use the same electrochemical-gradient logic as the sodium gradient a nerve cell maintains?
Your turn1 of 2
The electron transport chain spends much of its energy simply pumping protons from one side of a membrane to the other, rather than making any ATP directly at that point. A student assumes this pumping step must be a wasted intermediate with no real payoff of its own. Why does pumping protons across a membrane count as storing usable energy?ATP Synthase: The Rotating Motor
- How it worksHow does a flow of protons through ATP synthase's channel force its central rotor to spin?
- How it worksWalk me through how that spinning rotor's shape changes squeeze ADP and phosphate together into ATP.
- Worked exampleFind how many full rotations of ATP synthase are needed to make 300 ATP if each rotation yields about 3 ATP.
- Going furtherWhat happens to ATP synthase's rotation the moment the proton gradient driving it collapses?
Your turn1 of 2
ATP synthase makes about 3.2 ATP per full rotation. Find how many full rotations are needed to make 605 ATP.Calculating ATP Yield from One Glucose
- How it worksHow do the NADH and FADH2 made throughout glycolysis and the citric acid cycle get cashed in for ATP at the electron transport chain?
- Worked exampleFind the total ATP yield from one glucose molecule given about 2 ATP from glycolysis, 2 from the citric acid cycle, and 26 to 28 from oxidative phosphorylation.
- Going furtherWhat happens to the ATP yield per glucose when a cell uses the less efficient shuttle to move electrons into the mitochondrion?
Your turn1 of 2
Find the total ATP yield from one glucose molecule given 2 ATP from glycolysis, 2 from the citric acid cycle, and 28 ATP from oxidative phosphorylation.Cyanide and Metabolic Poisons
- How it worksHow does cyanide binding Complex IV block the entire electron transport chain sitting behind it?
- Going furtherWhat happens to a cell's ATP supply within minutes once its electron transport chain gets fully blocked?
- Ties togetherHow does cyanide's block of Complex IV connect back to the same chemiosmotic logic that ATP synthase depends on to keep spinning?
Your turn1 of 2
A tiny dose of cyanide can shut down energy production in almost every cell of the body at once. A student assumes cyanide must be attacking many different targets throughout each cell simultaneously to have such a widespread effect. What does cyanide actually target?