Trace hydrogen transfer by reduced NAD and FAD
Explain how reduced NAD and FAD carry hydrogen from respiratory pathways to carriers in the inner mitochondrial membrane.
Section 12.2 · Respiration
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
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Quick readiness check
What does NAD become after accepting hydrogen during dehydrogenation?
What you will learn
- Describe the role of reduced NAD and FAD as hydrogen carriers
- Connect dehydrogenation in the matrix to oxidative phosphorylation
Build the explanation
During glycolysis and the Krebs cycle, dehydrogenation transfers hydrogen to NAD and FAD, producing reduced NAD and reduced FAD. These reduced coenzymes carry hydrogen to carriers in the inner mitochondrial membrane. There, hydrogen contributes protons and energetic electrons to oxidative phosphorylation. The carrier molecules link earlier oxidation reactions with electron transport and the proton gradient that supports ATP synthesis.
Apply it carefully
NAD and FAD are carriers, not the final electron acceptor. Oxygen accepts electrons at the end of the electron transport chain. The syllabus requires the role of hydrogen transfer but not the names or detailed chemistry of every carrier in the membrane.
Pause and explain
Is oxygen the carrier that transports hydrogen from the Krebs cycle?
Worked example
What do reduced NAD and reduced FAD deliver to the inner mitochondrial membrane?
Show the worked solution
- They are reduced coenzymes carrying hydrogen.
- That hydrogen contributes protons and electrons for oxidative phosphorylation.
Answer They carry hydrogen to membrane carriers, where it contributes protons and energetic electrons.
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Explain how dehydrogenation in the Krebs cycle connects to oxidative phosphorylation.
Give me a hint
Follow hydrogen from a metabolic reaction to the membrane.
Compare my reasoning
- NAD and FAD accept hydrogen and become reduced.
- Reduced coenzymes carry hydrogen to inner-membrane carriers for oxidative phosphorylation.
Dehydrogenation transfers hydrogen to NAD and FAD, forming reduced coenzymes. These carry hydrogen to carriers in the inner mitochondrial membrane, linking Krebs-cycle oxidation with oxidative phosphorylation.
Look for these in your work
- Hydrogen accepted
- Reduced forms named
- Membrane destination stated
A student states that NAD and oxygen compete to carry the same electrons at the same step. Correct the sequence.
Give me a hint
Distinguish the earlier carrier from the final acceptor.
Compare my reasoning
- Reduced NAD carries hydrogen to membrane carriers.
- Oxygen is the final electron acceptor later in oxidative phosphorylation.
Reduced NAD carries hydrogen from earlier stages to carriers in the inner mitochondrial membrane. Oxygen acts later as the final electron acceptor, forming water; the two roles occur at different points in the pathway.
Look for these in your work
- NAD role correct
- Oxygen role correct
- Sequence distinguished
A cell has reduced coenzymes but cannot deliver hydrogen to the inner membrane. Predict one direct limitation to oxidative phosphorylation.
Give me a hint
Identify what the membrane stage receives from the carriers.
Compare my reasoning
- Reduced coenzymes normally transfer hydrogen to membrane carriers.
- Without that transfer, the electron transport chain receives less hydrogen-derived electron input and proton pumping is reduced.
The membrane carriers receive less hydrogen-derived input, so fewer energetic electrons pass along the chain and less energy is available to move protons across the membrane. The proton gradient and chemiosmotic ATP synthesis would be reduced.
Look for these in your work
- Carrier transfer identified
- Electron flow connected
- Proton-gradient consequence explained
Common mistakes
- Calling NAD or FAD the final electron acceptor
- Skipping the transfer to inner-membrane carriers
- Adding detailed carrier names beyond the stated syllabus scope
What can you explain now?
What does NAD become after accepting hydrogen during dehydrogenation?
Compare with the explanation
Reduced NAD
The reduced coenzyme carries hydrogen to the inner mitochondrial membrane.
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Make it stick
How do reduced NAD and FAD carry products of dehydrogenation to the inner membrane?
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