IB Biology HLCellular RespirationPaper 1 & 2~11 min read
Oxidation & Reduction
Respiration is often described as “burning” glucose. What is really happening is that electrons are being moved off glucose, one small handful at a time, and every one of those moves releases a little energy.
📚 What you need to know
Oxidation and reduction always happen together — they are redox reactions, and they involve electrons moving between molecules.
Oxidation is loss of electrons. It is also loss of hydrogen, gain of oxygen, and it releases energy (exergonic).
Reduction is gain of electrons. It is also gain of hydrogen, loss of oxygen, and it absorbs energy (endergonic).
A reducing agent donates electrons; an oxidising agent accepts them.
Electron carriers accept and donate electrons during respiration.
NAD+ is the primary electron carrier; FAD is the other one. Both are coenzymes.
Both act as oxidising agents: they take electrons and hydrogen ions from other molecules, becoming NADH and FADH2.
NAD exists in two forms: NAD+ (oxidised, an oxidising agent) and NADH (reduced, a reducing agent).
What is actually moving
In a redox reaction, one molecule hands electrons to another. Because nothing can lose electrons unless something else gains them, the two events are two halves of one reaction — hence the single word redox.
🧠
OIL RIG
Oxidation Is Loss • Reduction Is Gain — of electrons. Learn it with the word “electrons” attached, or you will end up applying it to oxygen and getting it backwards.
That last point trips almost everyone up. The molecule that gets oxidised is the reducing agent, because it is the one doing the reducing.
Four ways of saying the same thing
Oxidation
Reduction
Loss of electrons
Gain of electrons
Loss of hydrogen
Gain of hydrogen
Gain of oxygen
Loss of oxygen
Releases energy (exergonic)
Absorbs energy (endergonic)
In respiration the hydrogen definition is the useful one. You will rarely see a lone electron written in a pathway diagram, but you will constantly see 2H being pulled off a molecule. Whenever hydrogens are removed, that molecule has been oxidised — and whatever collected them has been reduced.
The electron carriers
Respiration does not move electrons straight from glucose to oxygen. It hands them to electron carriers, which ferry them to the final stage.
NAD+ (nicotinamide adenine dinucleotide) is the primary electron carrier in respiration.
FAD (flavin adenine dinucleotide) is the other one, used in the Krebs cycle.
Both are coenzymes — non-protein helpers that link one redox reaction to another.
Both act as oxidising agents: they accept electrons and hydrogen ions from molecules in the pathway, and in doing so they become the reduced forms, NADH and FADH2.
Reduction of the carriers
NAD+ + 2e− + 2H+ → NADH + H+
FAD + 2e− + 2H+ → FADH2
Later, in oxidative phosphorylation, they hand those electrons on and return to their original form, ready to be used again:
Oxidation of the carriers
NADH → NAD+ + 2e− + H+
FADH2 → FAD + 2e− + 2H+
Remember this diagram when you reach anaerobic respiration. If NADH cannot be turned back into NAD+, the whole pathway jams.
Careful with the word NAD. “NAD” is the collective name for the molecule in both of its forms. Write NAD+ when you mean the oxidised form that is about to collect hydrogens, and NADH or reduced NAD when you mean the loaded form on its way to the electron transport chain.
Worked examples
WE 1
Identify oxidation and reduction
During the link reaction, pyruvate loses hydrogen and NAD+ becomes NADH. State which molecule is oxidised, which is reduced, and which is the oxidising agent. (3 marks)
OxidisedPyruvate — it has lost hydrogen, and loss of hydrogen is oxidation.
Reduced
NAD+ — it has gained hydrogen and electrons to become NADH.
Oxidising agent
NAD+, because it is the molecule that accepts the electrons and therefore oxidises the pyruvate.
Pyruvate oxidised; NAD⁺ reduced and acting as the oxidising agentthe agent is named for what it does to the other molecule
WE 2
Explain the role of NAD
Explain the role of NAD in cell respiration. (4 marks)
Point 1: what it is
NAD is a coenzyme that acts as an electron carrier, linking one redox reaction to another.
Point 2: what it collects
It acts as an oxidising agent, accepting two electrons and hydrogen ions removed from substrates in glycolysis, the link reaction and the Krebs cycle.
Point 3: what it becomes
In doing so it is reduced to NADH.
Point 4: where it delivers
NADH carries them to the electron transport chain, releases them, and is reoxidised to NAD+ so it can be used again.
Collect, carry, deliver, recyclename all three stages that reduce NAD — it is an easy extra mark
WE 3
Apply the energy definitions
Explain why the oxidation of glucose during respiration releases energy. (2 marks)
Point 1: what oxidation does
Oxidation is the loss of electrons and hydrogen, and it is exergonic — energy is released to the surroundings.
Point 2: the link to ATP
Glucose is oxidised in a series of small steps, and the energy released at each step is used to make ATP rather than being lost all at once.
Oxidation is exergonic, and the pathway captures it as ATPexergonic, not exothermic — this is about free energy, not just heat
💡 Exam tips
Define oxidation and reduction by electrons first, then add the hydrogen and oxygen versions.
In respiration questions, look for hydrogen moving — that is where the redox is.
Write NAD+ and NADH (or reduced NAD) rather than just “NAD”.
Call NAD and FAD coenzymes and electron carriers; both terms appear in mark schemes.
State that redox reactions happen simultaneously if asked to define them.
Link oxidation to exergonic and reduction to endergonic.
⚠ Common mistakes
Getting the agents backwards. The oxidised molecule is the reducing agent.
Applying OILRIG to oxygen. It refers to electrons only.
Saying only oxidation occurs in respiration. Every oxidation is paired with a reduction.
Writing NADH2. The reduced forms are NADH and FADH2.
Calling NAD an enzyme. It is a coenzyme, and it is not a protein.
Thinking the carriers are used up. They cycle between oxidised and reduced forms constantly.
Up next: Glycolysis. Time to start the pathway itself — and the first oxidation you meet will be exactly the kind described here, with NAD+ waiting to collect the hydrogens.
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