C. Metabolic Processes · Metabolic ProcessesOxidation, Reduction and the Electron Carriers NAD⁺, FAD and NADP⁺
C2.1 C3.1 C3.3 A1.12About 65 minutes
Learning goals- I can define oxidation and reduction in terms of electrons, hydrogen and oxygen.
- I can identify the substance oxidized, the substance reduced, and the oxidizing and reducing agents in a biological redox reaction.
- I can explain how NAD⁺, FAD and NADP⁺ carry high-energy electrons, and write their oxidized and reduced forms.
- I can explain why electrons moving toward oxygen release energy, and why cells release it in small steps.
Words to know
oxidation
Loss of electrons. In biology it often appears as a loss of hydrogen atoms (or a gain of oxygen).
reduction
Gain of electrons. In biology it often appears as a gain of hydrogen atoms (or a loss of oxygen).
redox reaction
A reaction in which electrons are transferred: one substance is oxidized while another is reduced.
oxidizing agent
The substance that accepts electrons, causing another to be oxidized; it is itself reduced.
reducing agent
The substance that donates electrons, causing another to be reduced; it is itself oxidized.
electronegativity
How strongly an atom attracts shared electrons; oxygen is one of the most electronegative elements.
NAD⁺ / NADH
Nicotinamide adenine dinucleotide; oxidized form NAD⁺ accepts 2 electrons and 1 H⁺ to become NADH in respiration.
FAD / FADH₂
Flavin adenine dinucleotide; accepts 2 electrons and 2 H⁺ to become FADH₂ (in the Krebs cycle).
NADP⁺ / NADPH
Nicotinamide adenine dinucleotide phosphate; the electron carrier of photosynthesis, reduced to NADPH in the light-dependent reactions.
dehydrogenase
An enzyme that removes hydrogen atoms (electrons plus protons) from a substrate and usually passes them to NAD⁺ or FAD.
Where does the energy in food actually come from?
On a cold January night, a natural-gas furnace burns methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Your cells do something chemically similar with glucose, but without a flame. In both cases, energy is released because electrons move from fuel atoms (carbon and hydrogen) toward oxygen, which pulls on them very strongly.
Reactions that move electrons are called redox reactions. Every stage of cellular respiration and photosynthesis is built from them, so this lesson gives you the tools to follow the electrons.
Remember from Biochemistry
In the Biochemistry unit you learned that redox is one of the four main types of biochemical reaction. In Grade 11 Chemistry (if you took it) you learned that electronegativity describes how strongly an atom attracts shared electrons, and that oxygen is very electronegative. Both ideas are used here.
Oxidation and reduction
- Oxidation is the loss of electrons.
- Reduction is the gain of electrons.
Two memory aids: LEO says GER (Lose Electrons Oxidation, Gain Electrons Reduction) or OIL RIG (Oxidation Is Loss, Reduction Is Gain).
The two always happen together: electrons don't float around on their own. If one substance is oxidized, another is reduced.
- The substance that loses electrons is the reducing agent (it reduces the other one).
- The substance that gains electrons is the oxidizing agent.
In biology, follow the hydrogen. Electrons in organic molecules usually move together with protons, as hydrogen atoms (H = 1 e⁻ + 1 H⁺). So:
- Losing H (or gaining O) usually means oxidation.
- Gaining H (or losing O) usually means reduction.
In respiration, glucose (C₆H₁₂O₆) loses all its hydrogen atoms and ends up as CO₂: glucose is oxidized. Oxygen (O₂) gains hydrogen and becomes H₂O: oxygen is reduced.
Even when electrons are only partly shifted (shared unequally in a covalent bond), we count it as redox. In a C–H bond, electrons are shared almost equally. In C=O and O–H bonds, oxygen pulls them closer. Moving electrons from C–H bonds into bonds with oxygen lets them settle at a lower energy, and the difference is released.
Electron carriers: NAD⁺, FAD and NADP⁺
Cells don't hand electrons from glucose straight to oxygen. That would release a lot of energy at once, mostly as heat. Instead, enzymes called dehydrogenases strip pairs of hydrogen atoms (2e⁻ + 2H⁺) off fuel molecules and hand them to coenzymes that act as electron shuttles:
- NAD⁺ + 2e⁻ + 2H⁺ → NADH + H⁺ (one H⁺ stays in solution). NAD⁺ is the main electron carrier in glycolysis, pyruvate oxidation and the Krebs cycle.
- FAD + 2e⁻ + 2H⁺ → FADH₂. FAD is used in one step of the Krebs cycle.
- NADP⁺ + 2e⁻ + H⁺ → NADPH. NADP⁺ is the electron carrier of photosynthesis. A handy trick: P for Photosynthesis.
The reduced forms (NADH, FADH₂, NADPH) carry high-energy electrons. They are energy carriers, like ATP, but they carry energy as reducing power rather than as a phosphate group. NAD⁺ and FAD are made from B vitamins (niacin and riboflavin), which is one reason those vitamins are part of a healthy diet.
Falling electrons: a staircase, not a cliff
Picture electrons as balls on a staircase. Electrons in the C–H bonds of glucose are at the top (high free energy). Oxygen, very electronegative, is at the bottom. When electrons fall from glucose to oxygen, the energy difference is released.
- Burning glucose drops the electrons straight off the cliff: all 2870 kJ/mol comes out at once as heat and light.
- Cellular respiration lowers them step by step: glucose → NADH (or FADH₂) → a chain of carrier proteins (the electron transport chain) → O₂. Each small step releases a manageable amount of energy, which the cell uses to make ATP.
For NADH, the drop is large: NADH + ½O₂ + H⁺ → NAD⁺ + H₂O releases about 220 kJ/mol (ΔG ≈ −220 kJ/mol), several times the 30.5 kJ/mol needed to make one ATP. You will see how the cell captures this energy in the lesson on the electron transport chain.
In photosynthesis the staircase runs the other way: light energy lifts electrons from water (at the bottom) up to NADPH (near the top), and NADPH then delivers them to CO₂ to make sugar. That is why photosynthesis is endergonic and needs light.
Worked exampleLet's identify oxidation and reduction
In fermentation in muscle cells: pyruvate + NADH + H⁺ → lactate + NAD⁺. Pyruvate is C₃H₃O₃⁻ and lactate is C₃H₅O₃⁻. Which substance is oxidized, which is reduced, and which is the reducing agent?
- Compare the formulas: lactate has two more H than pyruvate (C₃H₅O₃⁻ vs C₃H₃O₃⁻). Gaining hydrogen means gaining electrons: pyruvate is reduced.
- NADH becomes NAD⁺: it loses 2 electrons (with an H⁺). NADH is oxidized.
- NADH donated the electrons, so it is the reducing agent. Pyruvate accepted them, so it is the oxidizing agent.
- Check: one substance is oxidized and one is reduced, as every redox reaction requires.
Pyruvate is reduced to lactate; NADH is oxidized to NAD⁺. NADH is the reducing agent and pyruvate is the oxidizing agent.
Worked exampleLet's use oxidation numbers to see why CO₂ is "used up" fuel
Assign an oxidation number to carbon in methane (CH₄), glucose (C₆H₁₂O₆, average per carbon) and carbon dioxide (CO₂). Use H = +1 and O = −2. What pattern do you see?
- CH₄: C + 4(+1) = 0, so C = −4.
- C₆H₁₂O₆: 6C + 12(+1) + 6(−2) = 0, so 6C = 0 and the average C = 0.
- CO₂: C + 2(−2) = 0, so C = +4.
- Going from glucose (0) to CO₂ (+4), each carbon loses electrons: it is oxidized. CO₂ is carbon's most oxidized state, so it has no more electrons to give: it is "spent" fuel.
Carbon is −4 in methane, 0 (average) in glucose and +4 in CO₂. The more reduced a carbon compound (more C–H bonds), the more energy its oxidation can release. That is why fats, with many C–H bonds, store more energy per gram than sugars.
Just for funWhy is the biochemist's favourite carrier so calm?
Because NAD⁺ never loses its cool: it just picks up a couple of electrons and gets reduced to NADH.
Hands-on activityWatch a redox reaction in your kitchen
You need: an apple, a knife (safety note: cut on a cutting board, slicing away from your fingers), a lemon or bottled lemon juice, 3 small plates, a timer or phone camera.
When a cut apple turns brown, an enzyme (polyphenol oxidase) uses oxygen from the air to oxidize compounds in the apple. Vitamin C (ascorbic acid) in lemon juice is a reducing agent: it is oxidized itself instead, protecting the apple compounds.
- Cut three apple slices of similar size.
- Plate A: leave in the open air. Plate B: brush or dip in lemon juice. Plate C: wrap tightly in plastic wrap or submerge in water (to limit oxygen).
- Photograph or describe the colour of each every 10 minutes for 40 minutes.
- Record your observations in a table with columns for time and each plate.
Questions:
- Which slice browned the most? Which substance acted as the oxidizing agent in that case?
- Explain the lemon juice result using the terms oxidized and reducing agent.
- What are the independent and dependent variables? Name two controlled variables.
What to look for: Plate A browns the fastest; oxygen (with the enzyme's help) is the oxidizing agent. Plate B stays pale because vitamin C donates electrons (it is the reducing agent and is itself oxidized). Plate C browns less because less oxygen reaches it. Independent variable: treatment; dependent: colour change; controlled: same apple, slice size, temperature, time.
Your turn
1Define oxidation and reduction in terms of electrons.
Oxidation is the loss of electrons; reduction is the gain of electrons.
2In C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, which reactant is oxidized and which is reduced?
Glucose is oxidized (it loses hydrogen atoms/electrons and becomes CO₂). Oxygen is reduced (it gains hydrogen/electrons and becomes H₂O).
3Write the equation for the reduction of NAD⁺.
NAD⁺ + 2e⁻ + 2H⁺ → NADH + H⁺.
4Which electron carrier is used in photosynthesis? Give its oxidized and reduced forms.
NADP⁺ (oxidized) and NADPH (reduced).
5What does a dehydrogenase enzyme do?
It removes hydrogen atoms (electrons and protons) from a substrate, oxidizing it, and usually passes the electrons to NAD⁺ or FAD.
6Why do cells pass electrons through many steps instead of straight to oxygen?
A single step would release a large amount of energy at once, mostly as heat, and could damage the cell. Many small steps let the cell capture the energy in useful amounts to make ATP.
7In the reaction FAD + 2H → FADH₂, is FAD oxidized or reduced? Is it acting as an oxidizing or a reducing agent for the substrate?
FAD is reduced (it gains hydrogen/electrons). Because it takes electrons from the substrate, it is acting as an oxidizing agent.
More practice: warm-up
1What does OIL RIG stand for?
Oxidation Is Loss, Reduction Is Gain (of electrons).
2Is a molecule that gains hydrogen atoms usually oxidized or reduced?
3Name the reduced form of FAD.
4Which is the final electron acceptor in aerobic respiration?
Oxygen (O₂), which is reduced to water.
5Which carries more usable energy: NAD⁺ or NADH?
NADH (the reduced form carries high-energy electrons).
More practice: on your own
1In the reaction malate + NAD⁺ → oxaloacetate + NADH + H⁺, identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent.
Malate loses hydrogen, so malate is oxidized (and is the reducing agent). NAD⁺ gains electrons, so NAD⁺ is reduced (and is the oxidizing agent).
2Calculate the oxidation number of carbon in ethanol (C₂H₅OH, average per carbon) and compare it with glucose.
2C + 6(+1) + (−2) = 0 → 2C = −4 → average C = −2. That is more reduced than glucose (0), so ethanol still holds a lot of chemical energy (it can be burned as fuel).
3Explain, using electronegativity, why the transfer of electrons from NADH to oxygen releases energy.
Oxygen is very electronegative: it attracts electrons strongly. Electrons moving from NADH to O₂ go from a high-energy, loosely held position to a low-energy, tightly held one. The difference in free energy (about 220 kJ/mol) is released.
4Fats have many C–H bonds and few C–O bonds. Use redox ideas to explain why fat releases more energy per gram than glucose.
Fats are more reduced: their carbons have many electrons in C–H bonds that can be moved to oxygen. Glucose already has many C–O bonds (it is partly oxidized). More electrons available to fall to oxygen means more energy released per gram (about 37 kJ/g for fat vs about 17 kJ/g for carbohydrate).
5Explain why the vitamin niacin is needed for cellular respiration.
Niacin is used to make NAD⁺. Without enough NAD⁺, dehydrogenases in glycolysis and the Krebs cycle have nothing to pass electrons to, so respiration and ATP production slow down.
Challenge
1NADH + H⁺ + ½O₂ → NAD⁺ + H₂O has ΔG ≈ −220 kJ/mol. If the cell captured this energy perfectly in ATP (30.5 kJ/mol each), how many ATP could it make? The cell actually makes about 2.5 ATP per NADH. Calculate the efficiency and explain the difference.
Ideal: 220 ÷ 30.5 ≈ 7.2 ATP. Actual: 2.5 ATP × 30.5 = 76.25 kJ; efficiency = 76.25 ÷ 220 ≈ 35%. The rest is released as heat, because every transfer step increases entropy (second law), and some of the energy is used to move substances across the membrane.
2In photosynthesis, water is split and its electrons end up in NADPH. Is water oxidized or reduced? Is this process exergonic or endergonic? Explain using the electron staircase.
Water loses electrons, so it is oxidized (and NADP⁺ is reduced). Electrons move from water, at the bottom of the staircase (held tightly by oxygen), up to NADPH. Moving electrons uphill needs energy, so the process is endergonic; light energy provides it.
3Some bacteria live in deep, oxygen-free mud and use sulfate (SO₄²⁻) instead of O₂ as their final electron acceptor, producing H₂S. Predict whether they get more or less energy per glucose than an aerobic cell, and explain.
Less. Sulfate is a weaker oxidizing agent (less strongly attracts electrons) than O₂, so the electrons fall a shorter distance down the staircase and less free energy is released per electron pair. Oxygen is the best common final acceptor, which is why aerobic respiration yields the most ATP.
Quiz
1. Oxidation is best defined as:
2. In cellular respiration, which substance is reduced?
3. Which is the reduced form of the photosynthetic electron carrier?
4. In the reaction succinate + FAD → fumarate + FADH₂, succinate is:
5. Why do cells transfer electrons to oxygen through many small steps?
6. The average oxidation number of carbon is 0 in glucose and +4 in CO₂. This shows that in respiration, carbon:
Big idea: Energy in metabolism moves with electrons. Fuel molecules are oxidized; electron carriers (NAD⁺, FAD, NADP⁺) shuttle the electrons; oxygen is the final acceptor in aerobic respiration (C2.1, C3.1).
How to help: Ask your teen to point out, in any equation from this unit, what loses hydrogen (oxidized) and what gains it (reduced). Remind your teen to use a cutting board when slicing the apple.
Common difficulties:
- Thinking the oxidizing agent is the substance oxidized. It is the opposite: the oxidizing agent is reduced.
- Mixing up NAD⁺/NADH (oxidized/reduced). The H tells you which is reduced.
- Confusing NADH (respiration) and NADPH (photosynthesis).
- Thinking oxidation always needs oxygen. It means loss of electrons, with or without oxygen.