Respiration in Plants — Glycolysis, the TCA Cycle and the ATP Ledger
Respiration is examined almost entirely as accounting and location: how much ATP, how much NADH, produced where. So the single most useful thing to build is a ledger you can reconstruct — glycolysis in the cytoplasm, the link reaction and TCA cycle in the mitochondrial matrix, the electron transport chain on the inner mitochondrial membrane. Get those three addresses right and a large family of questions collapses. The conceptual thread worth following is that respiration is a controlled, stepwise release of the energy that combustion would release all at once: the cell takes glucose apart in small steps so that the energy comes out in packets it can capture as ATP rather than as heat. And one idea from the end of the chapter reframes everything before it — respiration is amphibolic, not merely catabolic, because its intermediates are constantly withdrawn to build other molecules.
The TCA Cycle and Electron Transport
| Item | Per turn | Note |
|---|---|---|
| Starting reaction | Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C) | OAA is regenerated at the end, which is what makes it a cycle |
| NADH | 3 | At isocitrate, α-ketoglutarate and malate dehydrogenase steps |
| FADH₂ | 1 | Only at succinate → fumarate; succinate dehydrogenase is the one TCA enzyme in the inner membrane |
| GTP / ATP | 1 | At succinyl CoA → succinate — the only substrate-level phosphorylation in the cycle |
| CO₂ | 2 | Two decarboxylations; this is where respired CO₂ actually comes from |
🧠 The full ATP ledger, and the one place NEET expects specific numbers
Build the total from four rows. Glycolysis: net 2 ATP + 2 NADH. Link reaction (twice, once per pyruvate): 2 NADH. TCA cycle (twice): 6 NADH + 2 FADH₂ + 2 ATP. Oxidative phosphorylation then converts the reducing power, and NCERT uses the round figures 1 NADH → 3 ATP and 1 FADH₂ → 2 ATP. Adding up gives 36 ATP per glucose for a plant cell, which is the number to write unless a question specifies otherwise. Two cautions. First, the actual yield is lower — closer to 2.5 and 1.5 ATP respectively — because the proton gradient is not perfectly coupled, and some textbooks therefore quote 30–32; NEET follows NCERT, so use 36 but do not be thrown if a passage quotes another figure. Second, the glycolytic NADH made in the cytoplasm has to be shuttled into the mitochondrion, which costs something, and that is the technical reason the two accounting systems differ at all.| Component | What it does |
|---|---|
| Complex I — NADH dehydrogenase | Takes electrons from NADH to ubiquinone |
| Complex II — succinate dehydrogenase | Feeds in electrons from FADH₂, bypassing complex I — which is why FADH₂ yields less ATP |
| Ubiquinone → Complex III → cytochrome c | Mobile carriers passing electrons onward and pumping protons |
| Complex IV — cytochrome c oxidase | Hands electrons to O₂, the terminal acceptor, forming water |
| Complex V — ATP synthase | The proton gradient discharges through it, making ATP — oxidative phosphorylation |
⚠️ Why FADH₂ is worth less than NADH — and why oxygen matters even though it appears only at the end
FADH₂ delivers its electrons at complex II, downstream of complex I, so they pass through one fewer proton-pumping station and generate correspondingly less ATP — 2 against 3 in NCERT's figures. That is a structural reason, not an arbitrary constant, and stating it earns the mark on any "why" question. The second point catches more candidates: oxygen appears only in the very last step, accepting electrons at complex IV to form water, and yet without it the entire chain stops within seconds. The reason is that the carriers are a queue — if the last one cannot hand its electrons on, it stays reduced, and so does the one before it, all the way back. NADH then cannot be reoxidised, the TCA cycle halts for want of NAD⁺, and only glycolysis is left. So oxygen is not a participant in most of respiration, but it is the permission for all of it, and that distinction is exactly what the question tests.Respiratory Quotient and the Amphibolic Pathway
| Substrate | RQ | Why |
|---|---|---|
| Carbohydrate | 1.0 | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O — the gases balance exactly |
| Fat | Less than 1 (about 0.7) | Fats are hydrogen-rich and oxygen-poor, so oxidising them consumes extra oxygen |
| Protein | About 0.9 | Intermediate between the two |
| Organic acid | More than 1 | Already partly oxidised, so relatively little O₂ is needed |
🎯 Why respiration is called amphibolic rather than catabolic
Described as a pathway for breaking glucose down, respiration looks purely catabolic. But its intermediates are constantly being withdrawn for synthesis: acetyl CoA is the starting point for fatty acids, α-ketoglutarate and oxaloacetate are drawn off to make amino acids, and succinyl CoA goes towards porphyrins including the haem of chlorophyll and haemoglobin. So the same pathway serves breakdown and building simultaneously, which is what amphibolic means. Two consequences are worth stating. First, if intermediates are removed, they must be replenished or the cycle would run down — which is why a cell can feed carbon into the TCA cycle as well as take it out. Second, this is why respiratory substrates are not limited to carbohydrate: fats enter as acetyl CoA after β-oxidation, and proteins enter as various intermediates after deamination, which is exactly what the RQ table above is measuring. Calling respiration merely catabolic is the error the term amphibolic exists to correct.Key takeaways
- Locations: glycolysis in the cytoplasm, link reaction and TCA in the mitochondrial matrix, ETC on the inner membrane.
- Glycolysis gives a NET 2 ATP and 2 NADH — gross 4 ATP, minus 2 spent at hexokinase and phosphofructokinase.
- Fermentation exists to regenerate NAD⁺, not to make ATP; it releases under 7% of glucose energy.
- One TCA turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂. The only FADH₂ step is succinate → fumarate.
- Succinyl CoA → succinate is the only substrate-level phosphorylation in the TCA cycle.
- NCERT ledger: 1 NADH = 3 ATP, 1 FADH₂ = 2 ATP, total 36 ATP per glucose. FADH₂ yields less because it enters at complex II.
- Oxygen acts only at the very last step but its absence stops the whole chain, because the carriers are a queue.
- RQ: carbohydrate 1, fat about 0.7, protein about 0.9, organic acid above 1. Respiration is amphibolic, not merely catabolic.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The net ATP and NADH yield of glycolysis from one glucose molecule is:
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Answer: A — 2 ATP and 2 NADH
Four ATP are made but two are spent earlier at the hexokinase and phosphofructokinase steps, so the net is 2 along with 2 NADH. The 4-ATP option is the gross figure — a correct number attached to the wrong word — and 36 is the total for complete aerobic respiration, not glycolysis alone.The primary purpose of fermentation in an anaerobic cell is to:
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Answer: A — Regenerate NAD⁺ so that glycolysis can continue
Fermentation makes no ATP of its own — its function is to reoxidise NADH so that NAD⁺ is available and glycolysis does not stall, with ethanol or lactate as waste. Complete oxidation to CO₂ and water is aerobic respiration, which is precisely what an anaerobic cell cannot do.In one turn of the TCA cycle, FADH₂ is produced at which step?
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Answer: A — Succinate to fumarate
Succinate dehydrogenase is the only FADH₂-producing enzyme of the cycle, and notably the only TCA enzyme embedded in the inner mitochondrial membrane rather than free in the matrix. The isocitrate and malate steps give NADH, and succinyl CoA to succinate is the substrate-level phosphorylation that yields GTP.FADH₂ yields less ATP than NADH because it:
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Answer: A — Enters the chain at complex II, bypassing one proton-pumping complex
Both carry two electrons, but FADH₂ delivers them at complex II rather than complex I, so they pass through one fewer proton-pumping station and generate correspondingly less ATP. That is a structural consequence of where it enters, not a difference in how many electrons it holds.A germinating seed rich in fat would show a respiratory quotient of:
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Answer: A — Less than 1
Fats are hydrogen-rich and oxygen-poor, so oxidising them requires extra O₂ relative to the CO₂ released, giving an RQ of about 0.7. RQ exactly 1 belongs to carbohydrate and above 1 to organic acids, which are already partly oxidised and so need comparatively little oxygen.Respiration is described as an amphibolic pathway because:
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Answer: A — Its intermediates are withdrawn for biosynthesis as well as being broken down
Amphibolic means serving catabolism and anabolism at once: acetyl CoA leaves for fatty acid synthesis, α-ketoglutarate and oxaloacetate for amino acids, succinyl CoA for porphyrins. Occurring with or without oxygen is a different property, and being present in both kingdoms says nothing about the pathway's chemistry.If oxygen supply to a respiring cell is cut off, the electron transport chain stops because:
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Answer: A — The terminal carrier cannot pass on its electrons, so every carrier behind it stays reduced
Oxygen participates only at complex IV, but the carriers form a queue, so a blocked exit backs the whole line up and NADH can no longer be reoxidised — which in turn halts the TCA cycle. Glycolysis, by contrast, continues, since fermentation can regenerate NAD⁺ without oxygen.Using the NCERT convention of 3 ATP per NADH and 2 per FADH₂, the total ATP yield from complete aerobic oxidation of one glucose molecule is:
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Answer: A — 36
Glycolysis nets 2 ATP and 2 NADH, the link reaction adds 2 NADH, and two TCA turns add 6 NADH, 2 FADH₂ and 2 ATP; converting at 3 and 2 gives 36 in total. The figure 2 is glycolysis alone, and 18 is the ATP spent by the Calvin cycle per glucose — a number from the previous chapter that is offered here deliberately.