Respiration is the process by which cells break down glucose to release energy stored as ATP. CBSE Class 11 and NEET test this heavily. Expect two to three questions a year on specific enzymes, cycles and ATP yields.
If photosynthesis is the Earth’s power plant, respiration is every cell’s personal generator. Glucose contains chemical energy locked in C-H and C-C bonds. Respiration systematically breaks those bonds, captures the released energy in ATP, and leaves behind CO and water. The four-stage pathway — glycolysis, link reaction, Krebs cycle, ETC — is the most important metabolic sequence in biology.
Core Concepts
Glycolysis (cytoplasm)
The universal first step — occurs in every living cell, in the cytoplasm, and requires no oxygen (anaerobic).
Glucose (6C) is broken down into two molecules of pyruvate (3C each) through ten enzymatic steps.
Key intermediates and enzymes:
- Glucose → Glucose-6-phosphate (hexokinase, uses 1 ATP)
- Fructose-6-phosphate → Fructose-1,6-bisphosphate (phosphofructokinase, uses 1 ATP — this is the rate-limiting step)
- Each 3C fragment generates 2 ATP by substrate-level phosphorylation and 1 NADH
Net yield per glucose: 2 ATP + 2 NADH + 2 pyruvate.
(Gross: 4 ATP produced, but 2 ATP invested in the preparatory phase.)
Occurs in the cytoplasm. No oxygen required. Universal to all cells.
Link reaction (pyruvate decarboxylation)
Pyruvate enters the mitochondrial matrix and is:
- Decarboxylated (loses one CO)
- Oxidised (NAD reduced to NADH)
- Attached to coenzyme A → Acetyl CoA (2C)
Per glucose: 2 pyruvate → 2 acetyl CoA + 2 CO + 2 NADH.
This is an irreversible step catalysed by the pyruvate dehydrogenase complex — one of the largest enzyme complexes known.
Krebs cycle (citric acid cycle) — mitochondrial matrix
Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through eight enzymatic steps, citrate is oxidised back to oxaloacetate, releasing CO and capturing energy in reduced coenzymes.
Per turn of the cycle (per acetyl CoA):
- 3 NADH
- 1 FADH
- 1 GTP (= 1 ATP)
- 2 CO
Per glucose (2 turns): 6 NADH + 2 FADH + 2 ATP + 4 CO.
Key enzymes to know: citrate synthase (first step), isocitrate dehydrogenase (rate-limiting), alpha-ketoglutarate dehydrogenase (produces CO), succinate dehydrogenase (only enzyme embedded in inner membrane, produces FADH).
All six carbons of glucose are now fully oxidised to CO. The energy is stored in NADH and FADH, waiting for the ETC.
Electron transport chain (inner mitochondrial membrane)
The payoff stage — where most ATP is made. NADH and FADH donate electrons to a series of protein complexes:
| Complex | Accepts from | Pumps H? | Inhibitor |
|---|---|---|---|
| Complex I (NADH dehydrogenase) | NADH | Yes | Rotenone |
| Complex II (succinate dehydrogenase) | FADH | No | — |
| Complex III (cytochrome bc1) | Ubiquinone | Yes | Antimycin A |
| Complex IV (cytochrome c oxidase) | Cytochrome c | Yes | Cyanide, CO |
| ATP synthase (Complex V) | H gradient | — | Oligomycin |
Oxygen is the final electron acceptor at Complex IV: . This is why we breathe — to provide the electron dump at the end of the chain.
The ETC pumps H from the matrix into the intermembrane space, creating a proton gradient. ATP synthase (chemiosmosis, Peter Mitchell’s hypothesis) uses this gradient to drive ATP synthesis.
Yield per NADH: ~2.5 ATP. Per FADH: ~1.5 ATP (enters at Complex II, fewer protons pumped).
Total ATP yield
| Stage | NADH | FADH | ATP (direct) | ATP via ETC |
|---|---|---|---|---|
| Glycolysis | 2 | — | 2 | 5 (or 3)* |
| Link reaction | 2 | — | — | 5 |
| Krebs cycle | 6 | 2 | 2 | 18 |
| Total | 10 | 2 | 4 | 28 (or 26) |
Grand total: 30-32 ATP per glucose (modern estimate). Older textbooks say 36-38.
*The 2 cytoplasmic NADH from glycolysis cost 1 ATP each to shuttle into mitochondria (malate-aspartate shuttle gives 2.5 ATP/NADH; glycerol-phosphate shuttle gives 1.5 ATP/NADH).
Anaerobic respiration (fermentation)
When oxygen is absent, the ETC stops, NADH accumulates, and NAD runs out. Without NAD, glycolysis cannot continue. Fermentation regenerates NAD by reducing pyruvate:
- Lactic acid fermentation (in muscles, some bacteria): Pyruvate → Lactate. Reversible. Causes muscle cramps during intense exercise.
- Alcoholic fermentation (in yeast): Pyruvate → Acetaldehyde → Ethanol + CO. Used in brewing and bread-making.
Both yield only 2 ATP per glucose (from glycolysis). Much less efficient than aerobic respiration, but life-saving when oxygen is temporarily unavailable.
Respiratory quotient (RQ)
For carbohydrates: RQ = 1.0. For fats: RQ = ~0.7 (fats need more O). For proteins: RQ = ~0.8. For organic acids: RQ > 1.0.
RQ tells you what substrate is being respired. During a fast, RQ drops below 1.0 as the body switches from carbohydrate to fat oxidation.
Worked Examples
During intense exercise, oxygen supply to muscles falls short. Muscles switch to lactic acid fermentation, producing lactate. Lactate buildup lowers intracellular pH, interferes with enzyme function, and causes the painful cramping sensation. Rest restores oxygen supply, and lactate is either oxidised in the mitochondria or converted back to glucose in the liver (Cori cycle).
Cyanide binds to the iron in Complex IV (cytochrome c oxidase) of the ETC, blocking electron transfer to oxygen. Without electron flow, the proton gradient collapses and ATP synthesis stops. Cells with the highest metabolic rate — brain and heart — fail first. Death occurs within minutes. Treatment involves providing alternative electron acceptors (like methylene blue or hydroxocobalamin) that bypass Complex IV.
RQ = CO/O = 6/6 = 1.0. Carbohydrates always give RQ = 1 because they are already partially oxidised (contain oxygen in the molecule).
Fats are more reduced (more C-H bonds, fewer C-O bonds) than carbohydrates. More C-H bonds means more electrons to donate to the ETC, which means more ATP. One gram of fat yields about 9 kcal, while one gram of carbohydrate yields about 4 kcal. However, fat oxidation requires more oxygen (lower RQ), which is why during intense exercise (limited O), the body preferentially burns carbohydrates.
10 NADH × 2.5 ATP per NADH = 25 ATP (modern estimate). If using the older value of 3 ATP per NADH: 10 × 3 = 30 ATP. Check which convention your textbook uses — NCERT uses the older values for board exams.
Common Mistakes
Saying glycolysis needs oxygen. It does not — glycolysis is anaerobic and occurs in the cytoplasm. Only the ETC requires oxygen. Glycolysis happens in every cell, including RBCs (which lack mitochondria).
Confusing ATP yields — glycolysis gives a net of 2 ATP (not 4). Four ATP are produced but two are invested in the preparatory phase. Krebs cycle gives 2 GTP (= 2 ATP) per glucose, not 6.
Writing that the Krebs cycle happens in the cytoplasm. It occurs in the mitochondrial matrix. Glycolysis is in the cytoplasm. The ETC is on the inner mitochondrial membrane.
Forgetting that the link reaction also produces NADH and CO. Many students account for glycolysis and Krebs but forget the link reaction (2 NADH + 2 CO per glucose).
Saying anaerobic respiration produces no ATP. It produces 2 ATP per glucose (from glycolysis). It is much less efficient than aerobic (30-32 ATP), but it is not zero.
Exam Weightage and Strategy
Respiration carries 4-6 marks in CBSE Class 11 boards. NEET asks 2-3 questions per year, often on ATP yield, enzyme names, or the location of each stage. The chapter is highly numerical — memorise the yields for each stage. JEE does not test this topic.
Memorise the four stages (glycolysis → link → Krebs → ETC), their locations (cytoplasm → matrix → matrix → inner membrane), and ATP yield of each. Draw a single flow diagram with these numbers. The stages are fixed; PYQs only shuffle the numbers and ask you to trace them.
Practice Questions
Q1. What is the net ATP yield from one molecule of glucose under aerobic conditions? Show the contribution of each stage.
Glycolysis: 2 ATP + 2 NADH (= 5 ATP via ETC). Link reaction: 2 NADH (= 5 ATP). Krebs: 2 ATP + 6 NADH (= 15 ATP) + 2 FADH (= 3 ATP). Total: 2 + 2 + 5 + 5 + 15 + 3 = 32 ATP (or 30 if glycolytic NADH uses the glycerol-phosphate shuttle). Older textbooks give 36-38 using different NADH:ATP ratios.
Q2. Why do RBCs rely solely on glycolysis for ATP?
Mature RBCs lack mitochondria (lost during differentiation to maximise space for haemoglobin). Without mitochondria, they cannot perform the Krebs cycle or ETC. They rely entirely on glycolysis, producing 2 ATP per glucose via lactic acid fermentation. This is sufficient for their limited energy needs (maintaining membrane pumps and shape).
Q3. What is the Pasteur effect?
The Pasteur effect is the observation that glucose consumption drops dramatically when oxygen is introduced to anaerobic yeast. Under anaerobic conditions, yeast ferments rapidly (2 ATP per glucose). With oxygen, aerobic respiration kicks in (30+ ATP per glucose), so far less glucose is needed for the same energy output. Named after Louis Pasteur who first observed it.
Q4. Calculate the RQ for a tripalmitin (fat) with formula .
RQ = 51/72.5 = 0.703. This is typical for fats — RQ around 0.7. The low value reflects that fats are highly reduced and need more oxygen per carbon to be fully oxidised.
FAQs
What is the difference between respiration and breathing?
Breathing is the mechanical process of moving air in and out of the lungs. Respiration is the cellular process of breaking down glucose to release ATP. Breathing provides the oxygen for respiration and removes the CO produced. One is physical; the other is biochemical.
Why is oxygen called the final electron acceptor?
In the ETC, electrons flow through complexes I-IV, losing energy at each step (used to pump protons). At the end, these low-energy electrons must be removed — otherwise the chain backs up and stops. Oxygen accepts these electrons (along with H) to form water. No other molecule in aerobic organisms can do this job as efficiently.
What happens if ATP synthase is blocked?
If ATP synthase is inhibited (e.g., by oligomycin), H cannot flow back into the matrix. The proton gradient builds up, and eventually the ETC cannot pump more protons against the steep gradient. Electron transport slows, NADH accumulates, the Krebs cycle stalls, and the cell runs out of ATP. This is lethal.
Respiration is the engine of life. Every cell does it, every day, to turn food into ATP. Understand the four stages and you understand why you breathe.