Respiration in Plants (NEET Biology Class 11): Glycolysis, Krebs Cycle, ETS & the 38-ATP Balance Sheet
Glycolysis (EMP Pathway) & Fermentation
📌 NEET priority: Important. 2–3 questions a year, but almost all of them are exact numbers — the 38-ATP balance sheet, RQ values, and which ETS complex does what. These are free marks if you memorise them precisely.
ATP is the energy currency of the cell. Respiration breaks down organic substrates to release that energy in controlled steps, trapping it as ATP.
Glycolysis (EMP pathway) — in the cytoplasm
The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas — hence the EMP pathway. It occurs in the cytoplasm of all living organisms, and in anaerobic organisms it is the only respiratory process.
Glucose undergoes partial oxidation through a chain of ten enzyme-controlled reactions into two molecules of pyruvic acid. In plants, sucrose is first split by invertase into glucose and fructose.
ATP is USED at two steps: glucose → glucose-6-phosphate (enzyme hexokinase), and fructose-6-phosphate → fructose-1,6-bisphosphate. ATP is MADE at two steps: BPGA → PGA, and PEP → pyruvic acid. Since each happens twice (two 3-carbon molecules), that's 4 ATP made − 2 ATP used = net 2 ATP.
NADH + H⁺ is formed at exactly one step: when PGAL (3-phosphoglyceraldehyde) is converted to BPGA (1,3-bisphosphoglycerate).
Fermentation — the anaerobic fate of pyruvate
Type | Enzymes involved | End products | Where it happens |
Alcoholic fermentation | Pyruvic acid decarboxylase + alcohol dehydrogenase | Ethanol + CO₂ | Yeast, some microbes |
Lactic acid fermentation | Lactate dehydrogenase | Lactic acid (no CO₂) | Some bacteria; animal muscle when O₂ is inadequate |
Fermentation releases less than 7% of glucose's energy, with a net gain of only 2 ATP. It is also hazardous — the cell poisons itself. Yeast dies once alcohol reaches about 13%, which is why naturally fermented beverages cap out around there.
Aerobic Respiration: Krebs Cycle & the ETS
Aerobic respiration — link reaction and the Krebs cycle
Pyruvate enters the mitochondrial matrix and undergoes oxidative decarboxylation by pyruvate dehydrogenase (needs NAD⁺, Coenzyme A and Mg²⁺): pyruvic acid + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH + H⁺. From one glucose (2 pyruvate), this yields 2 NADH.
The TCA / citric acid / Krebs cycle — elucidated by Hans Krebs — begins when acetyl CoA (2C) condenses with oxaloacetic acid (OAA, 4C) and water to form citric acid (6C), catalysed by citrate synthase, releasing CoA.
Cycle sequence: citrate → isocitrate → (decarboxylation) α-ketoglutaric acid (5C) → (decarboxylation) succinyl-CoA (4C) → succinic acid → malic acid → back to OAA.
Per turn of the Krebs cycle: 3 NADH (three points where NAD⁺ is reduced), 1 FADH₂ (one point), 1 GTP — made during succinyl-CoA → succinic acid, a substrate-level phosphorylation, which is then converted to ATP. Also 2 CO₂ released.
Trap: continued operation of the cycle requires replenishment of OAA (the first member) and regeneration of NAD⁺ and FAD⁺ from NADH and FADH₂. Without those, the cycle halts.
Electron Transport System (ETS) — on the inner mitochondrial membrane
Complex | Name | Role |
Complex I | NADH dehydrogenase | Oxidises matrix NADH; passes electrons to ubiquinone |
Complex II | Succinate dehydrogenase | Feeds FADH₂ electrons (from succinate oxidation) to ubiquinone |
Complex III | Cytochrome bc₁ complex | Oxidises reduced ubiquinone (ubiquinol); passes electrons to cytochrome c |
Cytochrome c | (mobile carrier, not a complex) | Small protein on the OUTER surface of the inner membrane; shuttles electrons from complex III to IV |
Complex IV | Cytochrome c oxidase | Contains cytochromes a and a₃ plus two copper centres; passes electrons to O₂ |
Complex V | ATP synthase | F₁ headpiece (peripheral, makes ATP) + F₀ (integral, proton channel). 4H⁺ pass through F₀ per ATP |
Oxygen acts as the final hydrogen acceptor, forming water. Its role is limited to the terminal stage, but it is vital because it drives the whole system by removing hydrogen. Since the proton gradient here comes from oxidation-reduction energy (not light, as in photosynthesis), the process is called oxidative phosphorylation.
The Respiratory Balance Sheet — 38 ATP
The respiratory balance sheet — 38 ATP per glucose
Oxidation of one NADH gives 3 ATP; one FADH₂ gives 2 ATP. Working it through:
Stage | Location | ATP (direct) | NADH | FADH₂ | ATP after ETS |
Glycolysis | Cytoplasm | 2 (net) | 2 | — | 2 + (2 × 3) = 8 |
Link reaction (pyruvate → acetyl CoA) | Matrix | — | 2 | — | 2 × 3 = 6 |
Krebs cycle (×2 turns) | Matrix | 2 (from GTP) | 6 | 2 | 2 + (6 × 3) + (2 × 2) = 24 |
TOTAL per glucose | — | 4 | 10 | 2 | 38 ATP |
Important honesty from NCERT: this 38 figure is a theoretical exercise, valid only under four assumptions — (1) pathways run sequentially, one after another; (2) glycolytic NADH is transferred into the mitochondria; (3) no intermediates are withdrawn to make other compounds; (4) only glucose is respired. In a real cell, none of these hold strictly — pathways run simultaneously, substrates enter and leave as needed, and ATP is consumed as it's made.
Fermentation vs aerobic respiration
Fermentation is only a partial breakdown of glucose; aerobic respiration degrades it completely to CO₂ and H₂O.
Fermentation nets only 2 ATP per glucose; aerobic respiration nets 38.
NADH is oxidised back to NAD⁺ slowly in fermentation but vigorously in aerobic respiration.
Amphibolic Pathway, RQ & Why This Matters for NEET
The amphibolic pathway
Glucose is the favoured substrate, but others enter at different points: fats → broken to glycerol + fatty acids; fatty acids → degraded to acetyl CoA; glycerol → enters as PGAL; proteins → degraded by proteases, and amino acids (after deamination) enter as pyruvate, acetyl CoA, or at some point in the Krebs cycle.
Crucially, those same intermediates are withdrawn when the organism needs to synthesise fatty acids or proteins. Because the respiratory pathway serves both catabolism (breakdown) and anabolism (synthesis), it is properly called an amphibolic pathway — not merely a catabolic one.
Respiratory Quotient (RQ)
RQ = volume of CO₂ evolved ÷ volume of O₂ consumed, and its value tells you which substrate is being respired:
Substrate | RQ value | Why |
Carbohydrates | 1.0 | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O — equal volumes of CO₂ out and O₂ in |
Fats | Less than 1 (e.g. tripalmitin = 0.7) | Fats are less oxidised, so more O₂ is needed relative to CO₂ produced |
Proteins | About 0.9 | Intermediate between carbohydrates and fats |
Why this matters for NEET
High-value one-liners: glycolysis = EMP pathway, in the cytoplasm, 10 reactions, net 2 ATP; hexokinase phosphorylates glucose; Krebs cycle per turn = 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂; NADH = 3 ATP, FADH₂ = 2 ATP; total 38 ATP; 4H⁺ per ATP through F₀; RQ of carbohydrate = 1, fat = 0.7.
Trap: alcoholic fermentation releases CO₂, but lactic acid fermentation does NOT — a favourite distinguishing question. Also, in photosynthesis protons accumulate in the thylakoid lumen, but in respiration they accumulate in the intermembrane space — and cytochrome c sits on the outer surface of the inner membrane, not the outer membrane itself.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. Glycolysis is also known as the EMP pathway, named after:
(a) Embden, Mitchell and Palade
(b) Emerson, Meyerhof and Priestley
(c) Engelmann, Meyerhof and Parnas
(d) Embden, Meyerhof and Parnas
Show answer
Answer: (d) — The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J. Parnas — hence EMP pathway.
Q2. Glycolysis occurs in the:
(a) Chloroplast stroma
(b) Inner mitochondrial membrane
(c) Mitochondrial matrix
(d) Cytoplasm
Show answer
Answer: (d) — Glycolysis takes place in the cytoplasm and is present in all living organisms.
Q3. The net gain of ATP from glycolysis of one glucose molecule is:
(a) 2 ATP
(b) 4 ATP
(c) 8 ATP
(d) 38 ATP
Show answer
Answer: (a) — 4 ATP are synthesised and 2 are consumed, giving a net gain of 2 ATP.
Q4. The enzyme that phosphorylates glucose to glucose-6-phosphate is:
(a) Lactate dehydrogenase
(b) Citrate synthase
(c) Hexokinase
(d) Invertase
Show answer
Answer: (c) — Hexokinase catalyses the phosphorylation of glucose (and fructose) to glucose-6-phosphate.
Q5. Which of the following is produced in alcoholic fermentation but NOT in lactic acid fermentation?
(a) Pyruvic acid
(b) ATP
(c) Carbon dioxide
(d) NAD+
Show answer
Answer: (c) — Alcoholic fermentation yields ethanol AND CO2; lactic acid fermentation yields only lactic acid, with no CO2.
Q6. During fermentation, the percentage of glucose's energy released is:
(a) 100 per cent
(b) About 40 per cent
(c) About 70 per cent
(d) Less than 7 per cent
Show answer
Answer: (d) — Fermentation releases less than seven per cent of the energy in glucose, and not all of it is trapped as ATP.
Q7. Oxidative decarboxylation of pyruvate to acetyl CoA is catalysed by:
(a) Pyruvate dehydrogenase
(b) Alcohol dehydrogenase
(c) Citrate synthase
(d) Hexokinase
Show answer
Answer: (a) — Pyruvate dehydrogenase, requiring NAD+, Coenzyme A and Mg2+, converts pyruvate to acetyl CoA.
Q8. The Krebs cycle begins with the condensation of acetyl CoA with:
(a) Citric acid
(b) Succinic acid
(c) Malic acid
(d) Oxaloacetic acid
Show answer
Answer: (d) — Acetyl CoA (2C) condenses with oxaloacetic acid (4C) and water to form citric acid (6C), catalysed by citrate synthase.
Q9. In one turn of the Krebs cycle, the number of NADH molecules produced is:
(a) 1
(b) 2
(c) 3
(d) 4
Show answer
Answer: (c) — There are three points in the cycle where NAD+ is reduced to NADH, plus one point where FAD is reduced to FADH2.
Q10. GTP is synthesised during the conversion of:
(a) Citrate to isocitrate
(b) Succinyl-CoA to succinic acid
(c) Acetyl CoA to citrate
(d) Malic acid to OAA
Show answer
Answer: (b) — The succinyl-CoA to succinic acid step produces GTP by substrate-level phosphorylation, later converted to ATP.
Q11. Oxidation of one molecule of FADH2 through the ETS yields:
(a) 4 ATP
(b) 2 ATP
(c) 3 ATP
(d) 1 ATP
Show answer
Answer: (b) — NADH oxidation gives 3 ATP; FADH2 oxidation gives 2 ATP.
Q12. Complex IV of the electron transport system is:
(a) Cytochrome c oxidase
(b) NADH dehydrogenase
(c) Cytochrome bc1 complex
(d) Succinate dehydrogenase
Show answer
Answer: (a) — Complex IV is the cytochrome c oxidase complex, containing cytochromes a and a3 and two copper centres.
Q13. For each ATP produced by ATP synthase in mitochondria, the number of protons passing through F0 is:
(a) 3
(b) 2
(c) 4
(d) 6
Show answer
Answer: (c) — Four H+ pass through the F0 channel from the intermembrane space to the matrix for each ATP produced.
Q14. The total net gain of ATP from complete aerobic oxidation of one glucose molecule is:
(a) 38
(b) 18
(c) 36
(d) 2
Show answer
Answer: (a) — The theoretical net gain is 38 ATP per glucose under NCERT's stated assumptions.
Q15. The respiratory pathway is best described as:
(a) Purely catabolic
(b) An amphibolic pathway
(c) A photosynthetic pathway
(d) Purely anabolic
Show answer
Answer: (b) — Because respiratory intermediates are both broken down and withdrawn for synthesis, the pathway is amphibolic.
Q16. The respiratory quotient (RQ) when carbohydrates are completely oxidised is:
(a) 1.0
(b) 0.9
(c) 0.7
(d) Greater than 1
Show answer
Answer: (a) — Carbohydrate oxidation consumes and evolves equal volumes of O2 and CO2, giving RQ = 1.0.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q17. The RQ value for fats such as tripalmitin is approximately: (NEET PYQ)
(a) 0.9
(b) 0.7
(c) 1.0
(d) 1.5
Show answer
Answer: (b) — For tripalmitin, 102 CO2 evolved / 145 O2 consumed = about 0.7, i.e. less than 1.
Q18. In glycolysis, NADH + H+ is formed during the conversion of: (NEET PYQ)
(a) PGAL to BPGA
(b) PEP to pyruvate
(c) Fructose-6-phosphate to fructose-1,6-bisphosphate
(d) Glucose to glucose-6-phosphate
Show answer
Answer: (a) — The single NADH-forming step in glycolysis is the oxidation of PGAL to 1,3-bisphosphoglycerate (BPGA).
Q19. Cytochrome c in the electron transport system acts as: (NEET PYQ)
(a) A mobile carrier between complexes III and IV
(b) A fixed part of complex I
(c) The final electron acceptor
(d) An enzyme of the Krebs cycle
Show answer
Answer: (a) — Cytochrome c is a small mobile protein on the outer surface of the inner membrane, shuttling electrons from complex III to complex IV.
Q20. In aerobic respiration, the final acceptor of hydrogen is: (NEET PYQ)
(a) NAD+
(b) FAD
(c) Oxygen
(d) Cytochrome c
Show answer
Answer: (c) — Oxygen acts as the final hydrogen acceptor, forming water — its role is confined to the terminal stage but is vital.
Q21. Fatty acids, when respired, enter the respiratory pathway as: (NEET PYQ)
(a) Citric acid
(b) Acetyl CoA
(c) PGAL
(d) Pyruvate
Show answer
Answer: (b) — Fatty acids are first degraded to acetyl CoA before entering the pathway; glycerol enters as PGAL.
Q22. Yeast cells poison themselves to death when alcohol concentration reaches about: (NEET PYQ)
(a) 25 per cent
(b) 5 per cent
(c) 50 per cent
(d) 13 per cent
Show answer
Answer: (d) — Yeasts die once alcohol reaches roughly 13 per cent, capping naturally fermented beverages at that level.
Q23. In the Krebs cycle, the 5-carbon compound formed after the first decarboxylation is: (NEET PYQ)
(a) Malic acid
(b) Oxaloacetic acid
(c) Succinyl-CoA
(d) Alpha-ketoglutaric acid
Show answer
Answer: (d) — Isocitrate is decarboxylated to alpha-ketoglutaric acid (5C), which is then decarboxylated to succinyl-CoA (4C).
Active Recall Prompt
Write everything you can recall about Respiration in Plants, naming each part first: glycolysis (EMP pathway, location, the ATP-using and ATP-making steps, where NADH forms, net yield); fermentation (alcoholic vs lactic acid, the enzymes, the products, energy released); the link reaction (pyruvate to acetyl CoA); the Krebs cycle (the intermediates in order, and the NADH/FADH₂/GTP produced per turn); the electron transport system (the five complexes, cytochrome c, the role of oxygen); the respiratory balance sheet (ATP per NADH and FADH₂, and the total per glucose); the amphibolic pathway (how fats and proteins enter); and the respiratory quotient for each substrate. Begin each fact with its topic and end it with a full stop.