# Respiration in Plants (NEET Biology Class 11): Glycolysis, Krebs Cycle, ETS & the 38-ATP Balance Sheet Source: https://tracknation.in/story/respiration-in-plants-neet-biology-class-11-glycolysis-krebs-cycle-ets-the-38-atp-balance-sheet (TrackNation — free UPSC notes with PYQs) > NEET Class 11 Respiration in Plants, mined from NCERT: glycolysis (EMP pathway — its ten reactions, ATP-using and ATP-making steps, net yield), alcoholic vs lactic acid fermentation, aerobic respiration (oxidative decarboxylation, the full Krebs cycle with per-turn yields), the electron transport system complexes I–V with cytochrome c, the complete 38-ATP respiratory balance sheet (and NCERT's four assumptions behind it), the amphibolic pathway showing where fats and proteins enter, and respirat ## 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 **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 **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 **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 **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+ **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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.