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Glycolysis occurs in the cytoplasm of all living cells and is the partial oxidation of glucose to 2 molecules of pyruvic acid. Example: Glucose (6C)-2 Pyruvate (3C).
In glycolysis, one molecule of glucose yields 2 ATP (net gain) and 2 NADH + H?. Verify from NCERT.
Glycolysis involves 10 enzymatic steps; the key regulatory enzyme is phosphofructokinase (PFK), which catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
The link reaction (pyruvate decarboxylation) occurs in the mitochondrial matrix; pyruvate is converted to acetyl CoA by pyruvate dehydrogenase complex, releasing one CO? per pyruvate.
One molecule of pyruvate produces 1 NADH + H? and 1 acetyl CoA during the link reaction. Thus, per glucose: 2 NADH + 2 CO?.
Krebs cycle (TCA cycle) starts when acetyl CoA (2C) combines with oxaloacetate (4C) to form citric acid (6C), catalyzed by citrate synthase.
One turn of Krebs cycle produces: 3 NADH, 1 FADH?, 1 ATP (or GTP), and 2 CO?. Per glucose (2 acetyl CoA), double these values: 6 NADH, 2 FADH?, 2 ATP, 4 CO?.
Enzyme aconitase catalyzes the isomerization of citrate to isocitrate via cis-aconitate in the Krebs cycle.
Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to ?-ketoglutarate, producing the first NADH and one CO? per turn.
?-Ketoglutarate dehydrogenase complex converts ?-ketoglutarate to succinyl CoA, releasing one CO? and generating one NADH per turn.
Succinyl CoA synthetase produces one GTP (equivalent to ATP) during the conversion of succinyl CoA to succinate.
FAD acts as electron acceptor in the Krebs cycle during the oxidation of succinate to fumarate, forming FADH?; enzyme: succinate dehydrogenase (bound to inner mitochondrial membrane).
Oxidative phosphorylation occurs in the inner mitochondrial membrane and couples electron transport with ATP synthesis via chemiosmosis.
The electron transport chain (ETC) consists of four complexes: Complex I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome bc? complex), IV (cytochrome c oxidase).
Electrons from NADH enter at Complex I, while electrons from FADH? enter at Complex II; hence, NADH yields more ATP than FADH?.
Protons are pumped from the mitochondrial matrix to the intermembrane space by Complexes I, III, and IV, creating a proton gradient.
ATP synthase (Complex V) uses the proton motive force to synthesize ATP; for every 2.5 ATP per NADH and 1.5 ATP per FADH? (verify from NCERT).
Oxygen is the final electron acceptor in the ETC, combining with electrons and protons to form water at Complex IV.
Total theoretical ATP yield from one glucose molecule: 36 or 38 ATP in aerobic respiration (varies due to shuttle systems; verify from NCERT).
Anaerobic respiration in muscles produces lactic acid via lactate dehydrogenase; in yeast, pyruvate is converted to ethanol and CO? (alcoholic fermentation).
Intermediate — requires integration of multiple metabolic pathways and understanding of energy yields, but all concepts are directly from NCERT Class 11 Biology (Chapter 14: Respiration in Plants).
Trap: Assuming glycolysis produces CO?. Avoid: Glycolysis does not release CO?; decarboxylation begins only in the link reaction.
Trap: Believing FADH? produces same ATP as NADH. Avoid: FADH? enters ETC at Complex II, so it contributes to fewer proton pumps and yields ~1.5 ATP vs ~2.5 for NADH.
Trap: Thinking Krebs cycle occurs in cytoplasm. Avoid: Krebs cycle occurs exclusively in mitochondrial matrix in eukaryotes; in prokaryotes, it occurs in cytoplasm.
Q1. Where does glycolysis occur in plant cells? A. Mitochondrial matrix B. Inner mitochondrial membrane C. Cytoplasm D. Chloroplast stroma
Answer: C Explanation: Glycolysis occurs in the cytoplasm of all living cells, including plants. Why others fail: Mitochondrial matrix is for Krebs cycle, not glycolysis.
Q2. Which enzyme is responsible for the conversion of pyruvate to acetyl CoA? A. Hexokinase B. Pyruvate kinase C. Pyruvate dehydrogenase D. Phosphofructokinase
Answer: C Explanation: Pyruvate dehydrogenase complex catalyzes oxidative decarboxylation of pyruvate to acetyl CoA. Why others fail: Pyruvate kinase catalyzes the last step of glycolysis (PEP to pyruvate).
Q3. How many molecules of NADH are produced per glucose molecule during the Krebs cycle? A. 2 B. 4 C. 6 D. 8
Answer: C Explanation: Each acetyl CoA produces 3 NADH in one turn; two turns per glucose-6 NADH. Why others fail: Some students forget that two acetyl CoA molecules enter the cycle per glucose.
Q4. In oxidative phosphorylation, the proton gradient across the inner mitochondrial membrane is primarily generated by: A. ATP synthase B. Diffusion of H? into matrix C. Electron transport chain complexes D. Active transport of ATP
Answer: C Explanation: Complexes I, III, and IV pump protons into intermembrane space using energy from electron flow. Why others fail: ATP synthase uses the gradient, not creates it.
Q5. What is the net ATP yield from one molecule of glucose during glycolysis? A. 1 ATP B. 2 ATP C. 4 ATP D. 6 ATP
Answer: B Explanation: Glycolysis consumes 2 ATP and produces 4 ATP-net gain of 2 ATP. Why others fail: Students often select 4 ATP, forgetting the initial investment phase.
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