Respiration in Plants - Glycolysis, Krebs Cycle & ETS | Complete NEET Notes

Respiration in Plants is a high-weightage chapter from Class 11 NEET Biology, contributing 3-5 questions every year. It is conceptually dense with biochemical pathways, enzyme names, and ATP calculations that are frequently tested. A solid understanding of glycolysis, Krebs cycle, and the electron transport system is essential for scoring well.

This guide breaks down every pathway step-by-step with simplified tables, complete ATP accounting, and mnemonics aligned with NCERT.

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Overview of Cellular Respiration

Cellular respiration is the process by which organisms break down organic molecules (primarily glucose) to release energy in the form of ATP.

Overall Equation:

C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy (36-38 ATP)

Aerobic vs Anaerobic Respiration

The four stages of aerobic respiration:

1. Glycolysis (cytoplasm)
2. Pyruvate oxidation / Link reaction (mitochondrial matrix)
3. Krebs cycle / TCA cycle (mitochondrial matrix)
4. Electron Transport System + Oxidative phosphorylation (inner mitochondrial membrane)

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Glycolysis (Embden-Meyerhof-Parnas Pathway)

Glycolysis occurs in the cytoplasm of all living cells. It is common to both aerobic and anaerobic respiration. One molecule of glucose (6C) is broken down into two molecules of pyruvate (3C).

The 10 Steps of Glycolysis (Simplified)

Glycolysis Summary

NEET Tip: Phosphofructokinase (PFK) is the rate-limiting enzyme of glycolysis. It is allosterically inhibited by ATP and citrate, and activated by AMP and ADP.

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Fermentation (Anaerobic Pathway)

When oxygen is absent, pyruvate does not enter mitochondria. Instead, it undergoes fermentation in the cytoplasm to regenerate NAD+ so that glycolysis can continue.

Alcoholic vs Lactic Acid Fermentation

NEET Tip: Fermentation is incomplete oxidation of glucose. Only 2 ATP are produced because NADH is not oxidised through ETS.

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Pyruvate Oxidation (Link Reaction)

In the presence of oxygen, pyruvate enters the mitochondrial matrix through specific transporters and undergoes oxidative decarboxylation.

Reaction:

Pyruvate (3C) + CoA + NAD+ --> Acetyl CoA (2C) + CO2 + NADH

Enzyme: Pyruvate dehydrogenase complex (multi-enzyme complex requiring TPP, lipoic acid, FAD, NAD+, CoA as cofactors)

Per glucose molecule (2 pyruvate):

• 2 NADH produced
• 2 CO2 released
• 0 ATP directly

NEET Tip: This is an irreversible step and the gateway to the Krebs cycle. The pyruvate dehydrogenase complex links glycolysis to the Krebs cycle.

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Krebs Cycle (Tricarboxylic Acid Cycle / Citric Acid Cycle)

The Krebs cycle occurs in the mitochondrial matrix. Acetyl CoA (2C) enters the cycle by combining with oxaloacetate (4C) to form citrate (6C). Through a series of 8 reactions, the two carbons are released as CO2 and oxaloacetate is regenerated.

The 8 Steps of Krebs Cycle

Krebs Cycle Summary (Per Acetyl CoA)

NEET Tip: Succinate dehydrogenase (step 6) is the only Krebs cycle enzyme located in the inner mitochondrial membrane, not in the matrix. It is also Complex II of ETS.

Krebs Cycle Mnemonic

"Can I Keep Selling Sex For Money, Officer?"

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Electron Transport System (ETS) and Oxidative Phosphorylation

The ETS is located on the inner mitochondrial membrane. NADH and FADH2 produced in earlier stages donate their electrons to the chain, and the energy released is used to pump protons (H+) across the membrane, creating a gradient that drives ATP synthesis.

Components of ETS

Chemiosmosis (Peter Mitchell's Hypothesis)

1. NADH and FADH2 donate electrons to ETS
2. As electrons pass through complexes I, III, and IV, protons are pumped from matrix to inter-membrane space
3. This creates a proton motive force (electrochemical gradient)
4. Protons flow back through ATP synthase (F0-F1 complex) into the matrix
5. The energy of proton flow drives the synthesis of ATP from ADP + Pi

NEET Tip: FADH2 enters at Complex II, bypassing Complex I. Therefore, FADH2 produces only 2 ATP (fewer protons pumped) compared to 3 ATP from NADH. Some textbooks use the updated values of 2.5 ATP per NADH and 1.5 ATP per FADH2.

Key Points About ETS

• Oxygen is the final electron acceptor - it combines with electrons and H+ to form water
• Cyanide inhibits Complex IV (cytochrome c oxidase)
• Rotenone inhibits Complex I
• Antimycin A inhibits Complex III
• Oligomycin inhibits ATP synthase

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Complete ATP Accounting

ATP Yield Per Glucose Molecule (Aerobic Respiration)

Grand Total: 4 + 30 + 4 = 38 ATP (theoretical maximum)

Actual yield: 36 ATP (because 2 NADH from glycolysis require 1 ATP each to shuttle into mitochondria via the glycerol phosphate shuttle)

Updated values (some textbooks): 30-32 ATP using 2.5 ATP/NADH and 1.5 ATP/FADH2

Shuttle Systems for Cytoplasmic NADH

NEET Tip: The net ATP yield is 36 or 38 depending on which shuttle system is used. NEET typically accepts 36 ATP as the standard answer.

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Respiratory Quotient (RQ)

The Respiratory Quotient is the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration.

RQ = CO2 evolved / O2 consumed

NEET Tip: RQ greater than 1 indicates that the substrate is an organic acid. RQ of infinity indicates anaerobic respiration (no O2 consumed, but CO2 is still released in alcoholic fermentation).

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Amphibolic Pathway

The Krebs cycle is an amphibolic pathway because it functions in both catabolism (breakdown) and anabolism (synthesis).

• Catabolic role: Oxidises acetyl CoA to CO2, generating energy (NADH, FADH2, ATP)
• Anabolic role: Intermediates serve as precursors for biosynthesis
  • Oxaloacetate --> amino acids (aspartate)
  • alpha-Ketoglutarate --> amino acids (glutamate)
  • Succinyl CoA --> haem synthesis
  • Citrate --> fatty acid synthesis

NEET Tip: Since Krebs cycle intermediates are drawn off for biosynthesis, they must be replenished through anaplerotic reactions (e.g., pyruvate carboxylase converts pyruvate to oxaloacetate).

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Previous Year NEET Questions (PYQ Analysis)

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Practice MCQs

Q1. The net gain of ATP during glycolysis is:

• (a) 4 ATP
• (b) 8 ATP
• (c) 2 ATP ✓
• (d) 6 ATP

Glycolysis produces 4 ATP but consumes 2 ATP in the preparatory phase (hexokinase and PFK steps), giving a net gain of 2 ATP.

Q2. Krebs cycle takes place in:

• (a) Cytoplasm
• (b) Inner mitochondrial membrane
• (c) Mitochondrial matrix ✓
• (d) Outer mitochondrial membrane

All Krebs cycle enzymes are present in the mitochondrial matrix, except succinate dehydrogenase which is on the inner mitochondrial membrane.

Q3. The final electron acceptor in the electron transport chain is:

• (a) NAD+
• (b) FAD
• (c) Cytochrome c
• (d) Molecular oxygen ✓

Oxygen accepts electrons at Complex IV and combines with H+ ions to form water. Without oxygen, ETS stops completely.

Q4. The RQ value for fats is:

• (a) 1.0
• (b) Greater than 1
• (c) Less than 1 ✓
• (d) Zero

Fats have more hydrogen per molecule and require more O2 for complete oxidation than the CO2 they release, giving RQ less than 1 (approximately 0.7).

Q5. Which of the following is the rate-limiting enzyme of glycolysis?

• (a) Hexokinase
• (b) Phosphofructokinase ✓
• (c) Pyruvate kinase
• (d) Enolase

PFK catalyses the third step of glycolysis and is the main regulatory enzyme. It is inhibited by ATP and citrate, and activated by AMP.

Q6. During the conversion of succinyl CoA to succinate in the Krebs cycle:

• (a) NADH is produced
• (b) FADH2 is produced
• (c) GTP is produced by substrate-level phosphorylation ✓
• (d) CO2 is released

This is the only step in the Krebs cycle that produces ATP (as GTP) by substrate-level phosphorylation.

Q7. How many NADH molecules are produced per glucose molecule in the Krebs cycle?

• (a) 2
• (b) 4
• (c) 6 ✓
• (d) 8

Each turn of the Krebs cycle produces 3 NADH. Since two acetyl CoA molecules enter per glucose, total NADH from Krebs = 3 x 2 = 6.

Q8. Cyanide is a poison that inhibits:

• (a) Complex I of ETS
• (b) Complex II of ETS
• (c) Complex III of ETS
• (d) Complex IV of ETS ✓

Cyanide binds to cytochrome c oxidase (Complex IV), preventing the transfer of electrons to oxygen. This stops ATP production and is lethal.

Q9. The Krebs cycle is called an amphibolic pathway because:

• (a) It occurs in two locations
• (b) It involves both catabolism and anabolism ✓
• (c) It requires both aerobic and anaerobic conditions
• (d) It uses two different substrates

Amphibolic means it serves both catabolic (energy release) and anabolic (biosynthetic) functions. Krebs cycle intermediates are precursors for amino acids, fats, and haem.

Q10. In alcoholic fermentation, pyruvate is first converted to:

• (a) Lactic acid
• (b) Acetaldehyde ✓
• (c) Ethanol directly
• (d) Acetic acid

Pyruvate is first decarboxylated to acetaldehyde (by pyruvate decarboxylase), which is then reduced to ethanol (by alcohol dehydrogenase) using NADH.

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Memory Tricks and Mnemonics

Krebs Cycle intermediates - "Can I Keep Selling Sex For Money, Officer?" Citrate, Isocitrate, alpha-Ketoglutarate, Succinyl CoA, Succinate, Fumarate, Malate, Oxaloacetate

Glycolysis steps (energy investment vs payoff):

• Steps 1-5 = "Investment phase" (2 ATP spent)
• Steps 6-10 = "Payoff phase" (4 ATP + 2 NADH earned)

ETS inhibitors - "RACO":

• Rotenone --> Complex I
• Antimycin A --> Complex III
• Cyanide / CO --> Complex IV
• Oligomycin --> ATP synthase

RQ values - "Carbs are Fair, Fats are Lean":

• Carbohydrates: RQ = 1 (fair/equal exchange of CO2 and O2)
• Fats: RQ less than 1 (lean on CO2, heavy on O2)

ATP yield - "2, 2, 2 rule":

• Glycolysis: 2 ATP + 2 NADH
• Link reaction: 2 NADH (no ATP)
• Krebs: 2 ATP + 6 NADH + 2 FADH2

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Key Takeaways for NEET

1. Glycolysis occurs in the cytoplasm and does not require oxygen; net yield is 2 ATP + 2 NADH
2. PFK is the rate-limiting enzyme of glycolysis
3. Fermentation regenerates NAD+ so glycolysis can continue without oxygen
4. Pyruvate dehydrogenase complex converts pyruvate to acetyl CoA (link reaction)
5. Krebs cycle produces 2 ATP + 6 NADH + 2 FADH2 per glucose (two turns)
6. Succinate dehydrogenase is the only membrane-bound Krebs cycle enzyme (also Complex II of ETS)
7. ETS transfers electrons through Complexes I-IV; oxygen is the final acceptor
8. Chemiosmosis uses the proton gradient to drive ATP synthesis through ATP synthase
9. Total ATP per glucose: 36-38 (aerobic), 2 (anaerobic)
10. RQ = CO2/O2; carbohydrates = 1, fats < 1, organic acids > 1

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FAQs

Q: Why is glycolysis considered a universal pathway? A: Glycolysis occurs in all living organisms (prokaryotes and eukaryotes) in the cytoplasm without requiring oxygen. It is considered the most ancient metabolic pathway, predating the evolution of mitochondria and aerobic respiration.

Q: Why does anaerobic respiration produce much less ATP than aerobic respiration? A: In anaerobic respiration, only glycolysis occurs (2 ATP). The NADH produced is used to reduce pyruvate during fermentation rather than entering the ETS. Without the Krebs cycle and ETS, the majority of potential energy in glucose remains trapped in the end products (ethanol or lactic acid).

Q: What happens if oxygen is suddenly removed during aerobic respiration? A: ETS stops immediately because there is no final electron acceptor. NADH and FADH2 accumulate and cannot be reoxidised. The Krebs cycle and link reaction slow down and eventually stop. The cell switches to fermentation to regenerate NAD+ and maintain glycolysis. This is called the Pasteur effect.

Q: Why is the Krebs cycle called a cycle? A: Oxaloacetate (4C), which is the starting molecule, is regenerated at the end of each turn. Acetyl CoA (2C) enters, two carbons leave as CO2, and the 4C oxaloacetate is reformed to accept the next acetyl CoA. The intermediates are catalytic, not consumed.

Q: How does the ATP synthase enzyme work? A: ATP synthase (F0-F1 complex) acts as a molecular turbine. The F0 component is a proton channel in the inner mitochondrial membrane. As protons flow down their concentration gradient through F0, it causes rotation of the F1 component in the matrix. This mechanical rotation provides the energy to catalyse the binding of ADP and Pi to form ATP. Approximately 3-4 protons are needed to synthesise one ATP molecule.