Krebs Cycle (TCA Cycle)

• How pyruvate from glycolysis is converted to acetyl CoA (pyruvate oxidation).
• The 8 steps of the Krebs/TCA cycle in the mitochondrial matrix.
• Exact yield per acetyl CoA and per glucose: NADH, FADH₂, GTP, and CO₂.
• Key regulatory enzymes of the Krebs cycle and what controls them.
• Why the Krebs cycle is called an "amphibolic" pathway.

Level 1 — The Link Reaction and the Cycle

Pyruvate (3C) from glycolysis enters the mitochondrial matrix and is first converted to acetyl CoA (2C) in a reaction called pyruvate oxidation (or the link/transition reaction). Then acetyl CoA enters the Krebs cycle (also called the tricarboxylic acid cycle, TCA cycle, or citric acid cycle — named after Hans Krebs who described it in 1937).

The cycle is called "citric acid cycle" because the first product formed is citric acid (6C), a tricarboxylic acid. The cycle turns twice per glucose (once for each pyruvate/acetyl CoA). At each turn, 2 CO₂ are released, and the cycle regenerates oxaloacetate — which picks up the next acetyl CoA.

Main outputs per turn: 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂.

Level 2 — Pyruvate Oxidation, 8 TCA Steps, and Stoichiometry

Pyruvate Oxidation (Link Reaction — in mitochondrial matrix):

Pyruvate + CoA + NAD⁺ → Acetyl CoA (2C) + CO₂ + NADH

Enzyme: Pyruvate dehydrogenase complex (PDC — a multi-enzyme complex requiring 5 cofactors: TPP, lipoic acid, FAD, CoA, NAD⁺). Per glucose: 2 pyruvate → 2 acetyl CoA + 2 CO₂ + 2 NADH.

Krebs Cycle — 8 Steps (per acetyl CoA):

Yield per acetyl CoA (one turn): 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂

Total from Krebs cycle per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 GTP + 4 CO₂

Combined from pyruvate oxidation + Krebs (per glucose):

Note: These are in addition to the 2 NADH + 2 ATP from glycolysis.

Regulation:

• Isocitrate dehydrogenase (step 3): rate-limiting; inhibited by ATP and NADH (energy surplus); activated by ADP.
• α-ketoglutarate dehydrogenase (step 4): inhibited by succinyl CoA and NADH.
• Citrate synthase (step 1): inhibited by citrate (product inhibition), ATP, and NADH.

Amphibolic nature of Krebs cycle:

• Catabolic: breaks down acetyl CoA, releases CO₂, produces NADH/FADH₂.
• Anabolic: intermediates are withdrawn for biosynthesis (e.g., α-ketoglutarate → glutamate amino acids; oxaloacetate → aspartate; succinyl CoA → haem synthesis; citrate → fatty acid synthesis).

NEET-style Question:
Per molecule of glucose, how many CO2 molecules are released during pyruvate oxidation
and the Krebs cycle combined?

Step 1 — Pyruvate oxidation (link reaction)
  Glucose → 2 pyruvate (glycolysis — no CO2 released here).
  Each pyruvate loses 1 CO2 when converted to acetyl CoA.
  For 2 pyruvate: 2 CO2 released.

Step 2 — Krebs cycle (per turn, per acetyl CoA)
  Step 3: isocitrate → alpha-ketoglutarate: 1 CO2 released.
  Step 4: alpha-ketoglutarate → succinyl CoA: 1 CO2 released.
  Total per turn: 2 CO2.
  For 2 turns (per glucose): 4 CO2.

Step 3 — Add together
  Pyruvate oxidation: 2 CO2
  Krebs cycle: 4 CO2
  Total: 6 CO2

Step 4 — Verify with glucose formula
  C6H12O6 → 6 CO2 in complete aerobic respiration. ✓
  (The remaining CO2 and H2O accounting is completed via the ETC — O2 is consumed.)

Answer: 6 CO2 molecules (2 from pyruvate oxidation + 4 from the Krebs cycle)

• Write the equation for pyruvate oxidation, naming the enzyme complex involved.
• List the 8 intermediates of the Krebs cycle in order (oxaloacetate → citrate → isocitrate → α-ketoglutarate → succinyl CoA → succinate → fumarate → malate → oxaloacetate).
• State the net yield from one turn of the Krebs cycle (NADH, FADH₂, GTP, CO₂).
• Calculate the total NADH produced per glucose from pyruvate oxidation + Krebs cycle combined.
• At which step of the Krebs cycle does substrate-level phosphorylation occur? Name the enzyme.
• Why is succinate dehydrogenase considered part of the electron transport chain?
• Explain the amphibolic nature of the Krebs cycle with two examples of anabolic withdrawal.

• Figure 14.4 — Krebs cycle diagram showing all 8 steps, all intermediates, enzymes, and co-factor changes (NADH, FADH₂, GTP, CO₂) (NCERT Class 11, Chapter 14).
• Figure 14.3 — Pyruvate oxidation (link reaction) showing conversion of pyruvate → acetyl CoA with CO₂ and NADH production.
• Summary table/diagram of aerobic respiration showing glycolysis (cytoplasm), link reaction (mitochondrial matrix), Krebs cycle (mitochondrial matrix), and ETC (inner mitochondrial membrane).

• Name the enzyme that converts pyruvate to acetyl CoA.
• What is the first product formed when acetyl CoA enters the Krebs cycle?
• How many NADH molecules are produced per turn of the Krebs cycle?
• At which step of the Krebs cycle is FADH₂ produced? (Answer: Step 6 — succinate → fumarate, by succinate dehydrogenase)
• True or False: The Krebs cycle releases 6 CO₂ per glucose directly. (Answer: False — 4 CO₂ from Krebs, 2 from pyruvate oxidation)
• Name the rate-limiting enzyme of the Krebs cycle and state one molecule that inhibits it.
• Stretch: The Krebs cycle is described as "amphibolic." A student argues that if all the oxaloacetate is withdrawn for amino acid synthesis, the cycle will stop. Evaluate this claim and suggest how the cell might replenish oxaloacetate.

• Use the platform-native live simulation or PhET-style tool for this topic (number line, Venn, physics playground, molecule builder, sensor dashboard, etc.).
• Mirror / body / home activity: physically do the concept (count objects, measure, role-play) and photograph or describe for portfolio.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.