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✅ 15-Marks Answer: Define Glycolysis — Biochemical Pathway, Breakdown of Glucose to Pyruvate & Lactate, Energetics

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📌 DEFINITION

🔑 Key Point: Glycolysis occurs in ALL living cells — aerobic and anaerobic — and does NOT require oxygen (cytoplasmic). It is the starting point of carbohydrate catabolism.

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📌 LOCATION

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📌 OVERALL EQUATION

Glucose + 2NAD⁺ + 2ADP + 2Pᵢ → 2 Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O

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📌 THE TWO PHASES — FLOWCHART

GLUCOSE (C₆)
    │
    ▼
╔══════════════════════════════╗
║   PHASE 1: PREPARATORY       ║  ← Invests 2 ATP (energy input)
║   (Energy Investment Phase)  ║  ← "Priming the Pump"
╚══════════════════════════════╝
    │
    ▼  Fructose-1,6-bisphosphate (split into 2 × triose phosphates)
    │
    ▼
╔══════════════════════════════╗
║   PHASE 2: PAY-OFF PHASE     ║  ← Generates 4 ATP + 2 NADH
║   (Energy Generation Phase)  ║
╚══════════════════════════════╝
    │
    ▼
2 × PYRUVATE (C₃)

Net yield = 4 ATP − 2 ATP = 2 ATP

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📌 10 STEPS OF GLYCOLYSIS (Complete Table)

⭐ 3 Key Irreversible (Regulated) Enzymes: Hexokinase, PFK-1, Pyruvate Kinase

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📌 FATE OF PYRUVATE — FLOWCHART

                    PYRUVATE
                      │
         ┌────────────┼────────────────┐
         ▼            ▼                ▼
    AEROBIC        ANAEROBIC     GLUCONEOGENESIS
  (O₂ present)   (No O₂ / RBC)   (Fasting/liver)
         │            │
         ▼            ▼
  Acetyl-CoA →    LACTATE
    TCA Cycle    (Lactate Dehydrogenase)
  (more ATP)    NAD⁺ regenerated
                (Keeps glycolysis running)

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📌 AEROBIC vs ANAEROBIC GLYCOLYSIS

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📌 ENERGY CALCULATIONS (Energetics)

🔢 ATP Balance Sheet

✅ Net ATP = +2 per glucose molecule (from substrate-level phosphorylation) ✅ 2 NADH also produced (= 5 ATP more if oxidized in ETC under aerobic conditions)

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📌 REGULATION OF GLYCOLYSIS

        HIGH AMP/ADP (low energy) → Activates PFK-1 ↑ → Speeds Glycolysis
        HIGH ATP (excess energy)  → Inhibits PFK-1 ↓  → Slows Glycolysis
        HIGH Citrate              → Inhibits PFK-1 ↓  → Feedback inhibition
        INSULIN                   → Activates PFK-1 ↑ → Post-meal glycolysis
        GLUCAGON/Epinephrine      → Inhibits (liver PK) → Reduces glycolysis

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📌 SIGNIFICANCE / IMPORTANCE (Impression Points ✨)

1. Universal pathway — present in ALL organisms (aerobic + anaerobic)
2. Provides ATP rapidly — especially during hypoxia (ischemia, exercise)
3. Feeds TCA cycle — via pyruvate → Acetyl-CoA (pyruvate dehydrogenase)
4. Amphibolic role — provides biosynthetic precursors (G3P → lipids; pyruvate → amino acids)
5. Maintains RBC survival — RBCs depend entirely on anaerobic glycolysis (have no mitochondria)
6. Warburg Effect — Cancer cells preferentially use aerobic glycolysis even in presence of O₂ (clinically significant — basis of PET scan using radiolabeled glucose)
7. Malate-Aspartate Shuttle — transfers cytoplasmic NADH into mitochondria for ATP production
8. Feeder pathway — fructose, galactose, mannose all enter glycolysis at different points

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📌 CLINICAL SIGNIFICANCE (Extra Marks 🌟)

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📌 COMPLETE SUMMARY CYCLE CHART

GLUCOSE
  ↓ Hexokinase (−ATP)
Glucose-6-P  ←→  (Pentose Phosphate Pathway, Glycogen Synthesis)
  ↓ PGI
Fructose-6-P
  ↓ PFK-1 (−ATP) ⭐ RATE LIMITING
Fructose-1,6-bisP
  ↓ Aldolase
DHAP ←→ G3P (×2 from here)
  ↓ G3P Dehydrogenase (→ NADH)
1,3-BPG
  ↓ PGK (+ATP)
3-PG
  ↓ Mutase
2-PG
  ↓ Enolase
PEP
  ↓ Pyruvate Kinase (+ATP) ⭐
PYRUVATE
  ├─ Aerobic → Acetyl-CoA → TCA → 36 ATP
  └─ Anaerobic → LACTATE (only 2 ATP net)

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📌 NET SUMMARY TABLE

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📝 Tips for Writing This in Exam:

• Start with a clear definition (2 lines)
• Draw the 2-phase flowchart (5 marks visual)
• Write the 10-step table with enzymes (4 marks)
• Show fate of pyruvate fork (2 marks)
• Add regulation table (2 marks)
• End with 3-4 clinical points = Extra impression marks 🌟

1. Describe biosynthesis of cholesterol + role of cholesterol in the body
2. What are ketone bodies? Explain formation & importance

• Very Short Answer (2 marks) — multiple questions
• Short Answer Type Questions (5 marks) — multiple questions
• Long Answer Type Questions [15/10 marks] — 2 questions

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1. ① Define Glycolysis. Describe the biochemical pathway for breakdown of glucose to pyruvate & lactate. Write about the energetics.
2. ② Explain the reaction sequence, significance & energetics of TCA Cycle.
3. ③ Define Gluconeogenesis & explain the reactions involved in it with its significance.

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✅ Q2 — TCA Cycle (Krebs Cycle / Citric Acid Cycle)

"Explain the reaction sequence, significance & energetics of TCA Cycle"

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📌 DEFINITION

🔑 Discovered by Sir Hans Krebs in 1937 — Nobel Prize 1953

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📌 LOCATION & ENTRY POINT

Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH

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📌 THE 8 REACTIONS OF TCA CYCLE

        ACETYL-CoA (2C)
              +
        OXALOACETATE (4C)
              ↓ ① Citrate Synthase
         CITRATE (6C)           ← IRREVERSIBLE
              ↓ ② Aconitase
         cis-ACONITATE (6C)
              ↓ Aconitase
         ISOCITRATE (6C)
              ↓ ③ Isocitrate Dehydrogenase → NADH + CO₂  ← IRREVERSIBLE ⭐
         α-KETOGLUTARATE (5C)
              ↓ ④ α-Ketoglutarate Dehydrogenase → NADH + CO₂ ← IRREVERSIBLE ⭐
         SUCCINYL-CoA (4C)
              ↓ ⑤ Succinyl-CoA Synthetase → GTP (= ATP)
         SUCCINATE (4C)
              ↓ ⑥ Succinate Dehydrogenase → FADH₂
         FUMARATE (4C)
              ↓ ⑦ Fumarase (+ H₂O)
         MALATE (4C)
              ↓ ⑧ Malate Dehydrogenase → NADH
         OXALOACETATE (4C)   ← Cycle Regenerated ♻️

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📌 STEP-BY-STEP TABLE

⭐ 3 Irreversible steps (Regulated enzymes): Citrate Synthase, Isocitrate DH, α-KG DH

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📌 ENERGETICS — ATP BALANCE (Per Acetyl-CoA)

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📌 REGULATION OF TCA CYCLE

  LOW Energy (↑ADP/NAD⁺) → ACTIVATE cycle ↑
  HIGH Energy (↑ATP/NADH) → INHIBIT cycle ↓

  Citrate Synthase     → Inhibited by: ATP, NADH, Succinyl-CoA, Citrate
  Isocitrate DH ⭐    → Activated by: ADP, Ca²⁺ | Inhibited by: ATP, NADH
  α-KG DH ⭐          → Activated by: Ca²⁺ | Inhibited by: Succinyl-CoA, NADH

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📌 SIGNIFICANCE (Amphibolic Nature) ✨

                     TCA CYCLE
                        │
        ┌───────────────┼───────────────┐
        ▼               ▼               ▼
   ENERGY (ATP)    BIOSYNTHESIS    ANAPLEROSIS
   (NADH/FADH₂)   Precursors       (Replenishing
                                    intermediates)
        ↓               ↓               
  Oxidative      α-KG → Glu        Pyruvate → OAA
  Phosphory-     OAA → Asp         (Pyruvate Carboxylase)
  lation         Succinyl-CoA      
                 → Heme synthesis
                 Citrate → Fatty Acids

1. Energy production — Main source of NADH/FADH₂ for ETC → ATP
2. Amphibolic — Both catabolic AND anabolic (feeds biosynthesis)
3. Amino acid synthesis — α-KG → Glutamate; OAA → Aspartate
4. Heme synthesis — Succinyl-CoA is precursor of porphyrin ring
5. Fatty acid synthesis — Citrate carries Acetyl-CoA out of mitochondria
6. Gluconeogenesis — OAA → PEP → Glucose

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📌 CLINICAL SIGNIFICANCE

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✅ Q3 — Gluconeogenesis

"Define Gluconeogenesis & explain the reactions involved with its significance"

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📌 DEFINITION

🔑 "Making new glucose when dietary glucose is unavailable"

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📌 KEY FEATURES

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📌 PRECURSORS FLOWCHART

LACTATE ─────────────────────────────────┐
(from RBCs, muscle)                       │
                                          ▼
AMINO ACIDS (Alanine, Glutamine) ──► PYRUVATE / OAA ──► GLUCOSE
(from muscle protein)                     ▲
                                          │
GLYCEROL ─────────────────────────────────┘
(from fat breakdown, TG lipolysis)
→ Glycerol-3-P → DHAP → enters pathway

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📌 THE 3 KEY BYPASS REACTIONS

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📌 REACTION PATHWAY — FLOWCHART

PYRUVATE (cytoplasm)
    ↓ ① Pyruvate Carboxylase + CO₂ + ATP  [Mitochondria]
OXALOACETATE (OAA)
    ↓ ② PEPCK + GTP → CO₂
PHOSPHOENOLPYRUVATE (PEP)
    ↓ ③ Same as glycolysis (reverse) × 5 steps
FRUCTOSE-1,6-BISPHOSPHATE
    ↓ ④ Fructose-1,6-bisphosphatase (− Pᵢ, water)  ← BYPASS 2
FRUCTOSE-6-PHOSPHATE
    ↓ ⑤ Phosphoglucose Isomerase (reverse)
GLUCOSE-6-PHOSPHATE
    ↓ ⑥ Glucose-6-Phosphatase (−Pᵢ)  ← BYPASS 3  [ER membrane]
GLUCOSE (free, enters blood)

⚠️ Note: Steps ③–⑤ use the same enzymes as glycolysis (run in reverse)

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📌 ENERGETICS

Net equation (from 2 Pyruvate → 1 Glucose):
2 Pyruvate + 4 ATP + 2 GTP + 2 NADH → Glucose + 4 ADP + 2 GDP + 6 Pᵢ + 2 NAD⁺

Gluconeogenesis costs 6 ATP vs glycolysis which yields only 2 ATP — intentionally costly to prevent futile cycling

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📌 CORI CYCLE (Special Feature) ✨

MUSCLE (anaerobic)          LIVER
Glucose → Lactate  →→→→  Lactate → Glucose
    (glycolysis)          (gluconeogenesis)
         ↑_____________________________↑
                BLOOD

This cycle: Muscle generates lactate → Liver converts it back to glucose → sent back to muscle. Allows RBCs and exercising muscle to function without oxygen.

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📌 GLUCOSE-ALANINE CYCLE ✨

MUSCLE                    LIVER
Pyruvate + NH₃ → Alanine → Alanine → Pyruvate → Glucose
(protein breakdown)        (via gluconeogenesis)

Carries nitrogen (as alanine) from muscle to liver for urea synthesis, while recycling carbon skeleton for glucose.

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📌 REGULATION

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📌 SIGNIFICANCE

1. Maintains blood glucose during fasting/starvation
2. Brain survival — brain needs continuous glucose (can't use fatty acids well)
3. RBC function — RBCs have no mitochondria, depend on glucose
4. Recycles lactate — via Cori Cycle
5. Amino acid catabolism — provides route for carbon skeletons of amino acids to become glucose

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📌 CLINICAL RELEVANCE

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✅ Q4 — Biosynthesis of Cholesterol

"Describe the biosynthesis of cholesterol & add a note on role of cholesterol in the body"

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📌 DEFINITION

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📌 KEY FEATURES

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📌 BIOSYNTHESIS — FLOWCHART (4 Stages)

STAGE 1: SYNTHESIS OF MEVALONATE
══════════════════════════════════
3 × Acetyl-CoA
    ↓ Thiolase
Acetoacetyl-CoA (4C)
    ↓ HMG-CoA Synthase + Acetyl-CoA
HMG-CoA (3-Hydroxy-3-Methylglutaryl CoA) (6C)
    ↓ HMG-CoA Reductase + 2 NADPH  ← RATE LIMITING ⭐
MEVALONATE (6C)

STAGE 2: SYNTHESIS OF ISOPRENE UNITS (IPP)
═══════════════════════════════════════════
Mevalonate + 3 ATP
    ↓ Kinases + Decarboxylase
ISOPENTENYL PYROPHOSPHATE (IPP) (5C)  ← Active isoprene unit
    ↓ Isomerase
DIMETHYLALLYL PYROPHOSPHATE (DPP) (5C)

STAGE 3: SYNTHESIS OF SQUALENE
═══════════════════════════════
IPP (5C) + DPP (5C) → Geranyl-PP (10C) + NADPH
    + IPP (5C) → Farnesyl-PP (15C)
    + Farnesyl-PP (15C) + NADPH
    ↓ Squalene Synthase
SQUALENE (30C)

STAGE 4: CYCLIZATION → CHOLESTEROL
════════════════════════════════════
Squalene (30C)
    ↓ Squalene Epoxidase + O₂ + NADPH
Squalene-2,3-epoxide
    ↓ Cyclase
LANOSTEROL (30C)
    ↓ 20 reactions (ER enzymes)
       - Remove 3 methyl groups
       - Reduce double bonds
       - Shift double bond
CHOLESTEROL (27C) ✅

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📌 STAGE SUMMARY TABLE

Total: 18 ATP + 13 NADPH consumed per cholesterol molecule

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📌 REGULATION OF CHOLESTEROL SYNTHESIS

   HIGH intracellular cholesterol
         ↓
   ① Inhibits HMG-CoA Reductase (feedback)
   ② Decreases LDL receptor expression
   ③ Increases ACAT (esterifies excess cholesterol for storage)

   LOW intracellular cholesterol
         ↓
   ① SREBP activated → increases HMG-CoA Reductase gene expression
   ② Increases LDL receptors → uptakes more LDL cholesterol

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📌 ROLE OF CHOLESTEROL IN THE BODY ✨

                    CHOLESTEROL
                        │
       ┌────────────────┼──────────────────┐
       ▼                ▼                  ▼
 CELL MEMBRANES    PRECURSOR OF:      VITAMIN D
 (fluidity &       Steroid hormones   (7-dehydrocholesterol
  structure)       Bile acids         in skin → UV → Vit D₃)
       │                │
  Every cell      HORMONES:
  membrane        - Cortisol
  contains        - Aldosterone
  cholesterol     - Estrogen
                  - Testosterone
                  - Progesterone

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📌 CLINICAL SIGNIFICANCE

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✅ Q5 — Ketone Bodies

"What are Ketone Bodies? Explain formation & importance"

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📌 DEFINITION

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📌 THE THREE KETONE BODIES

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📌 CONDITIONS PROMOTING KETONE BODY SYNTHESIS

   Starvation / Fasting
   Uncontrolled Diabetes Mellitus (Type 1)
   Low-carbohydrate diet (ketogenic diet)
   Prolonged exercise
   Alcoholism
          ↓
   ↑ Fatty acid mobilization from adipose tissue
          ↓
   ↑ Acetyl-CoA in liver (from β-oxidation)
          ↓ (OAA depleted — used for gluconeogenesis)
   Acetyl-CoA cannot enter TCA cycle
          ↓
   → KETONE BODY SYNTHESIS ↑

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📌 KETOGENESIS (Formation) — FLOWCHART

FATTY ACIDS → β-Oxidation → ACETYL-CoA (×many)
                                  │
         ┌────────────────────────┘
         ↓
2 Acetyl-CoA
    ↓ ① Thiolase (reversal of β-oxidation)
ACETOACETYL-CoA (4C)
    ↓ ② HMG-CoA Synthase (mitochondrial) + Acetyl-CoA
HMG-CoA (6C)  ← RATE LIMITING STEP ⭐
    ↓ ③ HMG-CoA Lyase
ACETOACETATE + Acetyl-CoA
    │
    ├─ ④ Spontaneous decarboxylation → ACETONE (exhaled)
    │
    └─ ⑤ 3-Hydroxybutyrate Dehydrogenase + NADH
         ↓
    3-HYDROXYBUTYRATE (main circulating form)

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📌 STEP-BY-STEP TABLE

⚠️ Note: HMG-CoA Synthase in mitochondria = Ketogenesis; HMG-CoA Synthase in cytoplasm = Cholesterol synthesis (different compartments!)

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📌 KETOLYSIS (Use of Ketone Bodies by Peripheral Tissues)

3-HYDROXYBUTYRATE (from blood)
    ↓ 3-HB Dehydrogenase → NAD⁺ → NADH
ACETOACETATE
    ↓ + Succinyl-CoA (Thiophorase/Succinyl-CoA transferase) → Succinate
ACETOACETYL-CoA
    ↓ Thiolase
2 × ACETYL-CoA
    ↓ TCA Cycle
CO₂ + H₂O + ATP ✅

🚫 Liver cannot use its own ketone bodies — lacks Thiophorase enzyme (succinyl-CoA:acetoacetate CoA transferase). Liver makes ketones for others.

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📌 ENERGY YIELD FROM KETONE BODIES

Ketone bodies are efficient fuels — especially for brain during prolonged fasting (after 3–4 days, brain shifts from glucose to ketone bodies)

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📌 IMPORTANCE / SIGNIFICANCE ✨

              KETONE BODIES
                   │
     ┌─────────────┼──────────────┐
     ▼             ▼              ▼
ENERGY SOURCE  BRAIN FUEL    PROTEIN SPARING
(heart, muscle  (during       (reduces need to
 kidney use)   starvation)   break down muscle
                              protein in fasting)

1. Alternative fuel — Heart, renal cortex prefer ketone bodies over glucose
2. Brain adaptation — After 3–4 days fasting, brain uses ketone bodies (normally can't use fat) → spares glucose for RBCs
3. Protein sparing — Reduces muscle protein catabolism during starvation
4. Signal molecule — β-hydroxybutyrate acts as an HDAC inhibitor (epigenetic regulation)
5. Ketogenic diet — Used therapeutically in epilepsy, Type 2 diabetes management

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📌 REGULATION

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📌 CLINICAL SIGNIFICANCE

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📌 DKA vs Starvation Ketosis

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