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UNIT III — Lipid Metabolism

Complete Answers for Pharm D 1st Year

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✅ SECTION I — Very Short Answer (2 marks)

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Q1. What is the role of Citrate in Fatty Acid Synthesis?

Citrate = transport vehicle that carries acetyl-CoA out of mitochondria into cytoplasm for fat synthesis.

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Q2. What is Ketosis? Name 2 conditions that lead to Ketosis.

• Lots of fat is being broken down → acetyl-CoA builds up → liver converts it to ketone bodies faster than they can be used.

1. Diabetes mellitus (especially Type 1) — no insulin → glucose can't enter cells → body burns fat heavily
2. Prolonged starvation/fasting — glucose is low → fat is the main fuel → ketone body production increases

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Q3. What is Ketolysis? Give its significance.

• β-hydroxybutyrate → acetoacetate → acetoacetyl-CoA → 2 acetyl-CoA → enters TCA cycle → ATP

• Provides energy to brain and muscles during fasting/starvation (when glucose is scarce)
• Brain can use ketone bodies when glucose is unavailable — this is a survival mechanism
• Reduces dependence on glucose during prolonged fasting
• Each mole of acetoacetate yields ~22 ATP — efficient fuel source

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Q4. What is the functional significance of the Fatty Acid Synthase Complex?

• All enzymes needed to make a fatty acid are packed together in one complex → increases efficiency and speed
• The growing fatty acid chain is passed from one enzyme to the next without being released — no intermediate gets lost
• Uses NADPH and acetyl-CoA as raw materials
• End product: Palmitate (16-carbon fatty acid) — the primary product of synthesis in humans
• Located in cytoplasm of liver, adipose tissue, and mammary glands

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Q5. Name 3 unsaturated fatty acids. Write the structure of any one.

CH₃-(CH₂)₇-CH=CH-(CH₂)₇-COOH
         ↑
    double bond at position 9

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Q6. Write the structure of Cholesterol. What is the role of Cholesterol in our body?

• 3 six-membered rings (A, B, C) + 1 five-membered ring (D)
• A hydroxyl (–OH) group at carbon-3
• A double bond between C5–C6
• A hydrocarbon side chain at C17

1. Cell membrane component — maintains fluidity and structural integrity
2. Precursor of steroid hormones — cortisol, testosterone, estrogen, aldosterone
3. Precursor of bile acids — helps in fat digestion
4. Precursor of vitamin D — when skin is exposed to sunlight
5. Component of myelin sheath — protects nerve fibers

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Q7. What is β-oxidation of Fatty Acids?

• 1 FADH₂
• 1 NADH
• 1 Acetyl-CoA

Think of it like cutting a long chain link-by-link, 2 links at a time.

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Q8. Name 3 biologically important compounds derived from Cholesterol Catabolism.

1. Bile acids (cholic acid, chenodeoxycholic acid) — help in fat digestion and absorption
2. Steroid hormones — glucocorticoids (cortisol), mineralocorticoids (aldosterone), sex hormones (testosterone, estrogen, progesterone)
3. Vitamin D₃ (cholecalciferol) — formed in the skin from 7-dehydrocholesterol on UV exposure

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Q9. What is Hypercholesterolemia? Name 2 disorders that result from it.

1. Atherosclerosis — cholesterol deposits form plaques in artery walls → narrowing of arteries
2. Coronary Artery Disease (CAD) / Heart Attack — plaques in coronary arteries block blood flow to the heart

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Q10. What is Atherosclerosis?

1. LDL cholesterol deposits under the inner lining of arteries
2. Macrophages engulf it → become "foam cells"
3. Fibrous cap forms → plaque
4. Plaque narrows the artery → reduces blood flow
5. If plaque ruptures → blood clot → heart attack or stroke

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✅ SECTION II — Long Answer (15/10 marks)

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Q1. Describe the biosynthesis of Cholesterol. Add a note on the role of cholesterol in the body.

Biosynthesis of Cholesterol

Step-by-Step (Simple):

• 2 Acetyl-CoA → Acetoacetyl-CoA
• • another Acetyl-CoA → HMG-CoA (3-hydroxy-3-methylglutaryl CoA)
• HMG-CoA → Mevalonate (by HMG-CoA reductase — the key rate-limiting enzyme)
• ⭐ Statins (e.g., atorvastatin) block HMG-CoA reductase to lower cholesterol

• Mevalonate → IPP (isopentenyl pyrophosphate) — 5-carbon unit
• 6 × IPP → Squalene (30-carbon compound)

• Squalene (linear) → Lanosterol (first sterol ring structure)
• Lanosterol → Cholesterol (27 carbons) through multiple steps involving removal of methyl groups and reduction of double bonds

Role of Cholesterol in the Body:

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Q2. What are Ketone Bodies? Explain their formation and importance.

What are Ketone Bodies?

1. Acetoacetate — the primary ketone body
2. β-Hydroxybutyrate — the most abundant in blood (ratio 3:1 vs acetoacetate)
3. Acetone — minor product; exhaled via lungs (fruity breath in DKA)

Formation of Ketone Bodies (in liver mitochondria):

2 Acetyl-CoA → Acetoacetyl-CoA (by thiolase)
                    ↓ + Acetyl-CoA (HMG-CoA synthase)
               HMG-CoA
                    ↓ (HMG-CoA lyase)
               Acetoacetate + Acetyl-CoA
                    ↓ (β-hydroxybutyrate dehydrogenase)
               β-Hydroxybutyrate
                    ↓ (spontaneous)
               Acetone (+ CO₂)

Importance of Ketone Bodies:

1. Alternative fuel during fasting — muscles, heart, and brain use them when glucose is low
2. Brain fuel — the brain cannot use fatty acids but CAN use ketone bodies during prolonged starvation
3. Glucose sparing — conserves glucose for red blood cells (which must use glucose)
4. Each mole of acetoacetate → ~22 ATP — efficient energy source
5. Clinical marker — high ketones → indicate diabetic ketoacidosis (DKA) or starvation

When do ketone bodies become harmful?

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✅ SECTION III — Short Answer (5 marks)

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Q1. Explain the role of Palmitic acid synthesis starting from Acetyl-CoA.

Palmitate Synthesis (de novo fatty acid synthesis):

Steps:

• Requires biotin as cofactor
• Activated by citrate (high energy signal)
• Inhibited by palmitoyl-CoA (end-product feedback)

• Acetyl-CoA attaches to cysteine-SH of FAS
• Malonyl-CoA attaches to ACP (acyl carrier protein) on FAS

1. Reduction (NADPH used)
2. Dehydration
3. Reduction (NADPH used) → Chain grows by 2 carbons each cycle

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Q2. Explain the biosynthesis of Cholesterol. Write the role of Cholesterol in the body.

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Q3. What are Ketone Bodies? Explain their formation and importance.

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Q4. Describe briefly: Synthesis of Bile Acids & a note on Enterohepatic Circulation of Bile Salts.

Synthesis of Bile Acids:

• Site: Liver (hepatocytes)
• From: Cholesterol (27-carbon) → Primary bile acids (24-carbon)

1. Cholic acid (trihydroxy)
2. Chenodeoxycholic acid (dihydroxy)

• Glycocholic acid, Taurocholic acid, etc.

• Cholic → Deoxycholic acid
• Chenodeoxycholic → Lithocholic acid

Enterohepatic Circulation of Bile Salts:

Liver makes bile salts
        ↓
Stored in gallbladder
        ↓
Released into small intestine (duodenum) after meals
        ↓
Emulsify and help absorb dietary fats
        ↓
95% of bile salts reabsorbed in terminal ileum
        ↓
Via portal vein back to liver
        ↓
Liver takes them up and re-secretes into bile
        ↓ (cycle repeats 6-10 times/day)
Only 5% lost in feces (replenished by new synthesis)

• Conserves bile salts (body has only 3–5g but uses 20–30g/day due to recycling)
• Ensures fat digestion is continuously supported
• Interrupting this cycle (e.g., with cholestyramine/bile acid sequestrants) forces liver to make new bile acids from cholesterol → lowers blood cholesterol levels (used to treat hypercholesterolemia)

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Q4 (Right Column). Explain β-oxidation of Saturated Fatty Acids. Calculate ATP generated from complete oxidation of Palmitic Acid.

β-Oxidation of Saturated Fatty Acids:

Each Round of β-Oxidation (4 steps):

ATP Calculation for Palmitic Acid (C16):

(Using modern P/O ratios: FADH₂ = 1.5 ATP, NADH = 2.5 ATP, acetyl-CoA via TCA = 10 ATP)

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Q5. What is Fatty Liver? Name the causes responsible for fatty liver. What is Atherosclerosis?

Fatty Liver (Hepatic Steatosis):

1. Alcoholism — alcohol disrupts fat metabolism in liver; increases NADH → fatty acids accumulate
2. Obesity / Type 2 Diabetes — excess free fatty acids flood the liver
3. High-fat diet — excess dietary fat deposited in liver
4. Protein deficiency — lack of apoprotein B → VLDL cannot be formed → fat trapped in liver
5. Carbon tetrachloride (CCl₄) poisoning — liver toxin that damages fat export
6. Kwashiorkor (protein malnutrition) — impaired lipoprotein synthesis

Atherosclerosis:

• Begins with LDL cholesterol depositing in artery walls
• Macrophages consume cholesterol → become foam cells
• Fibrous plaque forms → narrows lumen → reduces blood flow
• Plaque can rupture → blood clot → heart attack or stroke

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1. Explain the role of Palmitic acid synthesis starting from Acetyl-CoA
2. Explain the biosynthesis of Cholesterol + role of cholesterol in the body
3. What are Ketone Bodies? Explain formation & importance
4. Describe synthesis of Bile Acids + note on Enterohepatic Circulation
5. Explain β-oxidation of saturated fatty acids + calculate ATP from complete oxidation of Palmitic acid

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SECTION III — Short Answer Type (5 Marks Each)

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① Role of Palmitic Acid Synthesis Starting from Acetyl-CoA

Acetyl-CoA + CO₂ → Malonyl-CoA Enzyme: Acetyl-CoA carboxylase (needs Biotin vitamin) This is the rate-limiting (slowest/controlling) step

Acetyl-CoA sits on one part of the Fatty Acid Synthase (FAS) complex Malonyl-CoA sits on ACP (Acyl Carrier Protein)

Acetyl + Malonyl join → 4-carbon chain + CO₂ is released

• 2 carbons are added
• 2 NADPH are used
• Chain grows: 4C → 6C → 8C → ... → 16C (Palmitate)

1 Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH → Palmitate + 7CO₂ + 8CoA + 14NADP⁺

• It is the main product of fatty acid synthesis in humans
• Can be elongated or converted to other fatty acids
• Stored as triglycerides in adipose tissue for future energy

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② Biosynthesis of Cholesterol + Role in the Body

4 Simple Stages:

2 Acetyl-CoA → Acetoacetyl-CoA → HMG-CoA (HMG = 3-Hydroxy-3-Methylglutaryl CoA)

Enzyme: HMG-CoA Reductase (rate-limiting enzyme) Statins block this enzyme → less cholesterol made Needs 2 NADPH

Mevalonate → IPP (5C unit) → 6 IPP combine → Squalene (30C)

Squalene (linear) → rings form → Lanosterol → Cholesterol (27C) Several steps, needs O₂ and NADPH

Role of Cholesterol in the Body:

Remember: Cholesterol is not all bad — it is ESSENTIAL for life. Problems arise only when it is in EXCESS.

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③ What are Ketone Bodies? Their Formation and Importance

What are Ketone Bodies?

Formation (in liver mitochondria):

Acetyl-CoA + Acetyl-CoA
        ↓ (Thiolase)
   Acetoacetyl-CoA
        ↓ + Acetyl-CoA (HMG-CoA Synthase)
       HMG-CoA
        ↓ (HMG-CoA Lyase)
   Acetoacetate ←→ β-Hydroxybutyrate
        ↓ (spontaneous)
      Acetone + CO₂

• Fasting / Starvation
• Uncontrolled Diabetes (Type 1)
• Low-carbohydrate diets

Importance:

1. Fuel for brain during starvation — brain normally uses glucose but switches to ketones after 2–3 days of fasting
2. Energy for muscles and heart — efficient fuel
3. Glucose sparing — saves glucose for RBCs (which can only use glucose)
4. 1 mole acetoacetate → ~22 ATP

When are they harmful?

• No insulin → uncontrolled fat breakdown → massive ketone production
• Blood becomes acidic → vomiting, confusion, coma → medical emergency
• Diagnosed by: fruity breath, high blood ketones, acidic blood pH

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④ Synthesis of Bile Acids + Enterohepatic Circulation

Synthesis of Bile Acids:

Cholesterol → 7α-hydroxycholesterol → ... → Primary bile acids

1. Cholic acid (3 –OH groups)
2. Chenodeoxycholic acid (2 –OH groups)

• Example: Glycocholic acid, Taurocholic acid

• Deoxycholic acid (from cholic)
• Lithocholic acid (from chenodeoxycholic)

Enterohepatic Circulation of Bile Salts (Simple Cycle):

LIVER makes bile salts
        ↓
Stored in GALLBLADDER
        ↓ (released after eating)
Enter SMALL INTESTINE (duodenum)
        ↓
Emulsify dietary fats → help absorb fat-soluble vitamins (A, D, E, K)
        ↓
~95% reabsorbed in TERMINAL ILEUM
        ↓
Travel back to liver via PORTAL VEIN
        ↓
LIVER reuses them → cycle repeats 6–10 times per day
        ↓
Only ~5% lost in stool (replaced by new synthesis)

• The body only has 3–5g of bile salts, but uses 20–30g per day — only possible because of recycling
• Bile acid sequestrants (e.g., cholestyramine) interrupt this cycle → liver must make new bile acids from cholesterol → blood cholesterol falls → used as a drug to treat high cholesterol
• If the terminal ileum is damaged (e.g., in Crohn's disease) → bile salts not reabsorbed → fat malabsorption → diarrhea

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⑤ β-Oxidation of Saturated Fatty Acids + ATP Calculation from Palmitic Acid

What is β-Oxidation?

Where?

• Mitochondrial matrix
• Fatty acid must first be activated to Acyl-CoA (uses 2 ATP)
• Then transported into mitochondria via the Carnitine shuttle

4 Steps of Each Round (easy to remember):

ATP Calculation for Palmitic Acid (16 carbons):

Palmitic acid gives 106 ATP — compare this to glucose which gives only ~30–32 ATP. Fat is energy-dense!

• Main source of energy during fasting
• Heart gets ~60–70% of its energy from fatty acid oxidation
• Defects in β-oxidation → accumulation of fatty acids → liver damage, muscle weakness

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UNIT III — Lipid Metabolism

Super Simple Language 😄

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VERY SHORT ANSWERS (2 marks)

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Q1. What is the role of Citrate in Fatty Acid Synthesis?

• Acetyl-CoA joins with another molecule → forms Citrate
• Citrate can come OUT of mitochondria easily
• Once outside, citrate breaks apart → releases acetyl-CoA → fat synthesis begins!

Simple line: Citrate = taxi that carries acetyl-CoA out of mitochondria so fat can be made.

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Q2. What is Ketosis? Name 2 conditions leading to ketosis.

1. Diabetes (Type 1) — no insulin → body can't use glucose → burns fat like crazy
2. Starvation / Long fasting — no food = no glucose → body burns fat for energy

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Q3. What is Ketolysis? Give its significance.

• Gives energy to brain, muscles, and heart when glucose is not available
• Helps the body survive fasting without eating glucose
• Brain can survive on ketones after 2–3 days of fasting — this is a survival superpower!

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Q4. What is the functional significance of the Fatty Acid Synthase Complex?

• All enzymes are together → work fast and efficiently
• The growing fat chain doesn't get lost — it stays attached to the machine
• Final product: Palmitate (16-carbon fat)
• Found in liver and fat tissue
• Uses acetyl-CoA (raw material) + NADPH (energy)

Simple line: FAS is a fat-making machine with all tools built in one place.

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Q5. Name 3 unsaturated fatty acids. Write the structure of any one.

1. Oleic acid (1 double bond) — found in olive oil
2. Linoleic acid (2 double bonds) — essential, must eat from food
3. Arachidonic acid (4 double bonds) — important for making prostaglandins

CH₃-(CH₂)₇-CH = CH-(CH₂)₇-COOH
               ↑
         double bond here (position 9)

• 18 carbons long
• 1 double bond at carbon 9
• Written as: 18:1 (Δ9)

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Q6. Write the structure of Cholesterol. What is the role of Cholesterol?

• 3 big rings + 1 small ring
• Has a –OH group (makes it slightly water-soluble)
• Has a long tail (hydrocarbon chain)
• Has 1 double bond
• Total: 27 carbons

Cholesterol is NOT just bad — your body NEEDS it. Problems start only when it's TOO much.

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Q7. What is β-oxidation of Fatty Acids?

• 1 Acetyl-CoA (used to make ATP)
• 1 NADH (used to make ATP)
• 1 FADH₂ (used to make ATP)

Simple line: β-oxidation = fat breakdown machine that chops fat into 2-carbon pieces to make energy.

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Q8. Name 3 biologically important compounds derived from Cholesterol catabolism.

1. Bile acids (e.g., cholic acid) — help digest fat in the intestine
2. Steroid hormones — cortisol (stress), testosterone (male hormone), estrogen (female hormone)
3. Vitamin D₃ — made in skin when sunlight hits cholesterol → strong bones

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Q9. What is Hypercholesterolemia? Name 2 disorders.

1. Atherosclerosis — fat plaques clog arteries like rust in a pipe
2. Coronary Artery Disease (Heart Attack) — clogged heart arteries → no blood to heart → heart attack

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Q10. What is Atherosclerosis?

1. LDL cholesterol enters artery wall
2. Immune cells (macrophages) eat the cholesterol → become "foam cells" (look foamy under microscope)
3. Foam cells pile up → form a plaque
4. Plaque makes the artery narrower → less blood flows
5. If plaque bursts → blood clot forms → blocks artery completely → Heart attack or stroke!

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SHORT ANSWER TYPE (5 marks)

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① Explain Palmitic Acid Synthesis Starting from Acetyl-CoA

Acetyl-CoA + CO₂ → Malonyl-CoA This is the first and most important step Enzyme = Acetyl-CoA Carboxylase (needs Vitamin Biotin) ⭐ This step is like the "ON switch" for fat making

Acetyl-CoA goes to one spot on FAS Malonyl-CoA goes to another spot (ACP)

Acetyl + Malonyl join together → 4-carbon chain CO₂ is released (that's why we added it in step 1 — just to help with the reaction)

1. Reduction (uses NADPH) — makes it less oxidized
2. Dehydration — removes water
3. Reduction (uses NADPH again) — fully reduces the chain

Each repeat adds 2 more carbons 4C → 6C → 8C → 10C → 12C → 14C → 16C = PALMITATE! 🎉

1 Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH → Palmitate + 7CO₂

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② Biosynthesis of Cholesterol + Its Role

4 Easy Stages:

Acetyl-CoA + Acetyl-CoA → Acetoacetyl-CoA Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA (HMG-CoA = 3-hydroxy-3-methylglutaryl CoA — just remember the name!)

Enzyme: HMG-CoA Reductase — this is the speed control of cholesterol making Statins (medicines) block this enzyme → cholesterol drops → heart is protected Needs 2 NADPH

Mevalonate (6C) → makes small 5-carbon units called IPP 6 IPP pieces join → Squalene (30C, looks like a long chain)

Squalene bends and folds into rings → Lanosterol (first ring structure) Lanosterol → slowly converted → Cholesterol (27C) Many steps, needs oxygen

Role of Cholesterol:

• 🔵 Cell walls — keeps them flexible
• 🟡 Bile acids — fat digestion
• 🟠 Hormones — testosterone, estrogen, cortisol
• ☀️ Vitamin D — sunlight + skin cholesterol = Vitamin D
• 🧠 Myelin — nerve protection

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③ Ketone Bodies — Formation and Importance

What are ketone bodies?

Formation (in liver mitochondria):

Step 1: Acetyl-CoA + Acetyl-CoA → Acetoacetyl-CoA

Step 2: Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA
        (enzyme: HMG-CoA Synthase)

Step 3: HMG-CoA → Acetoacetate + Acetyl-CoA
        (enzyme: HMG-CoA Lyase)

Step 4: Acetoacetate → β-Hydroxybutyrate (main one in blood)
        OR
        Acetoacetate → Acetone + CO₂ (exhaled)

Importance:

When it goes wrong — DKA (Diabetic Ketoacidosis):

No insulin → uncontrolled fat burning → too many ketones → blood becomes acidic → life-threatening emergency Signs: fruity breath, vomiting, confusion, coma

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④ Synthesis of Bile Acids + Enterohepatic Circulation

Bile Acid Synthesis:

1. Cholic acid
2. Chenodeoxycholic acid

• Glycine → Glycocholic acid
• Taurine → Taurocholic acid

• Deoxycholic acid
• Lithocholic acid

Enterohepatic Circulation — The Recycling System:

Imagine bile salts are reusable shopping bags. The liver makes them, they go to the intestine to carry fat, then they come back to the liver to be reused again and again!

🏭 LIVER makes bile salts
        ↓
💛 Stored in GALLBLADDER
        ↓ (you eat food)
🍔 Released into SMALL INTESTINE
        ↓
🧴 Break up fat droplets (like soap on grease)
        ↓
✅ 95% reabsorbed in TERMINAL ILEUM
        ↓
🚌 Travel back to liver in PORTAL VEIN
        ↓
🔄 LIVER reuses them (happens 6–10 times/day!)
        ↓
🗑️ Only 5% lost in poop (liver makes fresh to replace)

• Body only has 3–5g bile salts but uses 20–30g/day — only works because of recycling!
• Drug cholestyramine blocks reabsorption → liver forced to make new ones from cholesterol → blood cholesterol falls ✅
• Damage to terminal ileum (Crohn's disease) → bile salts lost → fat not digested → diarrhea

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⑤ β-Oxidation of Saturated Fatty Acids + ATP from Palmitate

What is β-oxidation?

4 Steps per Round (like a loop):

Acyl-CoA loses 2H → becomes Enoyl-CoA Makes FADH₂ (→ 1.5 ATP later)

Enoyl-CoA + water → 3-Hydroxyacyl-CoA No energy here — just reshaping

3-Hydroxyacyl-CoA → 3-Ketoacyl-CoA Makes NADH (→ 2.5 ATP later)

3-Ketoacyl-CoA → Acetyl-CoA (2C) + shorter chain Acetyl-CoA → goes to TCA cycle → makes 10 ATP

ATP Calculation for Palmitate (16 carbons):

• Glucose (6C) → ~32 ATP
• Palmitate (16C) → 106 ATP

Fat gives WAY more energy than sugar — that's why the body stores energy as fat, not sugar!

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📚 Sources: Basic Medical Biochemistry 6e | Lippincott Biochemistry 8e | Harper's Biochemistry 32e

💡 Tip for exam: Learn the key enzymes (HMG-CoA reductase, 7α-hydroxylase, Acetyl-CoA carboxylase) and key numbers (106 ATP from palmitate, 7 rounds of β-oxidation). Everything else you can explain in simple words!