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Lipids: Classification, Pufas, Lipoproteins, Ketone Body Synthesis & Utilization

Apr 25, 2023


Lipids are essential biomolecules involved in various physiological processes, including energy storage, cell membrane structure and function, and signalling pathways. A good understanding of lipids is therefore important for understanding how the body functions at a molecular level.

Besides, Lipids are one of the most heavily tested topics in the NEET PG exam, and questions related to lipid metabolism and function carry significant weightage.

Read this blog further to get a quick overview of this high-yield biochemistry topic for NEET PG exam preparations. In this blog we’ll cover classification of lipids, PUFAs, lipoproteins, ketone body synthesis and ketone body utilization.

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Lipid: Any compound which is insoluble in water & soluble in non-polar organic solvent

  • Made of fatty acid and alcohol joined by ester bond formed by removal of H2O

Classification of Lipids


Simple Lipids

  • Base alcohol is glycerol
Simple Lipids
Simple Lipids

Phospholipid (PL)

Phospholipid (PL)
Phospholipid (PL)

Phospholipases: Enzymes which hydrolyse phospholipids at different places



  • Complex phospholipid
Complex phospholipid
  • Phosphatidic acid = Glycerol + 2 FAs + P
  • So, product of hydrolysis of cardiolipins = 3 glycerol + 4 FA + 2 P
  • Can be antigenic due to its complex nature. 
  • E.g. Anti-Phospholipid Syndrome
    • Occurs due to Anticardiolipin antibodies formed in body
    • C/f are thrombotic condition and disease females with history of recurrent abortions

Phospholipids Types

1. GlyceroPhospholipids

Parent alcohol: Glycerol
2. SphingoPhospholipids
Parent alcohol: Sphingosine 
Sphingosine is an unsaturated amino alcohol
Phospholipids Types


  • Made of Alcohol + FA + Carbohydrate
  • Phosphate & base is never present
  • Alcohol glycerol is never present
  • Alcohol used is sphingosine (so, aka glycosphingolipids)


  1. Glucosylceramide / Glucocerebrosid
    • Sphingosine + FA + glucose
      • Sphingosine + FA = ceramide  
      • Ceramide + Glucose = Glucosylceramide or Glucocerebroside
    • Never found in CNS but always found in extra neural tissues
  2. Galactosylceramide/Galactocerebroside
    • Sphingosine + FA + Galactose
    • Ceramide + Galactose = Galactosylceramide/ Galactocerebroside
    • Always found in CNS


Polyunsaturated Fatty Acids (PUFAs) means FAs having 2 double bonds

  • aka Essential FAs (EFA) - they cannot be synthesized in our body and needs to be essentially taken in diet

Categories of PUFAs

Omega-3 category

Omega-6 category

  1. Cervonic acid/DHA (DocosaHexaenoic Acid)
  • 22 C (docosa) & 6 = (hexaenoic)
  • Source: Fish oil
  1. α-Linolenic acid
  • 18 C & 3 =
  • Source: Flaxseed oil, Soybean oil)
  1. Timnodonic acid/EPA (EicosaPentaenoic Acid)
  • 20 C (eicosa) & 5 = (pentaenoic)
  • Source: Fish oil
  1. γ–Linolenic acid
  • 18 C & 3 =
  • Source: oil of evening primrose
  1. Linoleic acid
  • 18 C & 2 =
  • Source: Safflower oil
  1. Arachidonic acid
  • 20 C & 4 =
  • Source: Animal Fats
  • Any fatty acid starting from “L” have same number of carbons e.g. α-Linolenic acid, γ–Linolenic acid and Linoleic acid all have 18 carbons
  • Smaller Name- Linoleic = 2 double bonds
  • Bigger name - Linolenic = 3 double bonds

Important information

  • DHA
    • is required for brain development of first 2-3 years of life so, Health drinks are fortified with DHA.
    • Breast milk always contains DHA.
  • α–Linolenic acid is precursor of ω-3 category means if α–Linolenic acid is taken in diet, other two ω-3 FAs can be made form it in the body.
  • Linoleic acid is precursor of ω-6 category and can be used to make other two ω-6 FAs in the body
  • Most Essential FA is - Linoleic acid as it can make arachidonic acid which is required for PGs and Leukotrienes synthesis.
  • PUFAs which are cardioprotective: ω-3 PUFAs

Lipid Transport

  • Polar substance is soluble in polar medium and non-polar substance is soluble in non-polar medium 
  • Transport medium in our body is blood and Blood is water based, hence it is polar
  • So, carbohydrate & proteins which are polar can easily dissolve in blood and thus can be easily transported from one place of body to another 
  • But dietary Lipids due to their non-polar nature cannot be dissolved directly in blood and need special transport structures known as Lipoproteins e.g. HDL, LDL, VLDL 



  • In Lipoproteins, Lipid is present towards the core surrounded by proteins in the periphery.

Lipids present in Lipoproteins

  • Triglyceride ? NP
  • Phospholipid ? Amphipathic
  • Cholesterol ? Amphipathic
  • Cholesterol ester (cholesterol + FA) ? NP
  • Proteins are called Apo-proteins
Lipids present in Lipoproteins

Arrangement of lipids in Lipoprotein

  • Non-polar Lipids (TG, Cholesterol ester)
    • Present embedded in core 
  • Amphipathic Lipids (Cholesterol, phospholipids)
    • Polar portion will be towards outside
    • Non-polar portion will be towards inside
Arrangement of lipids in Lipoprotein

Composition of various lipoproteins

LipoproteinLipid presentProtein/Apoprotein present
ChylomicronTG (exogenous)Apo B48
Chylomicron RemnantTG + CholesterolApo B48 + Apo E
VLDLTG (endogenous)Apo B100
VLDL Remnant (IDL)TG + CholesterolApo B100 + Apo E
LDLCholesterolApo B100 + Apo E
HDLCholesterol esterApo-A, Apo-C and Apo-E
Composition of various lipoproteins
Composition of various lipoproteins

Related Biochemistry Articles:

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High Density Lipoproteins (HDL)


  • HDL performs Reverse cholesterol Transport i.e. transport of excess cholesterol from peripheral tissues and blood vessels to Liver

How HDL adds Fatty acid

How HDL adds Fatty acid

Important Information

  • Exogenous TG is transported to peripheral tissues by Chylomicron.
  • Endogenous TG is transported from liver to peripheral tissues by: VLDL
  • Cholesterol is transported from liver to peripheral tissues by: LDL
  • Cholesterol is transported from peripheral tissues to liver by: HDL 

Lipoprotein lipase

  • Anabolic enzyme present in endothelium cells
  • Breaks TG present in chylomicrons and VLDL to convert them to their remnants.

Ligands on lipoproteins for uptake by liver

  • Ligands are proteins present on lipoprotein by which liver will recognise particular Lipoprotein
Ligands on lipoproteins for uptake by liver

Hyperlipoproteinemia (Fredrickson classification)






Common names


Lipoprotein Lipase

Or Apo C-II defect

Chylo > VLDL


Familial Hyperchylomicronemia

II a

LDL Receptor

or Apo B100



Familial Hypercholesterolemia

II b




Familial combined hyperlipoproteinemia


apo E

↑ Chylo–remnant

↑ VLDL remnant

Broad β diseases / Dys β lipoproteinemia

Important Information 

Clinical Features recognition to tackle hyperlipoproteinemia related clinical questions

  • Tendon xanthoma   ↑ Cholesterol
  • Eruptive xanthoma  ↑ TG
  • Palmar & Tubero eruptive xanthoma   ↑ Chylo-remnant & ↑ VLDL remnant
  • Milky plasma  ↑ Chylomicrons
  • Acute pain in abdomen [Acute pancreatitis]  ↑ TG

Important Information 

Lipoprotein ‘x’ 

  • Abnormal Lipoprotein found in two conditions:
    • LCAT deficiency
    • Cholestatic states 
  • Rich in amphipathic lipids (PL & cholesterol)
  • Poor in neutral lipids (TG & cholesterol ester)

Fatty Acid Synthesis


  • Anabolic pathway
  • Occurs in cytoplasm
  • Activated by Insulin

FA synthase complex

  • Main enzyme and a multienzyme complex (6 enzyme activities)
  • Dimer (composed of 2 identical subunits)

Important Information

Q. FA is synthesized from? 

Ans: Acetyl Co A and not malonyl CoA

  • Because extra Carbon of Malonyl CoA is not getting added in newly synthesized FA
  • 1st enzyme carboxylase adds one CO2 to form malonyl CoA but 2nd enzyme FA synthase removes the CO2. So, only CO2 of acetyl CoA are used.
  • But, Main Donor of carbon for fatty acid synthesis – Malonyl CoA


  • Used for the transport of Acetyl CoA from mitochondria to cytoplasm for fatty acid synthesis
  • Provides two things for FA synthesis
    • Acetyl CoA
    • NADPH

Ketone Body Synthesis And Ketone Body Utilisation


  • Both are catabolic pathway
  • Both occur in mitochondria
  • Both are activated by glucagon and inhibited by insulin


  • KB synthesis occurs only in Liver whereas KB utilization occurs in vital organs brain and heart & also in muscles but never occurs in liver.
Ketozene body synthesis pathway
Ketozene body utilisation pathway

Note: In the first step of KB utilization, high energy CoA is added without using ATP. So, the rule of business (i.e. activation using ATP) is not followed.

Important information

  • Thiolase is a common enzyme for 4 lipid metabolic pathways
    • Ketone body synthesis
    • Ketone body utilization
    • Cholesterol synthesis
    • β-oxidation of FA
  • Liver cannot use ketone bodies as it lacks Thiophorase.

Previous Year’s Question

Q. Active metabolite form in synthesis of fatty acid is? (AIIMS Nov 2017)

  1. Acetyl CoA
  2. Malonyl CoA
  3. Stearate
  4. Palmitate

Q. Major product of fatty acid synthesis is? (AIIMS Nov 2017)

  1. Acetyl CoA
  2. ATP
  3. Citrate
  4. Palmitate

Q. Which of the following is an essential fatty acid? (FMGE JUNE 2018) 

  1. Citric acid 
  2. Linoleic acid 
  3. Stearic acid 
  4. Palmitic acid

Q. Type I hyperlipoproteinemia is characterized by? (NEET Jan 2019)

  1. Elevated LDL
  2. Elevated HDL
  3. Elevated lipoprotein lipase
  4. Elevated chylomicrons.

Q. Ketone bodies are not utilized by? (JIPMER May 2019)

  1. Brain
  2. RBC
  3. Heart
  4. Skeletal muscle

Q. A patient has multiple tendon xanthomas. Serum cholesterol (398 mg/dl) & LDL (220 mg/dl) were found to be raised. Statins were given to this patient. What is the diagnosis? (NEET Sep 2021)

  1. Lipoprotein lipase deficiency 
  2. Familial hypercholesterolemia
  3. Tangier’s disease
  4. Huntington’s disease

Q. Eicosanoids are formed from? (INICET May 2022)

  1. Arachidonic acid
  2. Platelet aggregation
  3. 4 fused rings
  4. Arginine 

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