Biochemistry
MBBS Biochemistry question bank: biomolecules, enzymes, metabolism, molecular biology, vitamins, nutrition and clinical biochemistry.
12chapters144questions75High-Yield
Definition
Lipids are heterogeneous, water-insoluble biomolecules soluble in organic solvents; fatty acids are their basic building blocks.
Classification
- Simple — fats/oils (triglycerides), waxes
- Compound — phospholipids, glycolipids, lipoproteins
- Derived — fatty acids, steroids, cholesterol
Fatty Acids
- Long hydrocarbon chain + COOH group
- Saturated — no double bond (palmitic)
- Unsaturated — double bonds (oleic)
- Essential (linoleic, linolenic)
Lipids fall into simple, compound and derived classes. Type Example Simple Triglycerides Compound Phospholipids Derived Cholesterol Applied
- Saturated fat → atherosclerosis
- Omega-3 (cardioprotective)
🔑KEY POINTS TO REMEMBER- Lipids: simple, compound, derived.
- Fatty acids: saturated vs unsaturated; essential ones from diet.
- Saturated fat → atherosclerosis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Compound lipids contain additional groups besides fatty acids and alcohol; cholesterol is an important steroid lipid.
Compound Lipids
- Phospholipids — + phosphate (membranes)
- Glycolipids — + carbohydrate
- Lipoproteins — + protein (transport)
Cholesterol
- Steroid nucleus
- Component of cell membranes
- Precursor of bile acids, steroid hormones, vitamin D
Compound lipids carry extra groups; cholesterol is a key steroid. Lipid Extra group Phospholipid Phosphate Glycolipid Carbohydrate Lipoprotein Protein Applied
- ↑ LDL cholesterol → atherosclerosis
- Cholesterol gallstones
🔑KEY POINTS TO REMEMBER- Compound lipids: phospho-, glyco-, lipoproteins.
- Cholesterol → bile acids, steroid hormones, vitamin D.
- ↑ LDL → atherosclerosis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Amino acids are the building blocks of proteins, each with an amino group, a carboxyl group and a side chain (R).
General Structure
- Central carbon + –NH₂ + –COOH + R group + H
- 20 standard amino acids
- L-form found in proteins
Classification (by R group)
- Non-polar (aliphatic, aromatic)
- Polar uncharged
- Acidic (aspartate, glutamate)
- Basic (lysine, arginine, histidine)
Nutritional
- Essential vs non-essential
Amino acids differ in their side chains, which set their properties. Class Example Acidic Glutamate Basic Lysine Aromatic Phenylalanine Applied
- Zwitterion at isoelectric pH
- Aminoacidurias (phenylketonuria)
🔑KEY POINTS TO REMEMBER- Amino acid: –NH₂, –COOH, R group; 20 standard, L-form.
- Classified by R: polar, non-polar, acidic, basic.
- Essential vs non-essential.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Proteins are polymers of amino acids linked by peptide bonds, with four levels of structural organisation.
Levels of Structure
- Primary — amino acid sequence
- Secondary — α-helix, β-pleated sheet
- Tertiary — 3D folding
- Quaternary — multiple subunits
Classification
- By shape — fibrous, globular
- By composition — simple, conjugated
- By function — enzyme, structural, transport
Proteins build up through four hierarchical levels of structure. Level Bond / feature Primary Peptide bond Secondary Hydrogen bonds Tertiary Disulphide, ionic bonds Applied
- Sickle cell (primary structure change)
- Denaturation (loses higher structure)
🔑KEY POINTS TO REMEMBER- Four levels: primary, secondary, tertiary, quaternary.
- Secondary = α-helix / β-sheet (H-bonds).
- Sickle cell = single primary-structure change.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Plasma proteins are proteins in blood plasma — mainly albumin, globulins and fibrinogen — with diverse functions.
Types
- Albumin — osmotic pressure, transport
- Globulins — α, β, γ (immunity)
- Fibrinogen — clotting
Functions
- Maintain oncotic pressure (albumin)
- Transport (hormones, drugs)
- Immunity (immunoglobulins)
- Clotting and buffering
The three main plasma-protein groups handle osmosis, immunity and clotting. Protein Function Albumin Osmotic pressure Globulin Immunity Fibrinogen Clotting Applied
- Hypoalbuminaemia → oedema
- A:G ratio in disease
🔑KEY POINTS TO REMEMBER- Albumin (osmotic), globulins (immunity), fibrinogen (clotting).
- Albumin maintains oncotic pressure.
- Low albumin → oedema.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Essential fatty acids are polyunsaturated fatty acids that the body cannot synthesise and must obtain from the diet.
Examples
- Linoleic acid (omega-6)
- Linolenic acid (omega-3)
- Arachidonic acid (semi-essential)
Functions
- Membrane structure
- Precursors of prostaglandins / eicosanoids
- ↓ blood cholesterol
The body must take in linoleic and linolenic acid from food. Fatty acid Family Linoleic Omega-6 Linolenic Omega-3 Applied
- Deficiency → dermatitis, poor growth
- Omega-3 cardioprotective
🔑KEY POINTS TO REMEMBER- Essential FAs: linoleic (ω-6), linolenic (ω-3).
- Precursors of eicosanoids; cannot be synthesised.
- Deficiency → dermatitis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Phospholipids are compound lipids containing phosphate, the main structural component of cell membranes.
Structure
- Glycerol / sphingosine backbone
- Two fatty acids + phosphate + base
- Amphipathic (hydrophilic head, hydrophobic tail)
Examples & Roles
- Lecithin (phosphatidylcholine)
- Cephalin
- Surfactant (dipalmitoyl lecithin)
- Form bilayer membranes
The amphipathic phospholipid forms the membrane bilayer. Phospholipid Role Lecithin Membrane, surfactant Cephalin Clotting Applied
- Surfactant deficiency (RDS)
- Lung maturity (L:S ratio)
🔑KEY POINTS TO REMEMBER- Phospholipids: amphipathic, membrane-forming.
- Lecithin = surfactant; cephalin in clotting.
- Surfactant deficiency → RDS.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Essential amino acids cannot be synthesised by the body and must be supplied in the diet.
The Essential Ones
- Phenylalanine, Valine, Threonine
- Tryptophan, Isoleucine, Methionine
- Histidine, Leucine, Lysine
- (mnemonic: PVT TIM HaLL)
These amino acids must come from food since the body cannot make them. Type Examples Essential Lysine, leucine, valine Non-essential Alanine, glutamate Applied
- Deficiency → kwashiorkor
- Complete vs incomplete proteins
🔑KEY POINTS TO REMEMBER- Essential AAs (PVT TIM HaLL) must be dietary.
- Deficiency → protein-energy malnutrition.
- Complete proteins have all essentials.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The isoelectric pH (pI) is the pH at which an amino acid carries no net charge, existing as a zwitterion.
Zwitterion
- Has both + (–NH₃⁺) and – (–COO⁻) charges
- Net charge = zero (at pI)
- Amino acids are amphoteric
At Isoelectric pH
- No net charge → no migration in an electric field
- Minimum solubility
At its pI an amino acid is a neutral zwitterion that does not migrate. pH Charge Below pI Positive At pI Zero (zwitterion) Above pI Negative Applied
- Basis of electrophoresis
- Protein precipitation at the pI
🔑KEY POINTS TO REMEMBER- pI = pH of zero net charge (zwitterion).
- Zwitterion has both +NH₃ and –COO⁻.
- No migration at pI (electrophoresis basis).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Denaturation is the loss of the native higher-order structure of a protein without breaking peptide bonds.
Features
- Loss of secondary, tertiary, quaternary structure
- Primary structure intact (peptide bonds unbroken)
- Loss of biological activity
- Usually irreversible
Causes
- Heat, extremes of pH
- Heavy metals, organic solvents
- Urea, detergents
Denaturing agents unfold the protein while leaving peptide bonds intact. Structure Status Secondary/tertiary Lost Primary Intact Applied
- Cooking (egg white)
- Heat sterilisation
🔑KEY POINTS TO REMEMBER- Denaturation = loss of 2°/3°/4° structure; primary intact.
- Causes: heat, pH, heavy metals, urea.
- Loss of biological activity.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Cholesterol is a steroid lipid essential for membranes and as a precursor of important molecules.
Functions
- Membrane component (regulates fluidity)
- Precursor of bile acids
- Precursor of steroid hormones
- Precursor of vitamin D
Transport
- LDL — delivers cholesterol to tissues (‘bad’)
- HDL — returns cholesterol to liver (‘good’)
Cholesterol supports membranes and seeds bile acids, hormones and vitamin D. Lipoprotein Role LDL To tissues (bad) HDL To liver (good) Applied
- ↑ LDL → atherosclerosis
- Statins lower cholesterol
🔑KEY POINTS TO REMEMBER- Cholesterol: membrane + precursor (bile, hormones, vit D).
- LDL (bad) vs HDL (good).
- ↑ LDL → atherosclerosis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
A peptide bond is the covalent amide bond linking the carboxyl group of one amino acid to the amino group of another.
Features
- Formed by condensation (loss of water)
- –CO–NH– linkage
- Planar, rigid, partial double-bond character
- Usually trans configuration
Two amino acids join by losing water to form the peptide bond. Feature Detail Bond –CO–NH– Formation Condensation Nature Planar, rigid Applied
- Backbone of proteins
- Hydrolysed by proteases
🔑KEY POINTS TO REMEMBER- Peptide bond = amide (–CO–NH–), by condensation.
- Planar and rigid (partial double bond).
- Forms the protein backbone.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).