Biochemistry
MBBS Biochemistry question bank: biomolecules, enzymes, metabolism, molecular biology, vitamins, nutrition and clinical biochemistry.
12chapters144questions75High-Yield
Definition
The cell is the basic structural and functional unit of life; subcellular organelles carry out specialised biochemical functions.
Major Organelles
- Nucleus — DNA, transcription
- Mitochondria — ATP (oxidative phosphorylation)
- Endoplasmic reticulum — rough (protein), smooth (lipid)
- Golgi — modification, packaging
- Lysosomes — digestion (hydrolases)
Other Components
- Ribosomes — protein synthesis
- Peroxisomes — fatty-acid oxidation, H₂O₂
- Cytoskeleton
Each organelle performs a defined biochemical role within the cell. Organelle Function Nucleus DNA / transcription Mitochondria ATP Lysosome Digestion RER Protein synthesis Applied
- Lysosomal storage diseases
- Mitochondrial disorders
🔑KEY POINTS TO REMEMBER- Cell = basic unit; organelles have specialised functions.
- Nucleus (DNA), mitochondria (ATP), lysosomes (digestion).
- ER → Golgi handle protein/lipid processing.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Water is the biological solvent; pH measures acidity; buffers resist changes in pH.
Water
- Polar solvent with hydrogen bonding
- Medium for biochemical reactions
- High specific heat
pH
- pH = –log[H⁺]
- Range 0–14; blood pH 7.35–7.45
- Acid lowers pH, base raises pH
Buffers
- Weak acid + its conjugate base
- Resist pH change on adding acid/base
- Body buffers: bicarbonate, phosphate, protein
Buffers keep body fluids within a narrow physiological pH range. Buffer System Bicarbonate Main ECF buffer Phosphate Intracellular Protein Haemoglobin Applied
- Acidosis / alkalosis
- Bicarbonate buffer’s clinical importance
🔑KEY POINTS TO REMEMBER- Water = polar solvent; pH = –log[H⁺].
- Buffer = weak acid + conjugate base.
- Blood pH 7.35–7.45 (bicarbonate main buffer).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Carbohydrates are polyhydroxy aldehydes or ketones, classified by the number of sugar units, the simplest being monosaccharides.
Classification
- Monosaccharides — single sugar (glucose)
- Disaccharides — two units (sucrose)
- Oligosaccharides — 3–10 units
- Polysaccharides — many units (starch)
Monosaccharides
- By carbons: triose, pentose, hexose
- By group: aldose (glucose), ketose (fructose)
- Ring forms (pyranose / furanose)
Carbohydrates are grouped by how many sugar units they contain. Type Example Monosaccharide Glucose Disaccharide Sucrose Polysaccharide Starch Applied
- Glucose — main energy source
- Pentoses in nucleic acids
🔑KEY POINTS TO REMEMBER- Carbohydrates = polyhydroxy aldehydes/ketones.
- Mono-, di-, oligo-, polysaccharides.
- Monosaccharides: aldose vs ketose.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Disaccharides are two monosaccharides joined by a glycosidic bond; polysaccharides are polymers of many monosaccharides.
Disaccharides
- Maltose = glucose + glucose
- Lactose = glucose + galactose
- Sucrose = glucose + fructose (non-reducing)
Polysaccharides
- Starch — plant storage (amylose + amylopectin)
- Glycogen — animal storage (branched)
- Cellulose — structural (β-1,4 bonds)
Sugar units link by glycosidic bonds into di- and polysaccharides. Sugar Components Maltose Glucose + glucose Lactose Glucose + galactose Sucrose Glucose + fructose Applied
- Lactose intolerance
- Glycogen storage diseases
🔑KEY POINTS TO REMEMBER- Disaccharides: maltose, lactose, sucrose (non-reducing).
- Polysaccharides: starch, glycogen (storage), cellulose (structural).
- Lactose intolerance; glycogen storage diseases.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The plasma membrane is a selectively permeable lipid bilayer that controls transport in and out of the cell.
Structure (Fluid Mosaic Model)
- Phospholipid bilayer
- Proteins (integral, peripheral)
- Cholesterol, glycolipids
- Fluid mosaic arrangement
Transport
- Passive — diffusion, osmosis (no energy)
- Facilitated — via carriers
- Active — against gradient (ATP, e.g. Na⁺/K⁺ pump)
Substances cross passively down a gradient or actively against it. Type Energy Diffusion None Facilitated None (carrier) Active ATP Applied
- Na⁺/K⁺ pump (digoxin target)
- Cystic fibrosis (CFTR channel)
🔑KEY POINTS TO REMEMBER- Fluid mosaic: bilayer + proteins + cholesterol.
- Transport: passive, facilitated, active (ATP).
- Na⁺/K⁺ pump; CFTR in cystic fibrosis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Isomerism refers to compounds with the same molecular formula but different structures; monosaccharides show several types.
Types
- Stereoisomers — D & L forms
- Epimers — differ at one carbon (glucose / galactose)
- Anomers — α & β (ring form)
- Optical isomers — rotate plane-polarised light
Monosaccharides vary as stereoisomers, epimers and anomers. Isomer Example Epimer Glucose/galactose (C4) Anomer α / β glucose Aldose-ketose Glucose / fructose Applied
- Explains the diversity of sugars
🔑KEY POINTS TO REMEMBER- Stereoisomers (D/L), epimers (one C), anomers (α/β).
- Glucose/galactose = epimers.
- Optical isomers rotate light.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Benedict’s test detects reducing sugars, which have a free aldehyde or ketone group.
Principle
- Reducing sugar reduces Cu²⁺ (blue) → Cu⁺ (red)
- Colour change: blue → green → yellow → brick-red
- Positive: glucose, fructose, lactose, maltose
Non-reducing
- Sucrose (no free group) — negative
A reducing sugar turns the blue reagent to a brick-red precipitate on heating. Colour Sugar amount Blue None Green Trace Brick-red High Applied
- Urine glucose (glycosuria)
- Sucrose gives a negative result
🔑KEY POINTS TO REMEMBER- Benedict’s detects reducing sugars (free aldehyde/ketone).
- Cu²⁺ → Cu⁺; blue → brick-red.
- Sucrose is non-reducing (negative).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Glycosaminoglycans (GAGs), or mucopolysaccharides, are long unbranched chains of repeating disaccharides found in connective-tissue ground substance.
Features
- Repeating disaccharide units
- Highly negatively charged (sulphated)
- Bind water → form a gel
Examples
- Hyaluronic acid
- Chondroitin sulphate
- Heparin (anticoagulant)
- Keratan sulphate
Negatively charged GAG chains bind water to form the tissue gel. GAG Role Hyaluronic acid Lubrication Heparin Anticoagulant Chondroitin Cartilage Applied
- Mucopolysaccharidoses (Hurler syndrome)
- Heparin therapy
🔑KEY POINTS TO REMEMBER- GAGs = repeating disaccharides, sulphated, water-binding.
- Hyaluronic acid, chondroitin, heparin, keratan sulphate.
- Mucopolysaccharidoses.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The Henderson–Hasselbalch equation relates the pH of a buffer to the pKa and the ratio of conjugate base to acid.
Equation
- pH = pKa + log([base] / [acid])
- Bicarbonate: pH = pKa + log([HCO₃⁻] / [H₂CO₃])
- When [base] = [acid], pH = pKa
Significance
- Predicts the pH of a buffer
- Explains blood-pH regulation
The equation links buffer pH to the base-to-acid ratio. Ratio Effect on pH Base = acid pH = pKa ↑ base ↑ pH ↑ acid ↓ pH Applied
- Acid-base balance
- Bicarbonate : CO₂ ratio (20:1)
🔑KEY POINTS TO REMEMBER- pH = pKa + log([base]/[acid]).
- When base = acid, pH = pKa.
- Bicarbonate:CO₂ = 20:1 in blood.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The mitochondrion is the ‘powerhouse of the cell’, the site of ATP production by oxidative phosphorylation.
Structure
- Double membrane (outer + inner)
- Inner membrane folded into cristae
- Matrix — enzymes and its own DNA
Functions
- ATP synthesis (electron transport chain)
- TCA cycle (in the matrix)
- Fatty-acid oxidation
- Regulation of apoptosis
The TCA cycle and electron transport chain generate ATP here. Part Function Matrix TCA cycle Inner membrane ETC, ATP synthase Applied
- Mitochondrial diseases (maternal inheritance)
- Cyanide (blocks the ETC)
🔑KEY POINTS TO REMEMBER- Mitochondrion = powerhouse; double membrane, cristae, matrix.
- TCA cycle (matrix) + ETC (inner membrane) → ATP.
- Maternal inheritance; cyanide blocks ETC.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Lysosomes are membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.
Features
- Contain acid hydrolases (optimum pH ~5)
- Called ‘suicide bags’
- Digest worn organelles and bacteria
Functions
- Autophagy
- Phagocytosis (with white cells)
- Breakdown of macromolecules
Acid hydrolases inside lysosomes digest and recycle cell material. Feature Detail Enzymes Acid hydrolases pH ~5 Applied
- Lysosomal storage diseases (Tay-Sachs, Gaucher)
- Gout (crystals rupture lysosomes)
🔑KEY POINTS TO REMEMBER- Lysosomes = acid hydrolases (pH ~5), ‘suicide bags’.
- Autophagy + phagocytosis.
- Storage diseases: Tay-Sachs, Gaucher.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Dietary fibre is indigestible plant polysaccharide (mainly cellulose) that aids gastrointestinal function.
Types
- Soluble — pectin, gums (↓ cholesterol)
- Insoluble — cellulose (bulk, motility)
Benefits
- Prevents constipation (adds bulk)
- ↓ Cholesterol and blood glucose
- Prevents colon cancer, diverticulosis
Fibre adds bulk and slows absorption, benefiting the gut and metabolism. Fibre Effect Soluble ↓ cholesterol/glucose Insoluble Bulk, motility Applied
- Prevents constipation, colon cancer
- Useful in the diabetic diet
🔑KEY POINTS TO REMEMBER- Fibre = indigestible plant polysaccharide.
- Soluble (↓ cholesterol) vs insoluble (bulk/motility).
- Prevents constipation, colon cancer.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).