Biomolecules - Complete NEET Biology Notes 2026
Master Biomolecules for NEET 2026 with detailed notes on carbohydrates, proteins, lipids, nucleic acids, and enzymes. NCERT-aligned content with diagrams and previous year questions.
Key Takeaways
- 1Carbohydrates follow the formula Cn(H2O)n and are classified as mono-, di-, and polysaccharides
- 2Proteins are polymers of amino acids linked by peptide bonds
- 3Enzymes are biological catalysts that lower activation energy without being consumed
- 4DNA is double-stranded, RNA is single-stranded - key structural differences
- 5Lipids are hydrophobic molecules important for energy storage and membranes
Remember these points for your NEET preparation
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Biomolecules - Complete NEET Biology Notes
Biomolecules chapter contributes 4-6 questions in NEET and forms the foundation for understanding biochemistry. This guide covers all NCERT concepts essential for NEET 2026.
Introduction to Biomolecules
Living organisms are made up of various chemical compounds called biomolecules. These can be classified as:
| Category | Examples | Characteristics |
|---|---|---|
| Micromolecules | Sugars, amino acids, nucleotides | Low molecular weight, simple |
| Macromolecules | Proteins, nucleic acids, polysaccharides | High molecular weight, polymers |
Primary vs Secondary Metabolites:
- Primary: Essential for life (amino acids, sugars, fatty acids)
- Secondary: Not essential but provide ecological advantage (alkaloids, terpenes, pigments)
Carbohydrates
General Formula
Cn(H2O)n - hence the name "hydrates of carbon"
Classification
1. Monosaccharides (Simple Sugars)
| Sugar | Carbon atoms | Example | Source |
|---|---|---|---|
| Triose | 3C | Glyceraldehyde | Glycolysis intermediate |
| Tetrose | 4C | Erythrose | Pentose phosphate pathway |
| Pentose | 5C | Ribose, Deoxyribose | RNA, DNA |
| Hexose | 6C | Glucose, Fructose, Galactose | Most common |
Important Hexoses:
- Glucose (dextrose/grape sugar): Blood sugar
- Fructose (levulose): Sweetest natural sugar, found in fruits
- Galactose: Component of lactose
2. Disaccharides
| Disaccharide | Components | Bond | Source |
|---|---|---|---|
| Sucrose | Glucose + Fructose | α-1,2 | Sugarcane, beetroot |
| Maltose | Glucose + Glucose | α-1,4 | Germinating seeds |
| Lactose | Glucose + Galactose | β-1,4 | Milk |
NEET Tip: Sucrose is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bond.
3. Polysaccharides
| Polysaccharide | Monomer | Bond Type | Function |
|---|---|---|---|
| Starch | α-glucose | α-1,4 (amylose), α-1,4 & α-1,6 (amylopectin) | Plant storage |
| Glycogen | α-glucose | α-1,4 & α-1,6 (more branched) | Animal storage |
| Cellulose | β-glucose | β-1,4 | Plant cell wall |
| Chitin | N-acetylglucosamine | β-1,4 | Fungal cell wall, exoskeleton |
Key Difference: Starch and glycogen are digestible; cellulose is not (humans lack cellulase enzyme).
Proteins
Amino Acids - Building Blocks
General Structure:
- Central carbon (α-carbon) attached to:
- Amino group (-NH₂)
- Carboxyl group (-COOH)
- Hydrogen atom
- R group (side chain) - determines properties
Classification by R Group:
| Type | Property | Examples |
|---|---|---|
| Non-polar | Hydrophobic | Glycine, Alanine, Valine, Leucine |
| Polar uncharged | Hydrophilic | Serine, Threonine, Asparagine |
| Acidic | Negatively charged | Aspartic acid, Glutamic acid |
| Basic | Positively charged | Lysine, Arginine, Histidine |
| Aromatic | Ring structure | Phenylalanine, Tyrosine, Tryptophan |
Essential Amino Acids (must be obtained from diet): Mnemonic - PVT TIM HALL
- Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine
Peptide Bond
- Formed by condensation reaction between -COOH of one amino acid and -NH₂ of another
- Results in release of water
- Partial double bond character (planar, rigid)
Protein Structure
| Level | Description | Bonds Involved |
|---|---|---|
| Primary | Linear sequence of amino acids | Peptide bonds |
| Secondary | α-helix, β-pleated sheet | Hydrogen bonds |
| Tertiary | 3D folding | H-bonds, ionic, disulfide, hydrophobic |
| Quaternary | Multiple polypeptide chains | Same as tertiary |
Examples:
- Primary only: Insulin A & B chains (before folding)
- Secondary: Keratin (α-helix), Silk fibroin (β-sheet)
- Tertiary: Myoglobin (single chain, 3D)
- Quaternary: Hemoglobin (4 subunits: 2α + 2β)
Lipids
Characteristics
- Hydrophobic (water-insoluble)
- Soluble in organic solvents
- No true polymer structure
Classification
1. Simple Lipids
Fatty Acids:
| Type | Structure | Examples | Property |
|---|---|---|---|
| Saturated | No double bonds | Palmitic, Stearic | Solid at room temp |
| Unsaturated | Double bonds present | Oleic (1), Linoleic (2) | Liquid at room temp |
Triglycerides (Fats & Oils):
- Glycerol + 3 fatty acids
- Fats: Saturated (animal origin)
- Oils: Unsaturated (plant origin)
2. Compound Lipids
| Type | Components | Example | Function |
|---|---|---|---|
| Phospholipids | Glycerol + 2 FA + Phosphate | Lecithin | Cell membrane |
| Glycolipids | Lipid + Carbohydrate | Cerebrosides | Cell recognition |
| Lipoproteins | Lipid + Protein | LDL, HDL | Lipid transport |
3. Derived Lipids
- Cholesterol: Precursor for steroid hormones, vitamin D
- Steroids: Testosterone, estrogen, cortisol
Nucleic Acids
Nucleotide Structure
Nucleotide = Nitrogenous base + Pentose sugar + Phosphate group
Nitrogenous Bases:
| Purines (Double ring) | Pyrimidines (Single ring) |
|---|---|
| Adenine (A) | Cytosine (C) |
| Guanine (G) | Thymine (T) - DNA only |
| Uracil (U) - RNA only |
Base Pairing Rules (Chargaff's Rules):
- A pairs with T (2 hydrogen bonds) in DNA
- A pairs with U in RNA
- G pairs with C (3 hydrogen bonds)
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Structure | Double-stranded | Usually single-stranded |
| Location | Nucleus, mitochondria, chloroplast | Nucleus, cytoplasm, ribosomes |
| Function | Genetic information storage | Protein synthesis |
Types of RNA
| Type | Function | Location |
|---|---|---|
| mRNA | Carries genetic code | Nucleus → Cytoplasm |
| tRNA | Transfers amino acids | Cytoplasm |
| rRNA | Component of ribosomes | Ribosomes |
Enzymes
Definition
Biological catalysts that speed up reactions without being consumed.
Properties
- Highly specific (lock and key / induced fit)
- Increase reaction rate by lowering activation energy
- Not consumed in reaction
- Work optimally at specific pH and temperature
- Can be regulated
Classification (IUBMB)
| Class | Reaction Type | Example |
|---|---|---|
| Oxidoreductases | Oxidation-reduction | Dehydrogenases |
| Transferases | Transfer of groups | Kinases |
| Hydrolases | Hydrolysis | Lipases, Proteases |
| Lyases | Non-hydrolytic removal | Decarboxylases |
| Isomerases | Isomerization | Mutases |
| Ligases | Bond formation using ATP | Synthetases |
Factors Affecting Enzyme Activity
| Factor | Effect |
|---|---|
| Temperature | Increases to optimum, then denatures |
| pH | Optimal pH varies (pepsin: 2, trypsin: 8) |
| Substrate concentration | Increases until saturation |
| Enzyme concentration | Directly proportional |
| Inhibitors | Competitive or non-competitive |
Enzyme Inhibition
| Type | Mechanism | Effect on Km | Effect on Vmax |
|---|---|---|---|
| Competitive | Binds to active site | Increases | Unchanged |
| Non-competitive | Binds elsewhere | Unchanged | Decreases |
Co-factors and Co-enzymes
| Type | Nature | Examples |
|---|---|---|
| Prosthetic group | Tightly bound | Heme in hemoglobin |
| Co-enzyme | Loosely bound, organic | NAD⁺, FAD, Coenzyme A |
| Metal ion | Inorganic | Zn²⁺, Mg²⁺, Fe²⁺ |
Previous Year NEET Questions
Q1 (NEET 2023): Which of the following is a non-reducing sugar?
- (a) Maltose
- (b) Lactose
- (c) Sucrose ✓
- (d) Glucose
Q2 (NEET 2022): The most abundant protein in the animal world is:
- (a) Keratin
- (b) Collagen ✓
- (c) Hemoglobin
- (d) Insulin
Q3 (NEET 2021): Which of the following is an example of pyrimidine?
- (a) Adenine
- (b) Guanine
- (c) Cytosine ✓
- (d) None of the above
Q4 (NEET 2020): Competitive inhibitor of an enzyme:
- (a) Decreases Km
- (b) Increases Km ✓
- (c) Does not change Km
- (d) Increases Vmax
Q5 (NEET 2019): Essential amino acids are those which:
- (a) Are synthesized in the body
- (b) Must be provided in diet ✓
- (c) Are non-essential for growth
- (d) None of the above
Quick Revision Points
- Sweetest sugar: Fructose
- Blood sugar: Glucose
- Milk sugar: Lactose
- Storage polysaccharide: Starch (plants), Glycogen (animals)
- Structural polysaccharide: Cellulose (plants), Chitin (fungi)
- Most abundant organic compound: Cellulose
- Most abundant protein: Collagen (animals), RuBisCO (world)
- Saturated fats: No double bonds, solid
- Unsaturated fats: Double bonds, liquid
- DNA sugar: Deoxyribose (lacks -OH at 2' carbon)
FAQs
Q: Why is sucrose called a non-reducing sugar? A: In sucrose, both anomeric carbons (C1 of glucose and C2 of fructose) are involved in the glycosidic bond, so there's no free aldehyde or ketone group to reduce other compounds.
Q: What makes an amino acid essential? A: Essential amino acids cannot be synthesized by the human body in sufficient quantities and must be obtained from the diet. There are 9 essential amino acids for adults.
Q: Why do enzymes have optimal temperature and pH? A: Enzymes are proteins with specific 3D structures. Temperature and pH changes can alter hydrogen bonds and ionic interactions that maintain this structure. Beyond optimal conditions, the enzyme denatures and loses activity.
Q: What is the difference between DNA and RNA in terms of stability? A: DNA is more stable because: (1) deoxyribose lacks 2'-OH making it less reactive, (2) double-stranded structure provides redundancy, (3) thymine instead of uracil prevents deamination errors.
Q: Why is RuBisCO considered the most abundant protein? A: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is found in all photosynthetic organisms and makes up about 50% of leaf protein. Given the vast amount of plant biomass on Earth, it's the most abundant protein globally.