Biomolecules NEET Notes 2027: Key Points and PDF

Biomolecules is Chapter 9 of NCERT Class 11 Biology and part of Unit 3, Cell Structure and Function, of the NEET UG syllabus. These notes cover chemical analysis of living tissue, primary and secondary metabolites, proteins, polysaccharides, nucleic acids, the four levels of protein structure, and enzyme action, factors, classes and co-factors, with a free notes PDF.

Biomolecules NEET Notes

Section-wise key points from NCERT Chapter 9, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Biomolecules at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

How is the chemical composition of living tissue analysed?

A living tissue is ground in trichloroacetic acid (Cl3CCOOH) and strained, which splits it into an acid-soluble pool (filtrate) and an acid-insoluble fraction (retentate).

• Thousands of organic compounds are found in the acid-soluble pool. All carbon compounds obtained from living tissues can be called biomolecules.
• To find inorganic constituents, the tissue is weighed (wet weight), dried (dry weight) and fully burnt. Carbon compounds are oxidised to CO2 and water vapour; what remains is ash, which contains inorganic elements such as calcium and magnesium.
• Inorganic compounds such as sulphate and phosphate are also seen in the acid-soluble fraction.
• Qualitatively, living tissue and the earth’s crust contain the same elements, but carbon and hydrogen are relatively far more abundant in living organisms.

Small Biomolecules: Amino Acids, Lipids and Nucleotides

These low molecular weight compounds are the building blocks found in the acid-soluble pool.

Amino Acids

• Amino acids have an amino group and an acidic (carboxyl) group on the same carbon, the alpha-carbon, so they are called alpha-amino acids. They are substituted methanes.
• The four substituents on the alpha-carbon are hydrogen, carboxyl group, amino group and a variable R group. Only 20 types occur in proteins.
• R group examples: hydrogen (glycine), methyl (alanine), hydroxy methyl (serine).
• Acidic: glutamic acid; basic: lysine; neutral: valine; aromatic: tyrosine, phenylalanine, tryptophan.
• The -NH2 and -COOH groups are ionisable, so the structure changes with pH; the form carrying both charges is the zwitterionic form.

Lipids

• Lipids are generally water insoluble. A fatty acid has a carboxyl group attached to an R group of 1 to 19 carbons.
• Palmitic acid has 16 carbons and arachidonic acid 20 carbons, each counting the carboxyl carbon.
• Saturated fatty acids have no double bond; unsaturated ones have one or more C=C double bonds.
• Glycerol is trihydroxy propane. Fatty acids esterified with glycerol give mono-, di- and triglycerides, called fats and oils by melting point. Oils such as gingelly oil have a lower melting point and stay liquid in winter.
• Phospholipids contain phosphorus and a phosphorylated organic compound and are found in the cell membrane; lecithin is an example. Neural tissues have lipids with more complex structures.

Nitrogen Bases, Nucleosides and Nucleotides

What are primary and secondary metabolites?

Primary metabolites have identifiable functions in normal physiological processes; secondary metabolites, seen in plant, fungal and microbial cells, have roles that are not all understood yet.

Many secondary metabolites are useful for human welfare (rubber, drugs, spices, scents, pigments) and some have ecological importance.

Biomacromolecules and the Composition of Cells

Compounds in the acid-soluble pool weigh about 18 to 800 daltons (Da); the acid-insoluble fraction holds only proteins, nucleic acids, polysaccharides and lipids.

• Molecules below 1000 Da are micromolecules (biomolecules); those in the acid-insoluble fraction are macromolecules (biomacromolecules).
• Proteins, nucleic acids and polysaccharides weigh ten thousand daltons and above and are polymers.
• Lipids do not exceed 800 Da, but on grinding, cell membranes break into water-insoluble vesicles that separate with the acid-insoluble pool. So lipids are not strictly macromolecules.
• The acid-soluble pool roughly represents the cytoplasmic composition.

Proteins: Heteropolymers of Amino Acids

Proteins are polypeptides: linear chains of amino acids linked by peptide bonds. With 20 kinds of amino acids, a protein is a heteropolymer, not a homopolymer.

• Essential amino acids must come from the diet; non-essential amino acids are made by the body.
• Collagen is the most abundant protein in the animal world; RuBisCO (Ribulose bisphosphate Carboxylase-Oxygenase) is the most abundant protein in the whole biosphere.

Polysaccharides: Cellulose, Starch, Glycogen, Inulin and Chitin

Polysaccharides are long chains of sugars (monosaccharides) found in the acid-insoluble pellet.

Complex polysaccharides use building blocks such as glucosamine and N-acetyl galactosamine, and are mostly homopolymers.

Nucleic Acids: DNA and RNA

Nucleic acids are polynucleotides; with polysaccharides and polypeptides they form the true macromolecular fraction of a cell.

• A nucleotide has three parts: a heterocyclic compound (nitrogen base), a monosaccharide (sugar) and phosphoric acid (phosphate).
• Adenine and guanine are substituted purines; cytosine, uracil and thymine are substituted pyrimidines.
• The sugar is ribose or 2′ deoxyribose. Nucleic acid with deoxyribose is DNA; with ribose it is RNA.
• DNA and RNA function as genetic material and carry hereditary information from parents to progeny.

What are the four levels of protein structure?

Biologists describe protein structure at four levels: primary, secondary, tertiary and quaternary.

Enzymes: Nature, Action and Activation Energy

Almost all enzymes are proteins; some nucleic acids that behave like enzymes are called ribozymes.

• The folded tertiary structure forms crevices or pockets; the active site is the pocket into which the substrate fits.
• Inorganic catalysts work best at high temperature and pressure, while enzymes get damaged at high temperatures (say above 40 degrees C). Enzymes from thermophilic organisms of hot vents and sulphur springs stay active up to 80 to 90 degrees C.
• A physical change alters shape or state without breaking bonds; a chemical reaction breaks and forms bonds. Rate roughly doubles or halves for every 10 degrees C change.
• Carbonic anhydrase speeds CO2 + H2O to H2CO3 from about 200 molecules an hour to about 600,000 molecules a second, roughly 10 million times faster.
• A metabolic pathway is a multistep reaction: glucose becomes pyruvic acid through ten enzyme-catalysed reactions. Skeletal muscle forms lactic acid under anaerobic conditions; yeast forms ethanol during fermentation.
• Activation energy is the difference between the energy of the substrate and that of the transition state. Enzymes lower this energy barrier.
• If the product is at a lower energy level than the substrate, the reaction is exothermic.

Nature of Enzyme Action

The enzyme and substrate form a short-lived ES complex, then an enzyme-product complex: E + S to ES to EP to E + P. Formation of the ES complex is essential for catalysis.

• 1. The substrate binds to and fits into the active site.
• 2. Binding induces the enzyme to change shape and fit more tightly around the substrate.
• 3. The active site breaks the substrate’s chemical bonds and an enzyme-product complex forms.
• 4. The enzyme releases the products and is free to bind another substrate molecule.

Factors Affecting Enzyme Activity

Anything that alters the tertiary structure of the protein changes enzyme activity: temperature, pH, substrate concentration and specific chemicals.

Classification of Enzymes and Co-factors

Enzymes are divided into 6 classes, each with 4 to 13 subclasses, and named by a four-digit number.

Co-factors

Co-factors are non-protein constituents that make an enzyme catalytically active; the protein part is the apoenzyme. Catalytic activity is lost when the co-factor is removed.

Most-asked Facts for NEET: Biomolecules

Revise these one-line facts from the NCERT chapter just before the exam.

• Tissue is ground in trichloroacetic acid; filtrate = acid-soluble pool, retentate = acid-insoluble fraction.
• Water is the most abundant chemical in living organisms (70 to 90% of cell mass).
• Alpha-amino acids carry H, COOH, NH2 and an R group on the alpha-carbon; 20 types occur in proteins.
• Glycine: R = H; alanine: methyl; serine: hydroxy methyl.
• Palmitic acid has 16 carbons; arachidonic acid has 20 (including the carboxyl carbon).
• Lecithin is a phospholipid found in the cell membrane.
• Adenosine is a nucleoside; adenylic acid is a nucleotide.
• Abrin and ricin are toxins; concanavalin A is a lectin; vinblastin is a drug.
• Lipids are below 800 Da and are not strictly macromolecules.
• Collagen: most abundant protein in animals; RuBisCO: most abundant in the biosphere.
• GLUT-4 enables glucose transport into cells.
• Inulin is a polymer of fructose; cellulose is a homopolymer of glucose.
• Starch-iodine is blue; cellulose cannot hold iodine.
• Chitin forms the exoskeleton of arthropods.
• Adenine and guanine are purines; cytosine, uracil and thymine are pyrimidines.
• Only right-handed helices occur in proteins; haemoglobin has 2 alpha + 2 beta subunits.
• Ribozymes are nucleic acids with enzyme activity.
• Malonate is a competitive inhibitor of succinic dehydrogenase.
• Haem is the prosthetic group of peroxidase and catalase; zinc is the co-factor of carboxypeptidase; NAD and NADP contain niacin.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Calling lipids true macromolecules. They appear in the acid-insoluble fraction only because membranes break into insoluble vesicles.
• Saying inulin is a polymer of glucose. It is a polymer of fructose.
• Writing that cellulose gives a blue colour with iodine. Only starch, with its helices, holds I2.
• Mixing nucleosides and nucleotides: a nucleotide has the phosphate group; a nucleoside does not.
• Listing uracil or thymine as purines. Only adenine and guanine are purines.
• Calling a protein a homopolymer. Proteins are heteropolymers; cellulose is a homopolymer.
• Saying RuBisCO is the most abundant protein in animals. That is collagen; RuBisCO is most abundant in the whole biosphere.
• Thinking enzymes work better at high temperature like inorganic catalysts. Most are damaged above about 40 degrees C.
• Confusing prosthetic groups with co-enzymes: prosthetic groups are tightly bound, co-enzymes only transiently.
• Placing a competitive inhibitor elsewhere on the enzyme. It competes for the substrate-binding site because it resembles the substrate.

Biomolecules Notes PDF and Related Pages

Download the AglaSem notes for Biomolecules (key notes and important questions with answers) free: View PDF

• All chapters: NEET Biology notes
• Full topic list: NEET syllabus
• Practice: NEET question papers
• Source text: NCERT Chapter 9 (NCERT, Class 11 Biology)

NEET Biology Notes: All Chapters

Botany

• The Living World
• Biological Classification
• Plant Kingdom
• Cell: The Unit of Life
• Plant Growth and Development
• Anatomy of Flowering Plants
• Cell Cycle and Cell Division
• Respiration in Plants
• Photosynthesis in Higher Plants
• Morphology of Flowering Plants
• Principles of Inheritance and Variation
• Microbes in Human Welfare
• Sexual Reproduction in Flowering Plants
• Molecular Basis of Inheritance
• Organisms and Populations
• Ecosystem
• Biodiversity and Conservation

Zoology

• Animal Kingdom
• Biomolecules
• Breathing and Exchange of Gases
• Excretory Products and their Elimination
• Body Fluids and Circulation
• Structural Organisation in Animals
• Locomotion and Movement
• Reproductive Health
• Chemical Coordination and Integration
• Neural Control and Coordination
• Evolution
• Human Health and Disease
• Human Reproduction
• Biotechnology and its Applications
• Biotechnology: Principles and Processes

All chapters with their notes PDFs are on the NEET Biology notes page.

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