CBSE Class 11 Biology Revision Notes Chapter 9 Biomolecules

Biomolecules explains the chemical compounds found in living organisms and their role in cellular structure and function. In CBSE Class 11 Biology, this chapter covers metabolites, proteins, polysaccharides, nucleic acids, lipids and enzymes.

Biomolecules studies the organic and inorganic substances present in living tissues. Although living and non-living matter contain many similar elements, living organisms have a higher relative abundance of carbon, hydrogen and oxygen. These elements form molecules that support structure, energy storage, heredity, metabolism and enzyme activity.

Use CBSE Class 11 Biology Revision Notes Chapter 9 for 2026–27 to revise chemical composition, primary and secondary metabolites, biomacromolecules, proteins, polysaccharides, nucleic acids, protein structure and enzymes. The chapter also explains why water is the most abundant chemical in living organisms.

Key Takeaways

• Biomolecules: All carbon compounds obtained from living tissues are called biomolecules.
• Metabolites: Primary metabolites have identifiable functions, while many secondary metabolites are useful to humans and ecosystems.
• Biomacromolecules: Proteins, nucleic acids and polysaccharides are true macromolecules.
• Enzymes: Enzymes are biological catalysts that lower activation energy and increase reaction rate.

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Biomolecules Class 11 Biology Notes: Chapter Overview

Biomolecules are chemical compounds found in living organisms. They include small molecules such as amino acids, sugars and fatty acids, as well as large molecules such as proteins, polysaccharides and nucleic acids.

Living organisms contain both organic and inorganic substances. From a biological point of view, many organic compounds are classified as amino acids, nucleotide bases, fatty acids and sugars.

How to Analyse Chemical Composition in Class 11 Biology Chapter 9 Notes

To analyse chemical composition, a living tissue is ground in trichloroacetic acid. This forms a thick slurry that is strained to obtain two fractions.

The acid-soluble pool contains many organic compounds. The acid-insoluble fraction contains proteins, nucleic acids, polysaccharides and lipids associated with membranes.

Analysis of Inorganic Constituents

For inorganic analysis, a tissue sample is weighed, dried and burnt. Water evaporates during drying, while carbon compounds are oxidised and removed during burning.

Ash contains inorganic elements such as calcium and magnesium. Inorganic compounds such as sulphate and phosphate may also occur in the acid-soluble fraction.

Elements in Living and Non-Living Matter

Living and non-living matter may contain similar elements, but their relative abundance differs.

A closer comparison shows that carbon and hydrogen are relatively more abundant in living tissues than in the earth’s crust.

Primary and Secondary Metabolites in Biomolecules Class 11 Notes

Metabolites are biomolecules involved in life processes. They are grouped into primary metabolites and secondary metabolites.

Primary Metabolites

Primary metabolites are biomolecules with identifiable functions in normal physiological processes.

Primary metabolites are commonly found in animal tissues and play known roles in growth, development and cellular function.

Secondary Metabolites

Secondary metabolites are mostly found in plants, fungi and microbes. Their direct role in the host organism may not always be clear, but many are useful to humans and ecosystems.

Some secondary metabolites are used in medicines, perfumes, spices and dyes. Some also have ecological importance.

Biomacromolecules in CBSE Class 11 Biology Notes Chapter 9

Biomolecules are classified by molecular weight and solubility.

Proteins, nucleic acids and polysaccharides are true biomacromolecules because they are large polymeric substances.

Acid-Soluble and Acid-Insoluble Fractions

Together, these two fractions represent the chemical composition of living tissues.

Why Are Lipids Found in the Acid-Insoluble Fraction?

Lipids have molecular weights below 800 Da, so they are not true macromolecules. Still, they occur in the acid-insoluble fraction because they form cell membranes and other membrane structures.

When tissues are ground, membranes break into vesicles. These vesicles are not water-soluble, so they separate with the acid-insoluble fraction.

Average Chemical Composition of Cells

Water is the most abundant chemical in living organisms.

This table helps revise the relative abundance of major cellular components.

Amino Acids in Biomolecules Class 11 Biology Notes

Amino acids are organic compounds with an amino group and an acidic group attached to the same alpha carbon.

There are 20 types of amino acids found in proteins. Their properties depend mainly on the amino group, carboxyl group and R group.

Types of Amino Acids

Zwitterion in Amino Acids

Amino acids show ionisable –NH₂ and –COOH groups. In solutions of different pH, their structure changes.

A zwitterion is a dipolar form of an amino acid. It has both positive and negative charges within the same molecule.

Lipids in Class 11 Biology Chapter 9 Notes

Lipids are generally insoluble in water but soluble in non-polar solvents such as ether, chloroform and benzene.

Fatty Acids

Fatty acids have a carboxyl group attached to a hydrocarbon chain. They may be saturated or unsaturated.

Palmitic acid has 16 carbon atoms, including the carboxyl carbon. Arachidonic acid has 20 carbon atoms, including the carboxyl carbon.

Functions of Lipids

Proteins in Biomolecules Class 11 Notes

Proteins are polypeptides. They are linear chains of amino acids linked by peptide bonds.

A protein is a heteropolymer because it contains different types of amino acids.

Peptides and Polypeptides

The first amino acid of a polypeptide is called the N-terminal amino acid. The last amino acid is called the C-terminal amino acid.

Essential and Non-Essential Amino Acids

Dietary proteins supply essential amino acids.

Functions of Proteins

Collagen is the most abundant protein in the animal world. RuBisCO is the most abundant protein in the biosphere.

Polysaccharides in CBSE Class 11 Biology Notes Chapter 9

Polysaccharides are long chains of sugars. They are macromolecules present in the acid-insoluble fraction.

Starch, Glycogen and Cellulose

Starch can hold iodine molecules in its helical portion, giving a blue colour. Cellulose does not form such complex helices.

Chitin

Chitin is a complex polysaccharide found in the exoskeleton of arthropods. It contains amino-sugar derivatives such as N-acetyl glucosamine.

Nucleic Acids in Biomolecules Class 11 Biology Notes

Nucleic acids are polynucleotides. DNA and RNA are nucleic acids that function as genetic material.

Nucleotide Structure

A nucleotide has three components.

A nucleoside contains a nitrogenous base and sugar. A nucleotide contains a nucleoside plus phosphate.

Nitrogenous Bases

DNA contains adenine, guanine, cytosine and thymine. RNA contains adenine, guanine, cytosine and uracil.

DNA and RNA Difference

Structure of Proteins in Class 11 Biology Chapter 9 Notes

Protein structure is explained at four levels: primary, secondary, tertiary and quaternary.

Primary Structure of Protein

Primary structure is the linear sequence of amino acids in a polypeptide chain.

The primary structure determines how the protein folds.

Secondary Structure of Protein

Secondary structure refers to local folding of the polypeptide chain.

Hydrogen bonds help stabilise secondary structures. In proteins, only right-handed helices are observed.

Tertiary Structure of Protein

Tertiary structure is the three-dimensional folding of one polypeptide chain.

Tertiary structure is essential for many biological activities of proteins.

Quaternary Structure of Protein

Quaternary structure is found in proteins with more than one polypeptide chain or subunit.

The arrangement of these subunits forms the functional protein.

Enzymes in CBSE Class 11 Biology Revision Notes Chapter 9

Enzymes are biological catalysts. Almost all enzymes are proteins, but some nucleic acids also show catalytic activity and are called ribozymes.

All enzymes are proteins, but all proteins are not enzymes.

Enzyme Action

An enzyme works through its active site.

Enzymes increase reaction rate without being consumed.

Activation Energy and Enzymes

Activation energy is the energy needed to start a reaction. Enzymes lower activation energy, so reactions occur faster.

For example, carbonic anhydrase speeds up the formation of carbonic acid from carbon dioxide and water.

Factors Affecting Enzyme Activity

Enzyme activity depends on temperature, pH, substrate concentration and chemicals that bind to enzymes.

Temperature and pH

Each enzyme has an optimum temperature and optimum pH.

Substrate Concentration

With increasing substrate concentration, the reaction velocity rises at first. After a point, all enzyme active sites become occupied.

After Vmax, adding more substrate does not increase the reaction rate.

Enzyme Inhibition

Enzyme activity can decrease when a chemical binds to the enzyme. This process is called inhibition, and the chemical is called an inhibitor.

Competitive inhibitors reduce enzyme action by preventing the substrate from binding to the active site.

Classification of Enzymes in Biomolecules Class 11 Notes

Enzymes are classified based on the type of reaction they catalyse.

These six classes are important for quick enzyme revision.

Cofactors, Coenzymes and Prosthetic Groups

Some enzymes need non-protein components to become catalytically active. These components are called cofactors.

Examples of Cofactors

If the cofactor is removed, catalytic activity may be lost.

Biomolecules Class 11 Notes: Quick Comparison Table

Important Terms from Biomolecules Class 11 Biology Notes

NCERT-Based Exam Points

• Living and non-living matter may contain similar elements.
• Carbon and hydrogen are relatively more abundant in living organisms than in the earth’s crust.
• Trichloroacetic acid is used to analyse organic compounds in living tissue.
• The filtrate is called the acid-soluble pool.
• The retentate is called the acid-insoluble fraction.
• Ash contains inorganic elements.
• All carbon compounds obtained from living tissues are called biomolecules.
• Amino acids have an amino group and an acidic group on the alpha carbon.
• Proteinaceous amino acids are of 20 types.
• Lipids are generally water-insoluble.
• Phospholipids are found in cell membranes.
• Nitrogenous bases include adenine, guanine, cytosine, thymine and uracil.
• Nucleoside contains a base and sugar.
• Nucleotide contains base, sugar and phosphate.
• DNA and RNA function as genetic material.
• Primary metabolites have identifiable physiological functions.
• Secondary metabolites include alkaloids, flavonoids, rubber and essential oils.
• Biomacromolecules include proteins, nucleic acids and polysaccharides.
• Lipids are not strictly macromolecules.
• Water is the most abundant chemical in living organisms.
• Proteins are polypeptides.
• Proteins are heteropolymers of amino acids.
• Collagen is the most abundant protein in the animal world.
• RuBisCO is the most abundant protein in the biosphere.
• Starch is the storage polysaccharide in plants.
• Glycogen is the storage polysaccharide in animals.
• Cellulose is a structural polysaccharide in plant cell walls.
• Chitin occurs in arthropod exoskeletons.
• Proteins have primary, secondary, tertiary and quaternary structures.
• Almost all enzymes are proteins.
• Ribozymes are nucleic acids with catalytic activity.
• Enzymes lower activation energy.
• Enzyme activity depends on temperature, pH and substrate concentration.
• Malonate is a competitive inhibitor of succinic dehydrogenase.
• Enzymes are classified into six major classes.

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