Biomolecules Class 11 Chapter Notes

Key points

• Living tissues contain inorganic substances like water and minerals plus organic biomolecules like carbohydrates, proteins, lipids, nucleic acids and metabolites.
• The acid-soluble pool contains small biomolecules such as sugars, amino acids and nucleotides; the acid-insoluble fraction contains macromolecules such as proteins, nucleic acids and polysaccharides.
• Major elemental composition of living matter: carbon, hydrogen, oxygen and nitrogen dominate; phosphorus and sulphur are also important in nucleic acids, phospholipids and some amino acids.
• Monosaccharides join by glycosidic bonds to form oligosaccharides and polysaccharides; amino acids join by peptide bonds to form proteins; nucleotides join by phosphodiester bonds to form nucleic acids.
• Proteins are heteropolymers of amino acids and show primary, secondary, tertiary and quaternary levels of structure.
• Lipids are water-insoluble molecules, but they are not true polymers because they are not made of repeated monomer units in the same way as proteins or nucleic acids.
• DNA and RNA are polynucleotides; each nucleotide contains a nitrogenous base, a pentose sugar and a phosphate group.
• Enzymes are biological catalysts, mostly proteins, that lower activation energy, are substrate-specific and work best at optimum temperature and pH.
• Secondary metabolites such as alkaloids, pigments, essential oils, gums, drugs and toxins may not directly participate in normal growth but are important for defence, signalling and human use.

Important definitions

Biomolecule

Any chemical compound found in living organisms, including small molecules such as amino acids and large macromolecules such as proteins and nucleic acids.

Primary metabolite

A biomolecule directly involved in normal growth, development and physiological processes, such as amino acids, sugars, nucleotides and fatty acids.

Secondary metabolite

A compound not directly involved in basic metabolic functions but useful in ecological roles; examples include alkaloids, flavonoids, rubber, essential oils and toxins.

Macromolecule

A large biomolecule with high molecular weight, usually found in the acid-insoluble fraction, such as proteins, polysaccharides and nucleic acids.

Peptide bond

A covalent bond formed between the carboxyl group of one amino acid and the amino group of another with the removal of water.

Glycosidic bond

A covalent bond that joins two monosaccharide units by dehydration, forming disaccharides and polysaccharides.

Phosphodiester bond

A bond linking the phosphate group of one nucleotide to the sugar of the next nucleotide in DNA or RNA.

Zwitterion

A dipolar form of an amino acid carrying both positive and negative charges, commonly present around neutral pH.

Activation energy

The minimum energy required for reactants to reach the transition state and start a chemical reaction.

Active site

The specific region of an enzyme where the substrate binds and the catalytic reaction takes place.

Big picture: how to read this chapter

The chapter begins by asking what living organisms are chemically made of. If a tissue sample is ground in trichloroacetic acid, the filtrate forms an acid-soluble pool containing small biomolecules, while the residue contains the acid-insoluble fraction rich in macromolecules. This separation is important because CBSE often asks which molecules are small metabolites and which are macromolecules. The main organic biomolecules are carbohydrates, proteins, lipids and nucleic acids. Their function depends on their building blocks and bonds: sugars form glycosidic bonds, amino acids form peptide bonds, and nucleotides form phosphodiester bonds. Enzymes connect this chapter to metabolism because they control biochemical reactions by lowering activation energy without being consumed.

Chemical composition and pools of biomolecules

Biomolecule classes at a glance

Carbohydrates, proteins, lipids and nucleic acids: exam-ready notes

Protein structure levels

How a polymer forms by dehydration

1. Bring monomers close — Two monomer units such as amino acids, monosaccharides or nucleotides are positioned so their reactive groups face each other.
2. Remove water — A hydroxyl group from one molecule and hydrogen from the other are removed as H2O. This is why polymer formation is called dehydration or condensation.
3. Form the covalent bond — A peptide bond, glycosidic bond or phosphodiester bond forms depending on the type of monomer.
4. Repeat to form larger molecules — Repeated condensation produces polypeptides, polysaccharides or polynucleotides. Hydrolysis reverses the process by adding water.

Enzymes: high-yield revision sheet

Short worked examples

• How many peptide bonds and water molecules are involved in forming a tetrapeptide from 4 amino acids? A tetrapeptide has 4 amino acids joined in a chain, so peptide bonds = 4 - 1 = 3. Each peptide bond forms by removal of one water molecule, so 3 water molecules are released.
• A DNA segment has 30 nucleotides. How many phosphodiester bonds are present if it is a single straight strand? In one linear strand, phosphodiester bonds = number of nucleotides - 1 = 30 - 1 = 29. Do not double it unless two strands are specified.
• An enzyme works fastest at pH 7. What happens if pH is shifted to 2? The reaction rate usually decreases sharply because extreme pH changes ionic interactions and active-site shape. If the enzyme is denatured, activity may not recover fully.

Board exam traps and marking cues

• Do not write that all biomolecules are macromolecules. Sugars, amino acids, nucleotides and fatty acids are small biomolecules.
• Lipids are placed in the acid-insoluble fraction due to membrane association, but they are not true polymers.
• For bond-name questions, be exact: amino acids → peptide bond, sugars → glycosidic bond, nucleotides → phosphodiester bond.
• In protein-structure answers, mention both the level and stabilising force. Example: secondary structure is stabilised by hydrogen bonds.
• For enzyme graphs, label axes properly: reaction rate on Y-axis and substrate concentration, temperature or pH on X-axis as asked.
• If asked for examples, give NCERT-style examples: starch, glycogen, cellulose, collagen, insulin, DNA, RNA, alkaloids and flavonoids.

Quick revision checks

• Q: Name the bond joining two amino acids. A: Peptide bond.
• Q: Why are enzymes called biocatalysts? A: They speed up biochemical reactions by lowering activation energy without being consumed.
• Q: Which bases are purines? A: Adenine and guanine.
• Q: Give one reason why lipids are not true polymers. A: They are not made of long chains of repeating monomer units like proteins or nucleic acids.

Frequently Asked Questions