Difficulty: Intermediate | Prerequisites: Basic organic chemistry (functional groups, bonding), awareness of biological macromolecules.
Biomolecules sit at the intersection of organic chemistry and biology. This topic covers the four major classes of biological macromolecules: carbohydrates, proteins, nucleic acids and vitamins. It matters because CBSE tests it every year across Sections A, B and C, and the questions lean on precise definitions and classifications rather than heavy calculation. If you are coming in cold, make sure you are comfortable with functional groups (aldehyde, amine, carboxyl, hydroxyl) before reading on.
TL;DR
Vitamins split into fat-soluble (A, D, E, K, stored in the body) and water-soluble (B, C, excreted in urine). Proteins denature when heat or pH disrupts their structure. Nucleotides join via phosphodiester bonds to form DNA and RNA. Amino acids are amphoteric because they carry both -NH2 and -COOH groups.
Fat-soluble vitamins
Vitamins that dissolve in lipids and are stored in the liver and adipose tissue: A, D, E and K. They are not excreted in urine. In simple terms, these are the vitamins your body can stockpile, so you do not need them every single day.
Water-soluble vitamins
Vitamins that dissolve in water and cannot be stored in the body: the B-complex group and vitamin C. Excess is excreted in urine. Think of it as: what goes in must come out quickly, so you need a regular dietary supply.
Ascorbic acid (vitamin C)
A water-soluble vitamin essential for the synthesis of collagen. Its deficiency causes scurvy. In simple terms, it is the vitamin in citrus fruits that keeps your gums and connective tissues healthy.
Denaturation (of proteins)
The loss of a protein's native three-dimensional structure due to external stress such as heat, pH change or mechanical agitation. The primary structure (peptide bonds) remains intact. Think of it as: cooking an egg whites the protein but does not break its amino-acid chain.
Phosphodiester linkage
The covalent bond joining two nucleotides in a nucleic acid, where a phosphate group connects the 3' carbon of one sugar to the 5' carbon of the next. In simple terms, this is the backbone "link" that holds the rungs of the DNA ladder together.
Nucleotide
The monomer unit of nucleic acids, composed of a nitrogenous base, a pentose sugar and a phosphate group.
Amino acid
An organic molecule carrying both an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon (the alpha carbon).
Zwitterion
The dipolar ionic form of an amino acid in which the amino group is protonated (NH3+) and the carboxyl group is deprotonated (COO-). This is the dominant form at physiological pH.
Amphoteric behaviour
The ability of a substance to act as both an acid and a base. Amino acids are amphoteric because -NH2 accepts protons and -COOH donates them.
Oxime
The product formed when an aldehyde or ketone reacts with hydroxylamine (H2N-OH), giving R-C=N-OH. D-glucose forms an oxime because its open-chain form contains an aldehyde group.
Fat-soluble vitamins (A, D, E, K) are absorbed with dietary fats, transported via lipoproteins and stored in the liver and adipose tissue.
Because they accumulate, excessive intake can cause toxicity (hypervitaminosis).
Vitamin D is fat-soluble and is stored in the body. The assertion that "Vitamin D cannot be stored" is false, and this is a recurring CBSE trap.
Water-soluble vitamins (B-complex, C) dissolve in blood plasma and are filtered out by the kidneys.
Vitamin C (ascorbic acid) cannot be stored; any excess is excreted in urine. This is why a regular dietary supply is necessary.
Deficiency of vitamin C causes scurvy, characterised by bleeding gums and poor wound healing.
Vitamin B1 deficiency causes beriberi; vitamin B2 deficiency causes ariboflavinosis. These are common distractors in MCQs.
Denaturation disrupts the secondary, tertiary and quaternary structures of a protein but leaves the primary structure (the peptide bond sequence) intact.
Common triggers: heat, strong acids or bases, heavy metal salts, vigorous stirring.
Examples that appear in CBSE papers:
Coagulation of egg white on boiling (heat-induced denaturation).
Curdling of milk by acid (pH-induced denaturation).
Clotting of blood is not classified as simple denaturation in the CBSE context; it is an enzymatic cascade.
A nucleotide = nitrogenous base + pentose sugar + phosphate group.
Nucleotides are joined by phosphodiester linkages: the phosphate bridges the 3' carbon of one sugar to the 5' carbon of the next.
Glycosidic linkage joins the base to the sugar within a single nucleotide (not between nucleotides).
Peptide linkage joins amino acids in proteins, not nucleotides.
Hydrogen bonding holds the two strands of DNA together (A=T, G≡C) but does not join nucleotides in a single strand.
Amino acids carry both -NH2 (basic) and -COOH (acidic) groups, making them amphoteric.
At physiological pH, they exist as zwitterions: NH3+ and COO- on the same molecule.
In acidic solution, the amino acid is fully protonated (cation). In basic solution, it is fully deprotonated (anion).
D-glucose contains an aldehyde group in its open-chain form.
Reaction with hydroxylamine (H2N-OH) produces the corresponding oxime (glucoxime), confirming the presence of the carbonyl group.
This reaction is a nucleophilic addition at the C=O, with elimination of water.
Oxime formation: R-CHO + H2N-OH --> R-CH=N-OH + H2O
Phosphodiester bond: 3'-OH of sugar(n) + 5'-phosphate of sugar(n+1), with loss of water.
Zwitterion equilibrium: NH2-CHR-COOH ⇌ NH3+-CHR-COO-
Scurvy was historically the disease of sailors on long voyages who lacked fresh fruit. The British Navy's adoption of lime juice rations in the 18th century is why British sailors were called "limeys," and it remains one of the earliest examples of nutritional science in practice.
Protein denaturation is the principle behind cooking (egg whites solidify), cheese-making (curdling milk with acid or rennet) and sterilisation (autoclaving destroys microbial proteins).
Students often think vitamin D is water-soluble because it is associated with sunlight rather than dietary fat. It is not. Vitamin D is fat-soluble and is stored in the body.
Students confuse the bond joining nucleotides (phosphodiester) with the bond joining DNA strands (hydrogen bonds). These are different: phosphodiester is the backbone, hydrogen bonding is the cross-linking.
Blood clotting is sometimes listed alongside egg-white coagulation as denaturation. In CBSE's framework, coagulation of egg white and curdling of milk are the standard examples. Blood clotting involves an enzymatic cascade and is not treated as simple denaturation.
Some students assume amino acids are always neutral. They are not: their charge depends on pH.
⚠️ Vitamin classification (fat-soluble vs water-soluble) appears almost every year as an assertion-reason MCQ. Know the storage rule cold: A, D, E, K are stored; B and C are not.
⚠️ "Nucleotides are joined by" is a standard one-mark MCQ. The answer is phosphodiester linkage. Glycosidic, peptide and hydrogen bonding are the three distractors.
⚠️ Denaturation examples: coagulation of egg white and curdling of milk. CBSE uses "Both (1) and (2)" as the correct option when both are listed.
⚠️ The reaction of D-glucose with hydroxylamine to form an oxime tests whether you know glucose has an aldehyde group. This appears in Section C (3-mark questions).
True or false: Vitamin D is a water-soluble vitamin and is excreted in urine. ______
The bond that forms the backbone of DNA is called a ______ linkage.
True or false: Curdling of milk is an example of protein denaturation. ______
Scurvy is caused by the deficiency of ______ (vitamin name and chemical name).
An amino acid at physiological pH exists primarily as a ______.
Q: Which of the following is/are examples of denaturation of protein? (a) Coagulation of egg white (b) Curdling of milk (c) Clotting of blood (d) Both (a) and (b)
A: (d) Both (a) and (b). Coagulation of egg white is heat-induced denaturation; curdling of milk is acid/enzyme-induced denaturation. Blood clotting is an enzymatic cascade, not simple denaturation.
Q: Nucleotides are joined together by which type of linkage?
A: Phosphodiester linkage. A phosphate group bridges the 3' carbon of one sugar to the 5' carbon of the next.
Q: Why can vitamin C not be stored in our body?
A: Vitamin C is water-soluble. The body lacks mechanisms to store water-soluble vitamins; excess is excreted in urine. A regular dietary intake is therefore essential.
Q: Amino acids show amphoteric behaviour. Explain why.
A: Amino acids contain both an amino group (-NH2), which can accept protons (act as a base), and a carboxyl group (-COOH), which can donate protons (act as an acid). This dual capability makes them amphoteric.
Q: Write the product obtained when D-glucose reacts with hydroxylamine (H2N-OH).
A: D-glucose forms a glucoxime (an oxime). The aldehyde group of the open-chain form undergoes nucleophilic addition with hydroxylamine, producing R-CH=N-OH + H2O.
This connects to Organic Chemistry (Ch. 12, Aldehydes and Ketones) because glucose's reactions with hydroxylamine, HCN and Br2 water all depend on the aldehyde functional group. Understanding carbonyl reactivity from that chapter makes biomolecule reactions straightforward.
The vitamin classification ties into Solutions and Colligative Properties: water-soluble vitamins dissolve freely and contribute to osmotic effects in biological fluids, while fat-soluble ones are transported in lipid-based structures.
Tags: biomolecules, vitamins, fat-soluble vitamins, water-soluble vitamins, vitamin C, ascorbic acid, scurvy, denaturation, protein denaturation, coagulation, curdling, nucleotide, phosphodiester linkage, DNA backbone, amino acid, zwitterion, amphoteric, oxime, hydroxylamine, D-glucose, CBSE Class 12 Chemistry, Chapter 14