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Biomolecules looks like the easiest chapter in Class 12 chemistry: few equations, a lot of reading. Then a question asks which disaccharide reduces Tollens' reagent, or how many water molecules a pentapeptide loses, and the marks slip away. This post covers carbohydrates, proteins and nucleic acids with small calculations, the slips examiners look for, and a one-week revision plan.
What does the syllabus expect from Biomolecules?
The JEE (Main) 2026 syllabus lists Unit 19, Biomolecules: classification of carbohydrates, aldoses and ketoses, glucose and fructose, the monosaccharides that make up sucrose, lactose and maltose, the elementary idea of amino acids, the peptide bond, the four levels of protein structure, denaturation, enzymes, vitamins and the chemical make-up of DNA and RNA. All of this is in the NCERT Class 12 chapter on Biomolecules. Check the syllabus for your exam year, because NTA publishes a new one each cycle.
How do you know glucose is a straight-chain aldehyde?
NCERT gives a chain of evidence, and examiners like to ask for one link in it:
Prolonged heating with HI gives n-hexane, so the six carbons are in a straight chain.
Glucose forms an oxime with hydroxylamine, so it has a carbonyl group.
Bromine water, a mild oxidising agent, turns it into gluconic acid with the same six carbons, so the carbonyl is an aldehyde.
Acetylation gives a pentaacetate, so there are five -OH groups.
Learn these as reasons, not a list, and you can answer the question in any wording.
Which sugars are reducing, and why is sucrose not?
A reducing sugar reduces Fehling's solution and Tollens' reagent. NCERT states that all monosaccharides, aldose or ketose, are reducing sugars. For disaccharides, the question is whether a free aldehyde or ketone group can form.
Sucrose is glucose and fructose joined between C1 of glucose and C2 of fructose. Both reducing groups are used in the bond, so sucrose is non-reducing.
Maltose is two glucose units joined C1 to C4. The second glucose can still open at C1, so maltose is reducing.
Lactose is galactose and glucose joined C1 to C4. The glucose unit can open at C1, so lactose is reducing.
Worked example: invert sugar. Sucrose is dextrorotatory. Hydrolysis gives one glucose and one fructose for each sucrose. NCERT gives specific rotations of +52.5° for glucose and -92.4° for fructose. The two have the same molar mass, so equal moles means equal masses, and the mixture's specific rotation is roughly the average: (52.5 - 92.4) / 2 = -19.95°, about -20°. The sign has flipped from plus to minus, which is why the product is called invert sugar.
How many peptide bonds does a protein chain have?
A peptide bond is an amide, -CO-NH-, formed between the carboxyl group of one amino acid and the amino group of the next, with loss of water. A tripeptide has two. In general, n amino acids give n - 1 peptide bonds and lose n - 1 water molecules.
Worked example. Glycine (C2H5NO2, about 75.07 g/mol) and alanine (C3H7NO2, about 89.09 g/mol) join to make the dipeptide glycylalanine. One water molecule (18.02 g/mol) is lost, so the molar mass of the dipeptide is 75.07 + 89.09 - 18.02 = 146.14 g/mol. For a pentapeptide, count four peptide bonds and four waters, not five.
What does denaturation change, and what does it leave alone?
NCERT describes four levels of structure: primary (the sequence), secondary (the alpha-helix and beta-pleated sheet held by hydrogen bonds), tertiary (the overall fold) and quaternary (how several chains fit together). The alpha-helix was proposed by Pauling, Corey and Branson in 1951 as one of two hydrogen-bonded helical shapes for a polypeptide chain.
During denaturation, heat or a change in pH disturbs the hydrogen bonds. NCERT is precise: secondary and tertiary structures are destroyed, but the primary structure stays intact. Boiling an egg and the curdling of milk are its examples. The peptide bonds are not broken.
Proteins are not frozen shapes either. In a review of ten common misconceptions about proteins, Srebrenka Robic notes that students may think a protein stays fully folded all its life, when proteins in fact fluctuate and can unfold.
How do DNA and RNA differ, and how does base pairing give numbers?
Watson and Crick published the double helix in 1953, and shared the 1962 Nobel Prize in Physiology or Medicine with Wilkins. For the exam, keep the differences in one list:
Sugar: DNA has beta-D-2-deoxyribose, RNA has beta-D-ribose.
Bases: both have adenine, guanine and cytosine; DNA has thymine, RNA has uracil.
Strands: DNA is a double helix; RNA is usually a single strand that can fold back on itself.
Nucleoside is base plus sugar. Nucleotide is a nucleoside plus phosphate at the 5' position.
Worked example. In double-stranded DNA, adenine pairs with thymine and cytosine with guanine. If 20% of the bases are adenine, then thymine is also 20%. That leaves 60% for guanine and cytosine together, so each is 30%. This works only for double-stranded DNA, not for single-stranded RNA.
Common slips and how to avoid them
Calling sucrose reducing because it is a sugar. Check the bond.
Counting n peptide bonds for n amino acids. It is n - 1.
Writing that denaturation breaks peptide bonds. It disrupts the folding.
Putting uracil in DNA, or confusing nucleoside with nucleotide.
Mixing up vitamin solubility: A, D, E and K are fat-soluble; B-group and C are water-soluble.
A one-week revision routine
Day 1: Write the glucose evidence chain from memory, with reasons.
Day 2: Draw the sucrose, maltose and lactose links and mark each reducing or not.
Day 3: Ten peptide counting and molar mass questions.
Day 4: Explain protein structure levels and denaturation aloud.
Day 5: The DNA and RNA list, plus five base-percentage questions.
Days 6 and 7: Mixed past questions; fix every error at the level of the reason.
For the biology side of the same molecules, see our post on the molecular basis of inheritance. If one idea will not stick, such as what makes a carbon anomeric, the Fermi Concept Library covers every concept in chemistry and biology, each with its own lesson.
Tonight, close the book and write the reducing or non-reducing verdict for sucrose, maltose and lactose, with one line of reasoning each.
