CBSE Important Questions Class 12 Chemistry Chapter 10 – Biomolecules

Class 12 Chemistry Chapter 10 Biomolecules is part of the CBSE Class 12 Chemistry syllabus. Biomolecules are the compounds that make up living things and keep them working. Students learn about carbohydrates and their classification, proteins and amino acids, the structure of proteins, enzymes and how they work, vitamins and their deficiency diseases, and nucleic acids DNA and RNA.

These CBSE important questions for Class 12 Chemistry Chapter 10 cover the classification of carbohydrates, reducing and non-reducing sugars, the structure of glucose, the peptide bond and the levels of protein structure, denaturation, the difference between essential and non-essential amino acids, the role of enzymes, vitamins and their deficiency diseases, and the differences between DNA and RNA. Every question has a clear step-by-step solution in simple English, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. How are carbohydrates classified? Give one example of each class.

Answer: Carbohydrates are classified by how many simple sugar units they give on hydrolysis.
• Monosaccharides: cannot be hydrolysed any further. Examples: glucose, fructose, ribose.
• Oligosaccharides: give 2 to 10 monosaccharide units. Examples: sucrose and maltose (both give 2 units), raffinose (3 units).
• Polysaccharides: give a very large number of monosaccharide units. Examples: starch, cellulose, glycogen.
Carbohydrates are also classified by taste as sugars (sweet and crystalline, such as glucose and sucrose) and non-sugars (tasteless and amorphous, such as starch and cellulose).

Q2. What are reducing and non-reducing sugars? Give one example of each.

Answer: A reducing sugar is one that reduces Tollen's reagent and Fehling's solution. It has a free aldehyde or ketone group available.
Examples: all monosaccharides (glucose, fructose), and the disaccharides maltose and lactose.
A non-reducing sugar does not reduce these reagents, because its aldehyde and ketone groups are locked up in the glycosidic linkage and are not free.
Example: sucrose. In sucrose, the C1 of glucose is joined to the C2 of fructose, so both reactive groups are tied up.
On hydrolysis, however, sucrose gives glucose and fructose, which are reducing. This mixture is called invert sugar.

Q3. What is a peptide bond? Draw how two amino acids join.

Answer: A peptide bond is the −CO−NH− linkage formed when the −COOH group of one amino acid reacts with the −NH₂ group of another, with the loss of a molecule of water.
H₂N-CHR-COOH + H₂N-CHR′-COOH → H₂N-CHR-CO-NH-CHR′-COOH + H₂O
The product of two amino acids is a dipeptide, of three a tripeptide, and of many a polypeptide. When a polypeptide has more than about a hundred residues and a definite three-dimensional shape, it is called a protein.

Q4. Explain the primary, secondary, tertiary and quaternary structures of proteins.

Answer: • Primary structure: the exact sequence of amino acids in the polypeptide chain. Even a single change in this sequence can completely alter the protein, as happens in sickle cell anaemia.
• Secondary structure: the way the chain folds or coils, held by hydrogen bonds. The two common forms are the α-helix and the β-pleated sheet.
• Tertiary structure: the overall three-dimensional shape of the whole chain, held by hydrogen bonds, disulphide bridges, ionic bonds and van der Waals forces. It gives the fibrous or globular shape.
• Quaternary structure: the arrangement of two or more polypeptide chains together. Haemoglobin, with its four chains, is the standard example.

Q5. What is denaturation of proteins? Give two examples from daily life.

Answer: Denaturation is the loss of the biological activity of a protein when its secondary and tertiary structures are destroyed by heat, acid, alkali or heavy metal ions.
Importantly, the primary structure stays intact — only the folding is lost. The globular protein uncoils and becomes fibrous, and it can no longer do its job.
Two everyday examples:
1. Boiling an egg: the clear liquid egg white (albumin) turns into a white solid.
2. Curdling of milk: when curd is set or lemon juice is added, the milk protein casein denatures and separates out.
Denaturation is usually irreversible — a boiled egg cannot be turned back.

Q6. What are enzymes? Give two characteristics and one example.

Answer: Enzymes are biological catalysts. Chemically they are globular proteins that speed up the chemical reactions going on in living cells.
Two characteristics:
1. Highly specific: each enzyme catalyses only one particular reaction, or one type of reaction. This is often described as a lock and key fit.
2. Very efficient and work in mild conditions: they speed up reactions by a factor of 10⁶ to 10¹² and work at body temperature and neutral pH, where ordinary catalysts would need high heat and pressure.
Example: maltase, which catalyses the hydrolysis of maltose into glucose. Others are urease, which breaks down urea, and amylase in our saliva.

Q7. What are vitamins? Name two fat-soluble and two water-soluble vitamins with their deficiency diseases.

Answer: Vitamins are organic compounds needed in very small amounts in our diet. The body cannot make them (or makes too little), yet they are essential for normal growth and health.
Fat-soluble (A, D, E, K):
• Vitamin A — deficiency causes night blindness and xerophthalmia
• Vitamin D — deficiency causes rickets in children and osteomalacia in adults
Water-soluble (B group and C):
• Vitamin B₁ (thiamine) — deficiency causes beri-beri
• Vitamin C (ascorbic acid) — deficiency causes scurvy
Note that fat-soluble vitamins are stored in the liver, while water-soluble ones are not stored and must be taken regularly.

Q8. What is the difference between DNA and RNA?

DNA RNA
Sugar is 2-deoxy-D-ribose Sugar is D-ribose
Bases are A, G, C and thymine (T) Bases are A, G, C and uracil (U)
Double stranded helix Usually single stranded
Found mainly in the nucleus Found mainly in the cytoplasm
Carries and passes on hereditary information Carries out protein synthesis
Can replicate itself Does not replicate; made from DNA

Q9. What are essential and non-essential amino acids? Give one example of each.

Answer: Essential amino acids are those the body cannot make on its own, so they must be supplied through the diet. There are about ten of them.
Examples: valine, leucine, lysine, phenylalanine.
Non-essential amino acids are those the body can make for itself, so they need not come from food.
Examples: glycine, alanine, glutamic acid.
A diet short of essential amino acids leads to protein deficiency diseases such as kwashiorkor and marasmus, which is why a balanced diet matters.

Q10. What are the structural features of glucose that explain its reactions? Name two of its open chain reactions.

Answer: Glucose is D-(+)-glucose, an aldohexose with the formula C₆H₁₂O₆. Its open chain structure has one −CHO group, five −OH groups and a straight chain of six carbons.
Two reactions that prove this:
1. With hydroxylamine: glucose forms an oxime, which shows the presence of a carbonyl (−CHO) group.
2. With acetic anhydride: glucose gives glucose pentaacetate, which shows that there are exactly five −OH groups.
Also, on prolonged heating with HI it gives n-hexane, showing that all six carbons are in a straight chain.
What the open chain cannot explain: glucose does not give the Schiff's test and does not react with NaHSO₃, which led chemists to propose the cyclic pyranose structure.

Q11. What is the difference between starch and cellulose?

Answer: Both are polysaccharides made entirely of glucose units, but they differ in the way those units are joined.
• Starch has α-glycosidic linkages. It consists of amylose (a straight chain) and amylopectin (a branched chain). Our digestive enzymes can break α-linkages, so starch is our main source of food energy.
• Cellulose has β-glycosidic linkages and is a straight chain with no branching. Human beings have no enzyme that can break the β-linkage, so we cannot digest cellulose. It passes through as dietary fibre.
Cattle and other ruminants can digest cellulose because the bacteria in their stomachs produce the necessary enzyme.

Q12. What are nucleotides and nucleosides? How do they differ?

Answer: • A nucleoside is made of a nitrogenous base joined to a sugar (ribose or deoxyribose) at the C1 position.
Nucleoside = base + sugar
• A nucleotide is a nucleoside with a phosphate group attached as well, at the C5 position of the sugar.
Nucleotide = base + sugar + phosphate
The difference is therefore simply the phosphate group.
Nucleotides are the building blocks of nucleic acids: many of them join through phosphodiester linkages to form the long chains of DNA and RNA.

Related Study Material on Extramarks