CBSE Class 12 Chemistry (2026–27)
Chapter 11: Biomolecules
20 Important Questions with Answers
1. What are biomolecules? Classify them with examples.
Answer:
Biomolecules are naturally occurring organic compounds present in living organisms. They are essential for the structure and functioning of cells. Biomolecules are mainly classified into carbohydrates, proteins, nucleic acids, lipids, vitamins, and hormones. Carbohydrates such as glucose and starch serve as energy sources. Proteins like enzymes and hemoglobin perform structural and functional roles. Nucleic acids, DNA and RNA, store and transmit genetic information. Lipids include fats and oils that act as energy reserves. Vitamins regulate metabolic activities. Biomolecules are vital for growth, repair, reproduction, and maintenance of life processes in plants and animals.
2. Differentiate between monosaccharides and disaccharides.
Answer:
Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed into smaller carbohydrate units. Examples include glucose, fructose, and ribose. They are sweet, water-soluble, and serve as immediate energy sources. Disaccharides are carbohydrates formed by the condensation of two monosaccharide units linked through a glycosidic bond. Examples include sucrose, maltose, and lactose. On hydrolysis, disaccharides yield two monosaccharide molecules. Monosaccharides generally have the molecular formula (CH₂O)n, whereas disaccharides have more complex structures. Both play important biological roles, but monosaccharides are absorbed directly while disaccharides must first be hydrolyzed during digestion.
3. Explain the structure and importance of glucose.
Answer:
Glucose is an aldohexose sugar with the molecular formula C₆H₁₂O₆. It exists in both open-chain and cyclic forms. The cyclic structure is formed by the reaction between the aldehyde group and a hydroxyl group within the molecule. Glucose is the primary source of energy for living cells and is produced during photosynthesis. It is transported through blood and oxidized during respiration to release energy. Glucose also serves as a building block for larger carbohydrates such as starch, glycogen, and cellulose. Its structure was established through various chemical reactions and stereochemical studies.
4. What is mutarotation? Explain with reference to glucose.
Answer:
Mutarotation is the change in the specific rotation of an optically active sugar solution until it reaches a constant value. In glucose, this phenomenon occurs due to the interconversion of α-glucose and β-glucose through the open-chain form in aqueous solution. Freshly prepared α-D-glucose has a specific rotation of +112°, while β-D-glucose has +19°. Both forms gradually attain an equilibrium mixture with a constant rotation of about +52.7°. Mutarotation provides evidence for the cyclic structure of glucose and demonstrates the dynamic equilibrium between its different anomeric forms in solution.
5. What are reducing and non-reducing sugars? Give examples.
Answer:
Reducing sugars are carbohydrates that contain a free aldehyde or ketone group capable of reducing mild oxidizing agents such as Tollens’ reagent and Fehling’s solution. Examples include glucose, fructose, maltose, and lactose. Non-reducing sugars do not possess a free carbonyl group because it is involved in glycosidic bond formation. Therefore, they cannot reduce these reagents. Sucrose is the most common example of a non-reducing sugar. The distinction between reducing and non-reducing sugars is important in biochemical analysis and food chemistry, as reducing sugars participate in several chemical and biological reactions.
6. Describe the structure and biological significance of starch.
Answer:
Starch is a polysaccharide composed of α-D-glucose units. It is the major storage carbohydrate in plants. Starch consists of two components: amylose and amylopectin. Amylose is a linear polymer with α(1→4) glycosidic linkages, whereas amylopectin is a branched polymer containing both α(1→4) and α(1→6) linkages. Starch is insoluble in cold water and gives a blue color with iodine solution. It serves as an important source of dietary energy for humans and animals. During digestion, starch is hydrolyzed into glucose, which is utilized in cellular respiration.
7. What is cellulose? How does it differ from starch?
Answer:
Cellulose is a natural polysaccharide composed of β-D-glucose units linked by β(1→4) glycosidic bonds. It forms the structural component of plant cell walls and is the most abundant organic compound on Earth. Unlike starch, cellulose has a straight-chain structure with strong intermolecular hydrogen bonding. Starch contains α-glucose units and serves as a storage carbohydrate, while cellulose provides structural support. Humans cannot digest cellulose because they lack the enzyme cellulase. In contrast, starch can be digested and used as an energy source. Thus, cellulose acts mainly as dietary fiber in humans.
8. Explain glycogen and its role in living organisms.
Answer:
Glycogen is a highly branched polysaccharide composed of α-D-glucose units. It serves as the primary storage form of carbohydrates in animals and is mainly found in the liver and muscles. Glycogen contains α(1→4) glycosidic bonds in the main chain and α(1→6) linkages at branch points. Due to its branched structure, glycogen can be rapidly broken down to release glucose when energy is required. It helps maintain blood glucose levels and provides a readily available energy reserve. Glycogen is often referred to as “animal starch” because of its similarity to amylopectin.
9. What are amino acids? Why are they called zwitterions?
Answer:
Amino acids are organic compounds containing both an amino group (–NH₂) and a carboxyl group (–COOH). They are the basic building blocks of proteins. In aqueous solution, amino acids exist predominantly as zwitterions, which contain both a positively charged ammonium group (–NH₃⁺) and a negatively charged carboxylate group (–COO⁻). This internal ionic form results from the transfer of a proton from the carboxyl group to the amino group. Due to their zwitterionic nature, amino acids exhibit high melting points, are soluble in water, and show amphoteric behavior, acting as both acids and bases.
10. What is a peptide bond? How is it formed?
Answer:
A peptide bond is the amide linkage formed between the carboxyl group of one amino acid and the amino group of another amino acid. During this reaction, a molecule of water is eliminated, making it a condensation reaction. The resulting linkage is represented as –CO–NH–. Two amino acids joined by one peptide bond form a dipeptide, while many amino acids linked together form polypeptides and proteins. Peptide bonds are strong and stable, providing structural integrity to proteins. The sequence of amino acids connected through peptide bonds determines the unique properties and biological functions of proteins.
11. Differentiate between fibrous and globular proteins.
Answer:
Fibrous proteins have long, thread-like structures and mainly perform structural functions. They are generally insoluble in water. Examples include keratin in hair and collagen in connective tissues. Globular proteins have compact, spherical structures and are usually soluble in water. They perform dynamic biological functions such as catalysis, transport, and regulation. Examples include hemoglobin, enzymes, and insulin. Fibrous proteins provide strength and support, whereas globular proteins participate in metabolic activities. The difference in shape and solubility is due to the arrangement of amino acid chains and the extent of folding in the protein structure.
12. Explain the primary and secondary structures of proteins.
Answer:
The primary structure of a protein refers to the specific sequence of amino acids linked by peptide bonds in a polypeptide chain. This sequence determines the protein’s properties and biological function. The secondary structure arises due to hydrogen bonding between peptide groups within the chain. Common secondary structures include the α-helix and β-pleated sheet. These arrangements provide stability and shape to the protein molecule. Any alteration in the primary structure can affect the secondary structure and ultimately the function of the protein. Understanding protein structure is essential in biochemistry and molecular biology.
13. What is denaturation of proteins?
Answer:
Denaturation is the process by which a protein loses its native structure and biological activity due to external factors such as heat, changes in pH, chemicals, or radiation. During denaturation, the secondary, tertiary, and quaternary structures are disrupted, but the primary structure remains unchanged because peptide bonds are not broken. Examples include the coagulation of egg white during cooking and curdling of milk. Denatured proteins often become insoluble and lose their functional properties. Denaturation demonstrates the importance of protein structure in maintaining biological activity and proper cellular function.
14. What are enzymes? Mention their characteristics.
Answer:
Enzymes are biological catalysts that accelerate biochemical reactions without being consumed in the process. Most enzymes are proteins produced by living cells. They exhibit high specificity, meaning each enzyme acts on a particular substrate. Enzymes function efficiently under mild temperature and pH conditions and greatly increase reaction rates by lowering activation energy. They are required in small amounts and remain unchanged after the reaction. Enzyme activity can be influenced by factors such as temperature, pH, and inhibitors. Examples include amylase, pepsin, and urease. Enzymes play crucial roles in digestion, metabolism, and cellular regulation.
15. What are vitamins? Classify them.
Answer:
Vitamins are organic compounds required in small quantities for normal growth, metabolism, and maintenance of health. They are classified into two groups based on solubility. Fat-soluble vitamins include A, D, E, and K, which are stored in body tissues. Water-soluble vitamins include the B-complex group and vitamin C, which are not stored extensively and must be supplied regularly through diet. Deficiency of vitamins leads to various diseases such as night blindness, scurvy, and rickets. Vitamins act as coenzymes or regulators in many biochemical processes and are essential for maintaining overall health.
16. What is the difference between DNA and RNA?
Answer:
DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are nucleic acids involved in genetic processes. DNA contains deoxyribose sugar and the nitrogenous bases adenine, guanine, cytosine, and thymine. RNA contains ribose sugar and uracil instead of thymine. DNA is usually double-stranded and stores genetic information, whereas RNA is generally single-stranded and participates in protein synthesis. DNA is mainly found in the nucleus, while RNA occurs in both the nucleus and cytoplasm. Together, DNA and RNA ensure the storage, transmission, and expression of hereditary information in living organisms.
17. What are nucleosides and nucleotides?
Answer:
A nucleoside consists of a nitrogenous base attached to a pentose sugar through a glycosidic bond. Examples include adenosine and guanosine. When one or more phosphate groups are attached to a nucleoside, it forms a nucleotide. Nucleotides are the basic structural units of nucleic acids such as DNA and RNA. Besides their role in genetic material, nucleotides also function in energy transfer and cellular signaling. ATP (adenosine triphosphate) is a well-known nucleotide that serves as the energy currency of cells. Thus, nucleotides are biologically more complex and functional than nucleosides.
18. Explain the biological importance of carbohydrates.
Answer:
Carbohydrates are one of the most important biomolecules in living organisms. They serve as the primary source of energy, particularly glucose, which fuels cellular activities. Carbohydrates also act as storage compounds in the form of starch in plants and glycogen in animals. Structural carbohydrates such as cellulose provide strength to plant cell walls. Certain carbohydrates participate in cell recognition and communication processes. They are components of nucleic acids and several coenzymes. Adequate carbohydrate intake is essential for maintaining energy balance, supporting metabolism, and ensuring proper functioning of tissues and organs.
19. Why are proteins considered essential biomolecules?
Answer:
Proteins are essential biomolecules because they perform a wide range of biological functions. They act as enzymes, hormones, antibodies, transport molecules, and structural components of cells and tissues. Proteins are involved in growth, repair, and maintenance of body structures. Hemoglobin transports oxygen, antibodies provide immunity, and enzymes regulate metabolic reactions. Proteins are synthesized from amino acids according to genetic instructions encoded in DNA. Since many body functions depend on proteins, a deficiency can lead to impaired growth and health problems. Their versatility and functional diversity make proteins indispensable for life.
20. Explain the role of biomolecules in maintaining life processes.
Answer:
Biomolecules collectively support all life processes in living organisms. Carbohydrates provide energy, while lipids serve as energy reserves and components of cell membranes. Proteins perform structural, catalytic, and regulatory functions. Nucleic acids store and transmit genetic information necessary for inheritance and protein synthesis. Vitamins and hormones regulate metabolism and physiological activities. Biomolecules interact in a coordinated manner to ensure growth, reproduction, movement, immunity, and maintenance of cellular functions. Without biomolecules, essential biological reactions would not occur. Therefore, they form the chemical basis of life and are fundamental to the survival of all organisms.