Chapter 9: Biomolecules – Long Answer Type Questions
CBSE Class 11 Biology Long Answer Questions – Biomolecules (Chapter 9 | NCERT)
Course Details (Systematic Presentation)
Course: CBSE Class 11 Biology
Unit: III – Cell Structure and Function
Chapter: 9 – Biomolecules
Based on: NCERT Syllabus
Examination: CBSE Board Examination (Class XI)
Assessment Focus: Conceptual clarity, chemical nature, structures, and functions
Section A: Carbohydrates (Q1–Q5)
Q1. Describe the classification of carbohydrates with suitable examples.
Answer:
Carbohydrates are organic biomolecules composed of carbon, hydrogen, and oxygen, usually in the ratio (CH₂O)ₙ. Based on the number of sugar units, they are classified into monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are the simplest carbohydrates and cannot be hydrolysed further, for example glucose, fructose, and ribose. Oligosaccharides contain two to ten monosaccharide units; disaccharides like sucrose, lactose, and maltose are common examples. Polysaccharides are complex carbohydrates formed by polymerisation of many monosaccharide units, such as starch, cellulose, and glycogen. While starch and glycogen serve as energy storage molecules in plants and animals respectively, cellulose provides structural strength to plant cell walls. Thus, carbohydrates play diverse structural and functional roles in living organisms.
Q2. Explain the structure and biological significance of glucose.
Answer:
Glucose is a monosaccharide belonging to the group of aldohexoses with the molecular formula C₆H₁₂O₆. It exists in both linear and cyclic forms, but in aqueous solutions, the cyclic form predominates. In this form, glucose forms a six-membered ring known as glucopyranose. Glucose is a reducing sugar due to the presence of a free aldehyde group in its open-chain form. Biologically, glucose is the primary source of energy for living cells. It is oxidised during cellular respiration to release energy in the form of ATP. Glucose also acts as a building block for disaccharides like sucrose and polysaccharides such as starch, cellulose, and glycogen, making it central to carbohydrate metabolism.
Q3. Differentiate between starch, glycogen, and cellulose.
Answer:
Starch, glycogen, and cellulose are polysaccharides composed of glucose units, but they differ in structure and function. Starch is the primary storage carbohydrate in plants and consists of amylose and amylopectin, both made of α-glucose units. Glycogen is the storage polysaccharide in animals and is highly branched, allowing rapid release of glucose when energy is needed. Cellulose, in contrast, is a structural polysaccharide found in plant cell walls and is composed of β-glucose units linked by β-1,4 glycosidic bonds. Due to this linkage, cellulose forms strong fibres and cannot be digested by humans. These differences highlight how structural variations determine biological roles.
Q4. What are glycosidic bonds? Explain their formation.
Answer:
Glycosidic bonds are covalent bonds that link monosaccharide units to form disaccharides and polysaccharides. They are formed through a condensation reaction between the hydroxyl group of one monosaccharide and the hydroxyl group of another, resulting in the release of a water molecule. Depending on the carbon atoms involved and the orientation of the bond, glycosidic linkages may be α or β type. For example, in maltose, two glucose molecules are linked by an α-1,4 glycosidic bond, while in cellulose, β-1,4 linkages occur. These bonds play a crucial role in determining the structure, digestibility, and function of carbohydrates.
Q5. Explain the biological importance of polysaccharides.
Answer:
Polysaccharides perform essential structural and storage functions in living organisms. Storage polysaccharides like starch in plants and glycogen in animals serve as reservoirs of energy that can be mobilised when required. Structural polysaccharides such as cellulose in plants and chitin in arthropods provide mechanical strength and protection. Polysaccharides are generally insoluble, making them suitable for storage without affecting osmotic balance. They also contribute to dietary fibre in humans, aiding digestion and maintaining gut health. Thus, polysaccharides are vital for maintaining cellular structure, energy balance, and overall physiological stability.
Section B: Proteins (Q6–Q10)
Q6. Describe the structure of amino acids and peptide bond formation.
Answer:
Amino acids are the basic building blocks of proteins. Each amino acid contains a central carbon atom attached to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain or R group. The R group determines the chemical nature and properties of the amino acid. Amino acids are linked together by peptide bonds to form polypeptide chains. A peptide bond is formed when the carboxyl group of one amino acid reacts with the amino group of another through a condensation reaction, releasing a molecule of water. This linkage forms the backbone of protein structure and determines the primary structure of proteins.
Q7. Explain the levels of protein structure.
Answer:
Proteins exhibit four levels of structural organisation. The primary structure refers to the linear sequence of amino acids in a polypeptide chain. The secondary structure involves folding of the chain into α-helices or β-pleated sheets, stabilised by hydrogen bonds. The tertiary structure represents the three-dimensional folding of the protein, maintained by interactions such as hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. Some proteins also exhibit a quaternary structure, formed by the association of two or more polypeptide chains. These structural levels collectively determine the specific shape and biological function of proteins.
Q8. Differentiate between fibrous and globular proteins with examples.
Answer:
Fibrous and globular proteins differ in structure, solubility, and function. Fibrous proteins are elongated, insoluble in water, and primarily serve structural roles. Examples include keratin in hair and nails, and collagen in connective tissues. Globular proteins are compact, spherical, and generally soluble in water. They are involved in metabolic and regulatory functions. Examples include enzymes, hemoglobin, and hormones like insulin. The difference in their structure is due to the arrangement of amino acids and folding patterns, which directly influence their functional roles in living organisms.
Q9. What is protein denaturation? Explain its causes and effects.
Answer:
Protein denaturation refers to the loss of a protein’s native three-dimensional structure without breaking peptide bonds. It is caused by factors such as high temperature, extreme pH, heavy metals, or chemical agents. Denaturation disrupts secondary and tertiary structures by breaking hydrogen bonds and other weak interactions. As a result, the protein loses its specific shape and biological activity. For example, heating egg white causes denaturation of albumin, leading to coagulation. Denaturation highlights the importance of protein structure in maintaining biological function.
Q10. Explain the biological functions of proteins.
Answer:
Proteins perform a wide range of biological functions essential for life. They act as enzymes, catalysing biochemical reactions and regulating metabolic pathways. Structural proteins provide support and shape to cells and tissues. Transport proteins like hemoglobin carry molecules such as oxygen. Some proteins function as hormones, regulating physiological processes, while others act as antibodies involved in immune defense. Proteins also play roles in cell signaling, movement, and storage. Their functional diversity arises from variations in amino acid sequences and three-dimensional structures.
Section C: Lipids (Q11–Q15)
Q11. Define lipids and describe their general characteristics.
Answer:
Lipids are a diverse group of organic compounds that are insoluble in water but soluble in organic solvents such as ether and chloroform. They include fats, oils, phospholipids, waxes, and steroids. Lipids are rich in carbon and hydrogen and yield more energy upon oxidation than carbohydrates. They are hydrophobic or amphipathic in nature. Lipids serve as energy reserves, structural components of cell membranes, and protective layers. Due to their insolubility, lipids are not polymers like proteins or carbohydrates but play equally vital biological roles.
Q12. Explain the structure and functions of triglycerides.
Answer:
Triglycerides are the most common form of lipids found in living organisms. They are formed by the esterification of one glycerol molecule with three fatty acid molecules. Fatty acids may be saturated or unsaturated, influencing the physical properties of triglycerides. Triglycerides function primarily as long-term energy storage molecules. In animals, they are stored in adipose tissue, providing insulation and protection to organs. In plants, triglycerides are stored in seeds and serve as energy sources during germination.
Q13. Distinguish between saturated and unsaturated fatty acids.
Answer:
Saturated fatty acids contain only single bonds between carbon atoms and are fully saturated with hydrogen. They are usually solid at room temperature, such as fats found in butter. Unsaturated fatty acids contain one or more double bonds between carbon atoms, causing bends in their structure. They are generally liquid at room temperature, like oils. Unsaturated fatty acids are considered healthier due to their role in maintaining membrane fluidity and reducing cardiovascular risks.
Q14. Describe the role of phospholipids in cell membranes.
Answer:
Phospholipids are major structural components of biological membranes. Each phospholipid molecule has a hydrophilic head and two hydrophobic tails, making it amphipathic. In aqueous environments, phospholipids arrange themselves into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward. This arrangement forms a selectively permeable membrane that separates the cell from its surroundings. Phospholipids provide membrane fluidity, flexibility, and structural integrity, which are essential for proper cellular function.
Q15. Explain the biological significance of cholesterol.
Answer:
Cholesterol is a steroid lipid found in animal cell membranes. It plays a crucial role in maintaining membrane stability and fluidity, especially at varying temperatures. Cholesterol is also a precursor for the synthesis of steroid hormones, bile acids, and vitamin D. Although excessive cholesterol levels are associated with health risks, it is an essential biomolecule required for normal cellular and physiological functions.
Section D: Nucleic Acids (Q16–Q20)
Q16. Describe the structure of nucleic acids.
Answer:
Nucleic acids are polymers made of nucleotide units. Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group. Nucleotides are linked by phosphodiester bonds forming a sugar-phosphate backbone. DNA contains deoxyribose sugar and bases adenine, guanine, cytosine, and thymine, while RNA contains ribose sugar and uracil instead of thymine. The specific sequence of bases stores genetic information essential for heredity and protein synthesis.
Q17. Differentiate between DNA and RNA.
Answer:
DNA and RNA differ in structure and function. DNA contains deoxyribose sugar and thymine and is usually double-stranded, forming a double helix. RNA contains ribose sugar and uracil and is generally single-stranded. DNA stores genetic information, whereas RNA plays an active role in protein synthesis. These differences allow DNA to act as a stable genetic material and RNA to function as a messenger and translator of genetic information.
Q18. Explain complementary base pairing in DNA.
Answer:
Complementary base pairing refers to the specific pairing of nitrogenous bases in DNA. Adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. This specificity ensures accurate replication and transcription of genetic information. Complementary base pairing also maintains uniform width of the DNA double helix, contributing to its structural stability.
Q19. What is the role of RNA in protein synthesis?
Answer:
RNA plays a central role in protein synthesis. Messenger RNA carries genetic information from DNA to ribosomes. Transfer RNA brings specific amino acids to the ribosome, while ribosomal RNA forms the structural and catalytic core of ribosomes. Together, these RNA molecules ensure accurate translation of genetic code into functional proteins.
Q20. Explain the concept of genome.
Answer:
A genome is the complete set of genetic material present in an organism. It includes all DNA sequences that encode structural proteins, enzymes, and regulatory elements. The genome determines an organism’s characteristics and governs cellular activities. Understanding the genome is essential for studying heredity, variation, and evolution.
Section E: Enzymes and Metabolism (Q21–Q25)
Q21. Define enzymes and explain their properties.
Answer:
Enzymes are biological catalysts, mostly proteins, that increase the rate of biochemical reactions without being consumed. They are highly specific, reusable, and efficient. Enzymes function under optimum temperature and pH conditions and can be denatured under extreme conditions. Their specificity arises from the unique shape of the active site.
Q22. Explain the mechanism of enzyme action.
Answer:
Enzymes bind substrates at their active site to form an enzyme-substrate complex. According to the lock-and-key and induced-fit models, this interaction lowers activation energy, facilitating the reaction. After the product is formed, it is released, and the enzyme is free to catalyse another reaction.
Q23. What is metabolism? Explain its types.
Answer:
Metabolism is the sum total of all biochemical reactions occurring in a living organism. It includes anabolism, which involves synthesis of complex molecules requiring energy, and catabolism, which involves breakdown of molecules releasing energy. Both processes are essential for growth, repair, and maintenance.
Q24. Explain the role of ATP in metabolism.
Answer:
ATP acts as the universal energy currency of the cell. It stores energy in its high-energy phosphate bonds and releases it during metabolic reactions. ATP links energy-releasing and energy-requiring processes, making metabolism efficient and regulated.
Q25. Why are enzymes essential for life processes?
Answer:
Enzymes make biochemical reactions possible under normal cellular conditions by reducing activation energy. Without enzymes, metabolic reactions would occur too slowly to sustain life. They ensure regulation, coordination, and efficiency of cellular metabolism.