NCERT Solutions for Class 11 Biology Chapter 9 – Biomolecules
Chapter 9 Biomolecules is one of the most chemistry-intensive chapters in Class 11 Biology, and it forms a critical bridge between biology and biochemistry. This chapter explores the chemical building blocks of life — carbohydrates, proteins, lipids, nucleic acids, and enzymes — all of which are essential for every biological function. Must check the CBSE resources and NCERT Solutions.
For students preparing for NEET or CBSE board exams, biomolecules is a chapter that demands both conceptual clarity and factual precision. The sheer variety of molecules, their structures, and their biological roles can feel overwhelming without the right guidance. That is why well-crafted NCERT Solutions for Chapter 9 Biomolecules, available on Myclass24, are structured to break down each concept systematically. Whether you are studying the Michaelis-Menten kinetics of enzyme action or trying to understand the difference between fibrous and globular proteins, these solutions give you clear, exam-ready answers that align with the latest NCERT guidelines followed across all CBSE-affiliated schools in India.
Download PDF – NCERT Solutions for Class 11 Biology Chapter 9 Biomolecules
Biomolecules are the organic molecules produced by living organisms that are essential for sustaining life. The study of biomolecules is called biochemistry. Every living cell contains thousands of different types of biomolecules, all interacting with each other in a highly regulated manner.
Carbohydrates are made up of carbon, hydrogen, and oxygen in the ratio (CH₂O)n. They are the primary source of energy in living organisms. Monosaccharides like glucose and fructose are the simplest carbohydrates. Disaccharides like sucrose (glucose + fructose) and lactose (glucose + galactose) are formed by a glycosidic bond between two monosaccharides. Polysaccharides like starch, cellulose, and glycogen are long chains of monosaccharide units and serve storage or structural purposes.
Proteins are made up of amino acids linked by peptide bonds. There are 20 standard amino acids. The sequence of amino acids determines the primary structure of a protein. The secondary structure (alpha helix or beta pleated sheet) arises from hydrogen bonding. Tertiary structure refers to the 3D folding of the polypeptide chain, while quaternary structure involves the association of multiple polypeptide subunits. Proteins can be structural (collagen, keratin), enzymatic (amylase, pepsin), hormonal (insulin), or transport proteins (hemoglobin). Check out NCERT Solutions for Class 11 Biology and NCERT Solutions for Class 11 for the rest of the chapters.
Lipids are hydrophobic molecules that include fats, oils, waxes, and phospholipids. They are not polymers. Triglycerides are esters of glycerol and fatty acids. Saturated fatty acids have no double bonds and are solid at room temperature, while unsaturated fatty acids have one or more double bonds and are liquid (oils). Phospholipids form the bilayer of cell membranes because they have a hydrophilic head and a hydrophobic tail.
Nucleic acids — DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) — are the information molecules of the cell. DNA carries the genetic blueprint, while RNA helps translate that blueprint into proteins. Nucleotides are the monomers of nucleic acids, each consisting of a nitrogenous base, a pentose sugar, and a phosphate group. DNA has deoxyribose sugar and the bases adenine, thymine, guanine, and cytosine. RNA has ribose sugar and uracil replaces thymine.
| Biomolecule | Monomer | Function | Examples |
| Carbohydrates | Monosaccharides | Energy source, structure | Glucose, Starch, Cellulose |
| Proteins | Amino acids | Enzymes, structure, transport | Hemoglobin, Insulin, Collagen |
| Lipids | Fatty acids + glycerol | Energy storage, membrane | Triglycerides, Phospholipids |
| Nucleic acids | Nucleotides | Genetic information | DNA, RNA |
Enzymes are biological catalysts, almost always proteins (except ribozymes). They speed up chemical reactions without being consumed. The region of the enzyme where the substrate binds is called the active site. The lock and key model (Fischer, 1894) and the induced fit model (Koshland, 1958) explain enzyme-substrate interaction. The induced fit model is more accurate — it proposes that the enzyme changes shape slightly when the substrate binds.
Enzyme activity is affected by temperature, pH, substrate concentration, and inhibitors. The rate of enzyme-catalyzed reaction increases with substrate concentration up to a maximum velocity (Vmax) beyond which all enzyme active sites are saturated. The Km (Michaelis constant) is the substrate concentration at which reaction rate is half of Vmax — a lower Km means higher affinity for the substrate.
| Factor | Effect on Enzyme Activity |
| High Temperature | Denatures enzyme above optimum (usually ~37°C in humans) |
| Low Temperature | Slows activity by reducing molecular collisions |
| Acidic pH | May denature enzyme; pepsin active at pH ~2 |
| Alkaline pH | Trypsin active at pH ~8; most enzymes at pH 6–8 |
| Competitive inhibitor | Blocks active site; reversible with excess substrate |
| Non-competitive inhibitor | Binds allosteric site; changes enzyme shape permanently |
One of the most important topics in this chapter is the metabolic pool concept — the idea that various biomolecules in the cell are constantly being broken down and resynthesized, maintaining a dynamic equilibrium. Amino acids, for instance, can be derived from dietary intake or from the breakdown of body proteins. Similarly, carbohydrates and lipids can be interconverted through metabolic pathways.
Students should pay special attention to the primary metabolites (directly involved in growth and metabolism — amino acids, sugars, ATP) versus secondary metabolites (not directly involved in primary life processes — alkaloids, terpenes, rubber, resins), as this distinction is commonly tested. Myclass24 NCERT Solutions for Chapter 9 Biomolecules offer in-depth answers that cover all these nuances for both CBSE boards and competitive exams.