CBSE Class 11 Biology Chapter 9: Biomolecules NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 11 PDF

This chapter delves into the essential biomolecules that form the basis of life. The NCERT Solutions for Class 11 Biology, Chapter 9, Biomolecules, provide a detailed exploration of the elemental composition of living organisms and compare it with inanimate objects, highlighting the unique abundance of carbon, hydrogen, and oxygen in life forms. The solutions also clarify the presence and roles of various elements like silicon, magnesium, iron, and sodium in biological systems. A significant focus is placed on amino acids, explaining their dual functional groups and the concept of zwitterions. Furthermore, the chapter examines different types of sugars, comparing their carbon content with that of glucose. These solutions are designed to help students grasp complex biochemical concepts, understand the structure and function of key organic molecules, and prepare effectively for their board examinations by offering clear, step-by-step explanations and accurate answers.

Home

Quick info

Board	CBSE
Class	Class 11
Subject	Biology Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 9

Chapter summary

Chapter 9 of the CBSE Class 11 Biology syllabus focuses on Biomolecules. These NCERT Solutions cover the elemental composition of living and non-living matter, the occurrence and significance of elements like silicon, magnesium, iron, and sodium in organisms, and the structure and properties of amino acids, including the zwitterionic form. It also touches upon the classification and comparison of sugars based on their carbon count, using glucose as a reference. The solutions aim to provide a clear understanding of these fundamental biochemical concepts.

Learning outcomes

Understand the elemental composition of living organisms and compare it with inanimate objects.
Identify elements not commonly found in living organisms and explain the roles of essential elements.
Define amino acids and recognize their structural features.
Explain the concept of zwitterions and isoelectric pH for amino acids.
Compare the carbon count of different sugars with that of glucose.

Topics covered

Paper topics

Elemental Composition of Living Organisms
Comparison with Inanimate Objects
Abundance of Carbon, Hydrogen, and Oxygen
Inorganic Constituents of Tissues
Occurrence of Elements in Organisms
Role of Silicon, Magnesium, Iron, and Sodium
Amino Acids: Structure and Properties
Zwitterions and Isoelectric pH
Sugars: Classification and Carbon Count
Glucose, Fructose, Erythrose, Ribulose, Ribose

Important topics

Elemental Composition Comparison
Functions of Key Elements (Mg, Fe, Na)
Amino Acid Structure
Zwitterionic Form of Amino Acids
Sugar Carbon Counts

PDF preview

Read page by page below. PDF is streamed from the official NCERT website — no download button on this page.

Loading document …

Page of

Loading page …

Questions and Solutions

MCQ 1

It is said that elemental composition of living organisms and that of inanimate objects (like earth's crust) are similar in the sense that all the major elements are present in both. Then what would be the difference between these two groups? Choose a correct answer from among the following.

Living organisms have more gold in them than inanimate objects
Living organisms have more water in their body than inanimate objects
Living organisms have more carbon, oxygen and hydrogen per unit mass than inanimate objects
Living organisms have more calcium in them than inanimate objects

Solution: The correct answer is (c). While both living organisms and non-living matter are composed of the same elements, the key difference lies in their relative abundance. Extensive research has shown that living organisms, such as plants, animals, and microbes, contain a significantly higher percentage of organic compounds, specifically carbon, hydrogen, and oxygen, per unit mass. In contrast, the Earth's crust (non-living matter) has a higher percentage of other inorganic molecules like calcium and elements such as gold. The provided table illustrates the percentage composition of some elements in the human body versus their general abundance, highlighting this difference.

Element % Weight in Human Body

Hydrogen (H) 0.14 0.5

Carbon (C) 0.03 18.5

Oxygen (O) 46.6 65.0

Calcium (Ca) 3.6 1.5

Gold (Au) (trace) (trace)

MCQ 2

Many elements are found in living organisms either free or in the form of compounds. One of the following is not found in living organisms.

Silicon (b) Magnesium
Iron (d) Sodium

Solution: The correct answer is (a) Silicon. Silicon is not typically found freely in living organisms, although it does occur in the Earth's crust as oxides and silicates. While silicon is essential for plant life and is present in minute quantities in the human body, its exact function is still under investigation. In contrast, magnesium, iron, and sodium are commonly found in living organisms, often as ions, and play vital roles. Magnesium is essential for many enzyme actions, particularly those involving ATP. Iron is a critical component of hemoglobin and the electron transport chain. Sodium is vital for nerve impulse transmission, membrane permeability, and osmoregulation in animals.

MCQ 3

Amino acids have both an amino group and a carboxyl group in their structure. Which amongst the following is an amino acid?

Formic acid (b) Glycerol
Glycolic acid (d) Glycine

Solution: The correct answer is (d) Glycine. Glycine is the simplest amino acid, characterized by the presence of both an amino group (-NH2) and a carboxyl group (-COOH) attached to a central carbon atom. Formic acid is the simplest carboxylic acid. Glycerol is a type of alcohol (specifically, a triol). Glycolic acid is a carboxylic acid that also has a hydroxyl group attached.

H, N — C — CO O H

Amino group | carboxyl group H (Structure of glycine)

MCQ 4

An amino acid under certain conditions has both positive and negative charges simultaneously in the same molecule. Such a form of amino acid is called

acidic form (b) basic form
aromatic form (d) zwitterionic form

Solution: The correct answer is (d) zwitterionic form. A zwitterion is a neutral molecule that contains both a positive (cationic) and a negative (anionic) charge on different atoms within the same molecule. Amino acids exhibit this property. At low pH (acidic conditions), the amino group accepts a proton, becoming positively charged (

NH_3^+

). At high pH (alkaline conditions), the carboxyl group donates a proton, becoming negatively charged (

COO^-

). At a specific pH, known as the isoelectric pH, the amino acid exists in its electrically neutral zwitterionic form.

Cation (low pH) (Zwitterion at isoelectric pH) $NH_3^+$ — CH — COOH $\longrightarrow$ $^+H_3N$ — CH — $COO^-$ $\longrightarrow$ $H_2N$ — CH — $COO^-$ (high pH)

MCQ 5

Which of the following sugars have the same number of carbon atoms as present in glucose?

Fructose (b) Erythrose (c) Ribulose (d) Ribose

Solution: The correct answer is (a) Fructose. Glucose is classified as an aldohexose, meaning it is a six-carbon sugar (hexose) with an aldehyde functional group. Fructose is a ketohexose, which also contains six carbon atoms but has a ketone functional group. Erythrose is a tetrose (four carbons), Ribulose is a ketopentose (five carbons), and Ribose is a pentose (five carbons). Therefore, fructose is the only sugar listed that has the same number of carbon atoms as glucose.

Thinking Process

Glucose is an aldohexose. Its carbon is attached to a hydrogen atom by a single bond and to an oxygen atom by a double bond.

Common mistakes

Confusing the relative abundance of elements between living and non-living matter.
Misidentifying elements that are not found or are found in trace amounts in living organisms.
Incorrectly defining or identifying amino acids and their properties.
Failing to understand the concept of zwitterions and their formation.

Revision tips

Pay close attention to the comparison of elemental composition between living and non-living matter.
Memorize the functions of key elements like Magnesium, Iron, and Sodium in biological processes.
Clearly understand the structure of an amino acid and the conditions leading to zwitterion formation.
Practice identifying different types of sugars and their carbon numbers.

Practice MCQs

Q1. What is the primary difference in elemental composition between living organisms and inanimate objects like the Earth's crust?

Q2. Which of the following elements is generally NOT found in significant amounts or as a free element in living organisms?

Q3. Which of the following is the simplest amino acid, possessing both an amino group and a carboxyl group?

Q4. What is the term for an amino acid that simultaneously carries both positive and negative charges within the same molecule?

Q5. Which of the following sugars has the same number of carbon atoms as glucose?

Frequently asked questions

What is the main difference between the elemental composition of living organisms and inanimate objects?

Living organisms have a significantly higher percentage of carbon, hydrogen, and oxygen per unit mass compared to inanimate objects like the Earth's crust. While both share common elements, their relative abundance differs.

Which elements are essential for biological functions and where are they found?

Elements like Magnesium, Iron, and Sodium are crucial. Magnesium is vital for enzymes, Iron is key in hemoglobin and electron transport, and Sodium regulates nerve impulses and osmoregulation.

What is an amino acid?

An amino acid is an organic molecule that contains both an amino group (-NH2) and a carboxyl group (-COOH). They are the building blocks of proteins.

What is a zwitterion in the context of amino acids?

A zwitterion is a form of an amino acid where the molecule carries both a positive charge (on the amino group, -NH3+) and a negative charge (on the carboxyl group, -COO-) simultaneously, resulting in a net neutral charge.

How can I use these NCERT solutions for revision?

Review the explanations for each question, focusing on understanding the concepts behind the answers. Pay attention to the differences in elemental composition, the roles of specific elements, and the properties of amino acids and sugars.

Are all sugars the same in terms of carbon atoms?

No, sugars vary in their number of carbon atoms. For example, glucose and fructose are hexoses (6 carbons), while ribose and ribulose are pentoses (5 carbons).

Content reviewed by the NCERT Help team. Editorial Team and update policy

NCERT Solutions PDF PDF on NCERT Help. URL unchanged for search indexing.