Class 11 Biology NCERT Notes- Chapter 9: Biomolecules | EKADEMY | Online Courses, Live Classes, Lessons & Quizzes

Detailed Study Notes – Chapter 9: Biomolecules (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. The Chemical Foundation of Life

1.1 Elemental Composition of Living vs. Non-Living Matter

While all elements found in the earth’s crust are also present in living tissues, their relative abundance differs significantly. A chemical analysis reveals that living organisms have a notably higher concentration of carbon and hydrogen compared to non-living matter like the earth’s crust. Water is the most abundant chemical in any living organism, constituting 70-90% of the total cellular mass.

Table 1: Comparison of Elemental Abundance (% Weight)

Element	Earth’s Crust	Human Body
Hydrogen (H)	0.14	9.5
Carbon (C)	0.03	18.5
Oxygen (O)	46.6	65.0
Nitrogen (N)	very little	3.3
Sulphur (S)	0.03	0.3
Sodium (Na)	2.8	0.2
Calcium (Ca)	3.6	1.5
Magnesium (Mg)	2.1	0.1
Silicon (Si)	27.7	negligible

1.2 Methods of Chemical Analysis

To determine the chemical composition of living tissues, two primary methods are employed:

Organic Analysis: A living tissue (e.g., a vegetable or piece of liver) is ground in trichloroacetic acid (Cl₃CCOOH) to create a slurry. When strained, this separates into two fractions:
- Acid-Soluble Pool (Filtrate): Contains thousands of organic compounds, generally with molecular weights from 18 to 800 daltons (Da). These are referred to as micromolecules or simply biomolecules.
- Acid-Insoluble Fraction (Retentate): Contains macromolecules like proteins, nucleic acids, and polysaccharides.
Inorganic Analysis: This destructive method involves weighing a tissue sample (wet weight), then drying it to evaporate all water (dry weight). The tissue is then fully burnt, oxidizing all carbon compounds into gas. The remaining material, called ash, contains inorganic elements (like calcium, magnesium) and inorganic compounds (like sulphate, phosphate).

2. Micromolecules (Found in the Acid-Soluble Pool)

2.1 Amino Acids

Amino acids are the building blocks of proteins.

Structure: They are organic compounds containing an amino group (–NH₂) and an acidic carboxyl group (–COOH) attached to the same carbon, the α-carbon. They are essentially substituted methanes, with the four valency positions of the α-carbon occupied by a hydrogen atom, a carboxyl group, an amino group, and a variable R group.
Types: There are 20 types of amino acids that occur in proteins, distinguished by their R group. Examples include glycine (R=H), alanine (R=CH₃), and serine (R=CH₂OH).
Classification: Based on the number of amino and carboxyl groups, they can be acidic (e.g., glutamic acid), basic (e.g., lysine), or neutral (e.g., valine). Some are aromatic (e.g., tyrosine, phenylalanine).
Zwitterionic Form: Due to the ionizable nature of the amino and carboxyl groups, the structure of an amino acid can change with pH. The zwitterionic form has both a positive and a negative charge.

2.2 Lipids

Lipids are a diverse group of generally water-insoluble molecules.

Fatty Acids: A carboxyl group attached to an R group, which can range from a methyl group to a chain of up to 19 carbons.
- Saturated: No C=C double bonds.
- Unsaturated: One or more C=C double bonds.
- Examples: Palmitic acid (16 carbons), Arachidonic acid (20 carbons).
Glycerol: A simple lipid that is trihydroxy propane.
Glycerides: Formed when fatty acids are esterified with glycerol. They can be monoglycerides, diglycerides, or triglycerides.
- Fats and Oils: Classified based on melting point. Oils (e.g., gingelly oil) have lower melting points and remain liquid in winter.
Phospholipids: Lipids containing phosphorous and a phosphorylated organic compound. They are key components of cell membranes. Lecithin is a prominent example.
Complex Lipids: Found in tissues like neural tissue, possessing more complex structures.

2.3 Nitrogenous Bases, Nucleosides, and Nucleotides

These are carbon compounds containing heterocyclic rings.

Nitrogenous Bases:
- Purines: Adenine (A), Guanine (G).
- Pyrimidines: Cytosine (C), Thymine (T), Uracil (U).
Nucleosides: A nitrogenous base attached to a sugar (ribose or deoxyribose). Examples include adenosine, guanosine, and uridine.
Nucleotides: A nucleoside with a phosphate group esterified to the sugar. They are the building blocks of nucleic acids. Examples include adenylic acid and thymidylic acid.

3. Primary and Secondary Metabolites

Primary Metabolites: Biomolecules such as amino acids and sugars that have identifiable functions and play known roles in normal physiological processes. They are found in all animal tissues.
Secondary Metabolites: Thousands of other compounds found in plant, fungal, and microbial cells. Their direct function in the host organism is often not understood, but many are useful to humans (e.g., rubber, drugs, spices, pigments) or have ecological importance.

Table 2: Examples of Secondary Metabolites

Category	Examples
Pigments	Carotenoids, Anthocyanins
Alkaloids	Morphine, Codeine
Terpenoides	Monoterpenes, Diterpenes
Essential oils	Lemon grass oil
Toxins	Abrin, Ricin
Lectins	Concanavalin A
Drugs	Vinblastin, Curcumin
Polymeric substances	Rubber, Gums, Cellulose

4. Biomacromolecules (Found in the Acid-Insoluble Fraction)

Biomacromolecules have molecular weights of ten thousand daltons and above. With the exception of lipids, they are polymers.

Table 3: Average Composition of a Cell

Component	% of Total Cellular Mass
Water	70-90
Proteins	10-15
Nucleic Acids	5-7
Carbohydrates	3
Lipids	2
Ions	1

Note on Lipids: Lipids are not strictly macromolecules as their molecular weight does not exceed 800 Da. They are found in the acid-insoluble fraction because when a tissue is ground, cell membranes break into non-water-soluble vesicles that separate with the macromolecules.

4.1 Proteins

Structure: Proteins are polypeptides, which are linear chains of amino acids linked by peptide bonds. They are heteropolymers because they are built from 20 different types of amino acids.
Amino Acids: Can be essential (must be obtained from diet) or non-essential (can be synthesized by the body).
Functions:
- Enzymes: Trypsin
- Structural: Collagen (intercellular ground substance)
- Hormones: Insulin
- Transport: GLUT-4 (enables glucose transport)
- Immune: Antibody (fights infectious agents)
- Receptors: Sensory reception (smell, taste)
Abundance: Collagen is the most abundant protein in the animal world. RuBisCO (Ribulose bisphosphate Carboxylase-Oxygenase) is the most abundant protein in the whole biosphere.

4.2 Polysaccharides (Carbohydrates)

Polysaccharides are long chains (polymers) of sugars (monosaccharides).

Homopolymers: Composed of a single type of monosaccharide.
- Cellulose: A polymer of glucose; makes up plant cell walls.
- Starch: An energy storage polymer in plants. Its helical structure can hold iodine molecules, turning blue.
- Glycogen: The energy storage polymer in animals. It is a branched polymer of glucose.
- Inulin: A polymer of fructose.
Complex Polysaccharides: Can contain amino-sugars and chemically modified sugars.
- Chitin: Forms the exoskeleton of arthropods. It is a homopolymer of N-acetyl glucosamine.

4.3 Nucleic Acids

Nucleic acids are polynucleotides and are considered true macromolecules.

Building Block: The nucleotide, which has three components:
1. A heterocyclic nitrogenous base (A, G, C, T, U).
2. A monosaccharide sugar (either ribose or 2′ deoxyribose).
3. A phosphate group.
Types:
- Deoxyribonucleic Acid (DNA): Contains deoxyribose sugar.
- Ribonucleic Acid (RNA): Contains ribose sugar.
Function: DNA and RNA function as the genetic material, carrying hereditary information.

5. Levels of Protein Structure

The biological function of a protein is dependent on its unique three-dimensional structure, which is described at four levels.

Primary Structure: The linear sequence of amino acids in the polypeptide chain. It is defined by a beginning (N-terminus) and an end (C-terminus).
Secondary Structure: The local folding of the polypeptide chain into regular structures. The most common forms are the right-handed α-helix (like a revolving staircase) and the β-pleated sheet.
Tertiary Structure: The overall three-dimensional folding of the entire polypeptide chain, often described as a “hollow woolen ball.” This structure is absolutely necessary for the biological activity of proteins.
Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) in a single protein complex. For example, adult human haemoglobin consists of four subunits: two of the α-type and two of the β-type.

6. Enzymes: The Catalysts of Life

6.1 Nature of Enzymes

Almost all enzymes are proteins; some nucleic acids that act as enzymes are called ribozymes.
The tertiary structure of an enzyme creates a crevice or pocket called the active site, where the substrate binds.
Enzymes are highly efficient catalysts, increasing reaction rates dramatically. For example, carbonic anhydrase accelerates the formation of carbonic acid by about 10 million times.
They differ from inorganic catalysts by being sensitive to high temperatures (denatured above 40°C), though enzymes from thermophilic organisms can be stable up to 80°-90°C.

6.2 Mechanism of Enzyme Action

Enzymes work by lowering the activation energy of a reaction—the energy required to reach the high-energy transition state.

The substrate (S) binds to the active site of the enzyme (E), forming a transient enzyme-substrate (ES) complex.
The binding induces a conformational change in the enzyme, fitting it more tightly around the substrate.
The enzyme facilitates the chemical reaction, transforming the substrate into a product (P) while it is bound in an enzyme-product (EP) complex.
The product is released, and the enzyme returns to its original state, ready for another catalytic cycle. E + S ⇌ ES → EP → E + P

6.3 Factors Affecting Enzyme Activity

Temperature and pH: Each enzyme has an optimal temperature and pH at which it exhibits maximum activity. Activity decreases outside this narrow range. Low temperatures cause temporary inactivation, while high temperatures cause permanent denaturation.
Substrate Concentration: The reaction velocity increases with substrate concentration until the enzyme becomes saturated with substrate, at which point the reaction reaches its maximum velocity (Vmax).
Inhibition:
- Competitive Inhibition: An inhibitor molecule that structurally resembles the substrate competes for the active site. This prevents the substrate from binding and reduces enzyme activity. An example is the inhibition of succinic dehydrogenase by malonate.

6.4 Enzyme Classification

Enzymes are divided into 6 major classes based on the type of reaction they catalyse:

Oxidoreductases: Catalyse oxidation-reduction reactions.
Transferases: Transfer a functional group between substrates.
Hydrolases: Catalyse hydrolysis of various bonds.
Lyases: Remove groups from substrates by means other than hydrolysis, leaving double bonds.
Isomerases: Catalyse the inter-conversion of isomers.
Ligases: Catalyse the joining of two molecules.

6.5 Co-factors

Many enzymes require a non-protein component, or co-factor, to be catalytically active. The protein portion of such an enzyme is called the apoenzyme.

Prosthetic Groups: Tightly bound organic compounds. Example: haem in peroxidase and catalase.
Co-enzymes: Transiently bound organic compounds, often derived from vitamins. Example: NAD and NADP contain the vitamin niacin.
Metal Ions: Form coordination bonds with the active site and/or substrate. Example: Zinc is a cofactor for carboxypeptidase.

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Q&A Section

Short-Answer Questions

(Answer in 2-3 sentences based on the provided text.)

What is the main difference in elemental composition between living tissues and the earth’s crust?
Describe the two fractions obtained when a living tissue is ground in trichloroacetic acid.
What is “ash” in the context of chemical analysis, and what does it contain?
What are the four substituent groups attached to the α-carbon of an amino acid?
How are amino acids classified as acidic, basic, or neutral?
What is a zwitterion?
Differentiate between saturated and unsaturated fatty acids.
What are triglycerides, and how do fats and oils differ?
Define nucleosides and nucleotides, highlighting their key difference.
What is the defining characteristic of primary metabolites?
Give three examples of secondary metabolites and their categories.
Why are lipids, despite their low molecular weight, found in the acid-insoluble fraction?
What is a heteropolymer? Why is protein considered a heteropolymer?
Name the most abundant protein in the animal world and the most abundant protein in the biosphere.
How does starch differ from cellulose in terms of structure and its reaction with iodine?
What is chitin and where is it found?
Describe the primary structure of a protein. What are the N-terminus and C-terminus?
Explain the tertiary structure of a protein and why it is significant.
What is the quaternary structure of a protein? Use haemoglobin as an example.
What is an enzyme’s active site?
How do enzymes accelerate the rate of a chemical reaction?
Explain what happens to enzyme activity at temperatures below and above the optimum temperature.
What is Vmax in the context of enzyme kinetics?
Describe competitive inhibition with an example.
What is an apoenzyme, and what are the three kinds of co-factors?

Multiple-Choice Questions (MCQs)

Which pair of elements is significantly more abundant in the human body than in the Earth’s crust? a) Oxygen and Silicon b) Carbon and Hydrogen c) Sodium and Calcium d) Nitrogen and Magnesium
The acid-soluble pool obtained from chemical analysis contains compounds with a molecular weight range of approximately: a) 10,000 daltons and above b) 1,000 to 5,000 daltons c) 800 to 1,000 daltons d) 18 to 800 daltons
Which of the following is an example of an acidic amino acid? a) Lysine b) Valine c) Glutamic acid d) Alanine
Lecithin, a key component of the cell membrane, is an example of a: a) Triglyceride b) Saturated fatty acid c) Phospholipid d) Nucleoside
Adenine and Guanine are classified as: a) Pyrimidines b) Nucleosides c) Purines d) Polysaccharides
Which of the following is considered a secondary metabolite? a) Glucose b) Glycine c) Morphine d) Insulin
According to the average composition of cells, which macromolecule is most abundant after water? a) Lipids b) Proteins c) Nucleic acids d) Carbohydrates
The bond that links amino acids together in a polypeptide chain is called a: a) Glycosidic bond b) Ester bond c) Peptide bond d) Hydrogen bond
Cellulose is a homopolymer composed of which monosaccharide? a) Fructose b) Ribose c) Glucose d) Galactose
The right-handed α-helix is an example of which level of protein structure? a) Primary b) Secondary c) Tertiary d) Quaternary
Adult human haemoglobin is composed of: a) 4 identical subunits b) 2 α-subunits and 2 β-subunits c) A single long polypeptide chain d) 2 polypeptide subunits
Nucleic acids that behave like enzymes are called: a) Apoenzymes b) Co-enzymes c) Ribozymes d) Zymogens
Enzymes from thermophilic organisms can retain catalytic power at temperatures up to: a) 40°C b) 50°-60°C c) 80°-90°C d) 100°C
The energy barrier that a substrate must overcome to be converted into a product is called: a) Potential energy b) Kinetic energy c) Activation energy d) Free energy
When an enzyme is saturated with substrate, the reaction is said to have reached: a) Optimum pH b) Vmax c) The transition state d) Equilibrium
Malonate inhibiting succinic dehydrogenase is an example of: a) Non-competitive inhibition b) Feedback inhibition c) Allosteric regulation d) Competitive inhibition
Enzymes that catalyse the linking together of two compounds are called: a) Hydrolases b) Lyases c) Ligases d) Isomerases
Haem is an example of which type of co-factor? a) Metal ion b) Co-enzyme c) Prosthetic group d) Apoenzyme
The vitamin niacin is an essential component of which co-enzyme? a) Haem b) Zinc ion c) Flavin adenine dinucleotide (FAD) d) Nicotinamide adenine dinucleotide (NAD)
What is the most abundant chemical in living organisms? a) Protein b) Water c) Carbon d) Nucleic Acid

Essay Questions

Describe in detail the process of analysing the chemical composition of a living tissue to identify its organic and inorganic constituents.
Explain the four levels of protein structure, providing details on the types of bonds or interactions that stabilise each level and the overall significance of the final 3D structure.
Compare and contrast the four major classes of biomacromolecules: proteins, polysaccharides, nucleic acids, and lipids. Discuss their monomeric units (where applicable), general structure, and primary functions in a living cell.
What are enzymes? Discuss their chemical nature and explain the detailed mechanism by which they catalyse biochemical reactions, including the concepts of the active site, enzyme-substrate complex, and activation energy.
Elaborate on the various factors that affect enzyme activity, including temperature, pH, and substrate concentration. Explain the concept of enzyme inhibition, focusing on competitive inhibition.
Differentiate between primary and secondary metabolites. Provide examples for each and discuss their known roles and importance to living systems and human welfare.
What is a nucleotide? Describe its three distinct chemical components and explain how these building blocks polymerise to form nucleic acids like DNA and RNA.
Discuss the diversity and key functions of lipids in living organisms, covering fatty acids, triglycerides, and phospholipids.
Explain the classification system for enzymes. List the six major classes and describe the type of reaction each class catalyses.
What are co-factors, and why are they essential for the activity of some enzymes? Differentiate between the three main types of co-factors: prosthetic groups, co-enzymes, and metal ions, providing an example for each.

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Answer Keys

Short-Answer Questions – Answer Key

Living tissues have a significantly higher relative abundance of carbon and hydrogen compared to other elements. In contrast, the earth’s crust is predominantly composed of elements like oxygen and silicon.
The two fractions are the filtrate (acid-soluble pool) and the retentate (acid-insoluble fraction). The filtrate contains small biomolecules (micromolecules), while the retentate contains larger macromolecules like proteins and nucleic acids.
“Ash” is the remainder of a living tissue after it has been completely burnt. It contains the inorganic elements (like calcium, magnesium) and inorganic compounds (like sulphate, phosphate) present in the tissue.
The four substituent groups are a hydrogen atom, a carboxyl group (–COOH), an amino group (–NH₂), and a variable group designated as the R group.
They are classified based on the number of amino and carboxyl groups in their structure. Acidic amino acids have more carboxyl groups, basic amino acids have more amino groups, and neutral ones have an equal number.
A zwitterion is the form of an amino acid that has both a positive charge (on the amino group) and a negative charge (on the carboxyl group). This form occurs at a specific pH due to the ionizable nature of these functional groups.
Saturated fatty acids have no carbon-carbon double bonds in their R group hydrocarbon chain. Unsaturated fatty acids have one or more carbon-carbon double bonds.
Triglycerides are lipids where three fatty acids are esterified to a glycerol molecule. Fats have higher melting points and are solid at room temperature, while oils have lower melting points and remain liquid.
A nucleoside is a nitrogenous base attached to a sugar. A nucleotide is a nucleoside that also has a phosphate group esterified to the sugar. The presence of the phosphate group is the key difference.
The defining characteristic of primary metabolites is that they have identifiable functions and play known roles in the normal physiological processes of an organism. They are essential for life and growth.
Examples include: Alkaloids (Morphine), Pigments (Carotenoids), and Drugs (Vinblastin). These are produced by plants, fungi, or microbes and often have uses for human welfare.
Although lipids are small molecules, they are arranged into cell membranes. During grinding, these membranes break into non-water-soluble vesicles, which then separate along with the other macromolecules in the acid-insoluble fraction.
A heteropolymer is a polymer made of different types of monomeric units. A protein is a heteropolymer because it is built from a combination of 20 different types of amino acids.
The most abundant protein in the animal world is collagen. The most abundant protein in the whole biosphere is Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO).
Starch forms helical secondary structures which can hold iodine molecules, causing it to turn blue. Cellulose does not contain complex helices and therefore cannot hold iodine.
Chitin is a complex polysaccharide, a homopolymer found in the exoskeletons of arthropods. Its building blocks are amino-sugars like N-acetyl galactosamine.
The primary structure is the specific sequence of amino acids in a protein chain. The first amino acid in the sequence is called the N-terminal amino acid, and the last amino acid is the C-terminal amino acid.
The tertiary structure is the overall 3-dimensional folding of the polypeptide chain upon itself, resembling a hollow woollen ball. This structure is absolutely necessary for the protein to carry out its biological activities.
Quaternary structure is the arrangement of multiple folded polypeptide subunits into a functional protein. Adult human haemoglobin is an example, consisting of four subunits: two α-type and two β-type.
An enzyme’s active site is a crevice or pocket in its tertiary structure into which the substrate fits. It is the site of catalytic activity.
Enzymes accelerate reactions by lowering the activation energy. They provide an alternative reaction pathway that requires less energy for the substrate to be converted into the product.
Below the optimum temperature, enzyme activity is low because the enzyme is in a temporarily inactive state. Above the optimum, high temperatures destroy enzymatic activity by denaturing the protein’s tertiary structure.
Vmax is the maximum velocity or rate of an enzymatic reaction. It is reached when the enzyme molecules are saturated with substrate, and a further increase in substrate concentration does not increase the reaction rate.
Competitive inhibition occurs when an inhibitor molecule, which closely resembles the substrate, competes for the enzyme’s active site. An example is malonate inhibiting succinic dehydrogenase by competing with its substrate, succinate.
An apoenzyme is the protein portion of an enzyme that requires a co-factor to be active. The three kinds of co-factors are prosthetic groups, co-enzymes, and metal ions.

Multiple-Choice Questions – Answer Key

b) Carbon and Hydrogen
d) 18 to 800 daltons
c) Glutamic acid
c) Phospholipid
c) Purines
c) Morphine
b) Proteins
c) Peptide bond
c) Glucose
b) Secondary
b) 2 α-subunits and 2 β-subunits
c) Ribozymes
c) 80°-90°C
c) Activation energy
b) Vmax
d) Competitive inhibition
c) Ligases
c) Prosthetic group
d) Nicotinamide adenine dinucleotide (NAD)
b) Water

Essay Questions – Model Answers

(Answers will be provided soon…)