Biomolecules: Complete Chapter Revision for Class 11 NEET by PW

Biomolecules form an essential part of living organisms and include compounds such as carbohydrates, proteins, lipids and nucleic acids. To revise this chapter effectively, you need to understand more than their names and definitions. You should be familiar with their basic units, structures, classifications, chemical linkages, important examples and biological roles.

PW offers topic-wise explanations for Biomolecules for easier revision. You can revise the chemical analysis of living tissue first and then move through carbohydrates, amino acids and proteins, lipids, nucleic acids and enzymes. Important distinctions, such as nucleoside vs nucleotide, peptide vs glycosidic linkage, reducing vs non-reducing sugars, and apoenzyme vs holoenzyme, are also covered for quick recall.

Chemical Analysis of Living Tissue

Chemical analysis helps you identify the different substances present in living tissues. Before studying individual biomolecules, understand how biological material is separated into different fractions.

The general process involves:

1. Washing: The tissue is washed to remove unwanted external material.

2. Drying: Excess water is removed from the sample.

3. Grinding: The tissue is crushed to break open cells and release their internal constituents. This is referred to as physical digestion.

4. Chemical treatment: The ground tissue is treated with trichloroacetic acid.

5. Filtration: The mixture is filtered to separate the acid-insoluble and acid-soluble fractions.

Retentate and Filtrate

After filtration, you obtain two fractions with different types of molecules:

Remember: The terms retentate and filtrate describe what happens during filtration. The retentate remains on the filter, whereas the filtrate passes through it.

Chemical tests can then be used to identify specific substances. For example, Fehling's test and Benedict's test are used to detect reducing sugars.

Organic and Inorganic Components of Living Tissue

Living organisms contain both organic molecules and inorganic substances. The inorganic fraction includes water and various mineral ions that participate in important biological processes.

Some important ions and their roles are:

1. Calcium ions (Ca²⁺): Important for the development and maintenance of bones and teeth.

2. Sodium, potassium and chloride ions: Help maintain ionic balance and osmotic conditions.

3. Hydrogen and bicarbonate ions: Participate in maintaining acid-base balance.

4. Magnesium and manganese ions: Can act as cofactors for specific enzymes.

Ash Analysis

Ash analysis helps determine the inorganic mineral content of a biological sample.

The tissue is first dried and weighed and is then heated strongly. During heating, most organic components are converted into gaseous products, while inorganic mineral components remain behind as ash.

If the mass of fresh tissue is x and the mass after dehydration is y, then:

Mass of water = x − y

The remaining ash can contain inorganic substances such as sodium, potassium, calcium, magnesium, chloride, carbonate, phosphate and sulphate.

Key point: Water is classified as an inorganic molecule, even though it is essential for virtually every biological process.

Biomicromolecules and Biomacromolecules

Biomolecules can be broadly discussed as smaller biomicromolecules and larger biomacromolecules.

Biomicromolecules

These are relatively small molecules with low molecular masses. Examples include:

• Amino acids

• Simple sugars

• Nucleotides

• Vitamins

They are generally present in the acid-soluble fraction.

Biomacromolecules

These are large molecules with high molecular masses. Major examples include:

• Proteins

• Nucleic acids

• Polysaccharides

Many biomacromolecules are polymers formed by joining smaller units through covalent bonds.

Important Exception: Lipids

Do not classify lipids as true polymers simply because they are large biomolecules.

Lipids are not true polymers because they are not formed by the repeated joining of identical monomeric units. They may occur in the acid-insoluble fraction because of their association into membrane fragments and aggregates.

Carbohydrates

Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that produce such substances on hydrolysis.

They are important sources of energy and also perform structural roles in living organisms. Common dietary sources include rice, wheat, pulses and milk.

Aldoses and Ketoses

Carbohydrates can be classified according to the nature of their carbonyl group:

1. Aldoses: Contain an aldehyde group.

2. Ketoses: Contain a ketone group.

For example, glucose is an aldose, whereas fructose is a ketose.

The formula Cₓ(H₂O)ᵧ is commonly associated with carbohydrates, but it is not a universal formula for every carbohydrate. Therefore, the formula alone cannot be used to identify a compound as a carbohydrate.

Structural Forms of Monosaccharides

Monosaccharides can exist in both:

• Open-chain form

• Cyclic or ring form

In aqueous solution, many monosaccharides predominantly occur in cyclic forms while remaining in equilibrium with their open-chain forms.

Classification Based on Number of Carbon Atoms

The prefixes tri-, tetr-, pent-, hex- and hept- indicate the number of carbon atoms present.

Glycosidic Linkage

A glycosidic linkage joins sugar units to form larger carbohydrates. It is formed through a condensation reaction involving the elimination of water.

The position of the linkage influences the structure of the carbohydrate:

• 1→4 linkage: Commonly associated with linear chains.

• 1→6 linkage: Can produce branching.

Important Carbohydrates to Revise

1. Maltose: Consists of two glucose units.

2. Sucrose: Consists of glucose and fructose and is a non-reducing sugar.

3. Lactose: Consists of glucose and galactose.

4. Starch: Main storage polysaccharide of plants; contains amylose and amylopectin.

5. Glycogen: Major storage polysaccharide in animals; highly branched and stored mainly in the liver and muscles.

6. Cellulose: Structural polysaccharide of plant cell walls and contains β-D-glucose units.

7. Chitin: Structural polysaccharide present in arthropod exoskeletons and fungal cell walls.

8. Inulin: A fructose polymer found in the roots of plants such as dahlia and chicory.

Starch, Amylose and Amylopectin

Starch contains two components:

• Amylose: Essentially unbranched and forms a helical structure.

• Amylopectin: Branched polysaccharide.

Glycogen has a more extensively branched structure than amylopectin.

Why Is Sucrose Non-Reducing?

Sucrose is a non-reducing sugar because the anomeric carbons of its glucose and fructose units are involved in the glycosidic linkage. Therefore, there is no free anomeric group available for the usual reducing behaviour.

Amino Acids

Amino acids are the building units of proteins. An α-amino acid contains an amino group, a carboxyl group, a hydrogen atom and a variable R group attached to the same α-carbon.

The R group determines the specific characteristics of an amino acid.

Examples include:

1. Glycine: R = H

2. Alanine: R = CH₃

3. Serine: R = CH₂OH

Classification of Amino Acids

You can classify amino acids in different ways.

Based on Dietary Requirement

1. Essential amino acids: Cannot be synthesised adequately by the body and therefore must be obtained from the diet.

2. Non-essential amino acids: Can be synthesised by the body.

3. Semi-essential amino acids: May need to be obtained from the diet during particular stages such as growth.

Based on Chemical Properties

1. Neutral amino acids: Contain one amino group and one carboxyl group.

2. Acidic amino acids: Have more carboxyl groups than amino groups. Examples include aspartic acid and glutamic acid.

3. Basic amino acids: Have more amino groups than carboxyl groups. Examples include lysine and arginine.

4. Aromatic amino acids: Contain an aromatic ring, such as phenylalanine, tyrosine and tryptophan.

Zwitterionic Form

Amino acids can exist as zwitterions, which contain both positive and negative charges within the same molecule but have an overall neutral charge.

The amino group can accept a proton to form −NH₃⁺, while the carboxyl group can lose a proton to form −COO⁻.

Therefore, a zwitterion contains:

• A positively charged ammonium group

• A negatively charged carboxylate group

• Overall zero charge

Proteins

Proteins are polymers of α-amino acids joined through peptide bonds. They perform structural, transport, regulatory and catalytic functions in living organisms.

Peptide Bond

A peptide bond is an amide linkage formed between the carboxyl group of one amino acid and the amino group of another amino acid, with the elimination of water.

The two ends of a polypeptide chain are:

• N-terminal: Has the free amino group.

• C-terminal: Has the free carboxyl group.

Levels of Protein Structure

Protein structure can be understood at four levels:

The three-dimensional arrangement of a protein is closely associated with its biological function. Haemoglobin is an important example of a protein with quaternary structure.

Lipids

Lipids are biomolecules that are generally insoluble in water but soluble in organic solvents.

Many simple lipids contain glycerol and fatty acids.

Glycerol and Fatty Acids

Glycerol is a three-carbon alcohol containing three hydroxyl groups.

Fatty acids contain a long hydrocarbon chain and a carboxyl group.

They are classified as:

1. Saturated fatty acids: Contain only single carbon-carbon bonds.

2. Unsaturated fatty acids: Contain one or more carbon-carbon double bonds.

Glycerides

Fatty acids combine with glycerol through ester linkages.

Other important lipid classes include:

• Phospholipids: Contain phosphate and are major components of cell membranes.

• Glycolipids: Contain carbohydrate groups.

• Derived lipids: Include cholesterol and steroid hormones.

NEET point: Lipids are important biomolecules, but they are not true polymers.

Nucleic Acids

Nucleic acids are biomolecules involved in the storage, transmission and expression of hereditary information. The two major nucleic acids are DNA and RNA.

Nucleotide and Nucleoside

A nucleotide consists of:

1. Pentose sugar

2. Nitrogenous base

3. Phosphate group

A nucleoside contains only:

1. Pentose sugar

2. Nitrogenous base

So, the addition of a phosphate group converts a nucleoside into a nucleotide.

DNA and RNA

Nucleotide Linkages

The nitrogenous base is attached to the 1′ carbon of the sugar through an N-glycosidic linkage.

The phosphate group is generally attached to the 5′ carbon.

Adjacent nucleotides are joined through 3′–5′ phosphodiester linkages.

Base Pairing in DNA

DNA contains two antiparallel polynucleotide strands.

The complementary base pairs are:

1. Adenine pairs with thymine through two hydrogen bonds.

2. Guanine pairs with cytosine through three hydrogen bonds.

This complementary arrangement helps DNA maintain and transmit genetic information accurately.

Enzymes

Enzymes are biological catalysts that increase the rate of biochemical reactions without being consumed in the reaction.

Most enzymes are proteins, particularly globular proteins. However, certain RNA molecules also possess catalytic activity and are called ribozymes.

How Do Enzymes Work?

The substrate binds to a specific region of the enzyme called the active site. The enzyme-substrate interaction provides a pathway with lower activation energy, allowing the reaction to occur more rapidly.

The basic sequence can be remembered as:

Enzyme + Substrate ⇌ Enzyme-Substrate Complex → Enzyme + Product

Classification of Enzymes

Enzymes are classified according to the reactions they catalyse:

1. Oxidoreductases: Catalyse oxidation-reduction reactions.

2. Transferases: Transfer functional groups between molecules.

3. Hydrolases: Catalyse bond cleavage using water.

4. Lyases: Catalyse the removal or addition of groups without hydrolysis.

5. Isomerases: Catalyse rearrangements within a molecule to form an isomer.

6. Ligases: Join two molecules, generally using energy.

Enzyme Inhibition

Enzyme inhibitors reduce enzyme activity.

Competitive inhibition

• The inhibitor competes with the substrate for the active site.

• Malonate is an example.

Non-competitive inhibition

• The inhibitor binds at a site other than the active site.

• This can alter the enzyme's conformation and reduce its activity.

• Cyanide inhibits cytochrome c oxidase.

Enzyme Cofactors

Some enzymes require a non-protein component for their activity.

1. Apoenzyme: Protein part of an enzyme.

2. Cofactor: Non-protein component required for enzyme activity.

3. Holoenzyme: Complete active enzyme consisting of the apoenzyme and its required cofactor.

Cofactors can occur as:

• Prosthetic groups

• Coenzymes

• Metal ions

For example, NAD and NADP function as coenzymes, while Zn²⁺ can act as a metal-ion cofactor for certain enzymes.

Biomolecules: Quick Revision Points

Before you move on from this chapter, make sure you can recall these key points:

1. Carbohydrates: Polyhydroxy aldehydes, polyhydroxy ketones or compounds that produce them on hydrolysis.

2. Glucose: Aldose; fructose: ketose.

3. Sucrose: Non-reducing sugar.

4. Starch: Plant storage polysaccharide.

5. Glycogen: Animal storage polysaccharide.

6. Cellulose: Structural polysaccharide of plant cell walls.

7. Proteins: Polymers of α-amino acids.

8. Peptide bond: Amide linkage joining amino acids.

9. Lipids: Not true polymers.

10. Nucleoside: Sugar + base.

11. Nucleotide: Sugar + base + phosphate.

12. DNA: Contains thymine.

13. RNA: Contains uracil.

14. A–T: Two hydrogen bonds.

15. G–C: Three hydrogen bonds.

16. Nucleotides: Joined by 3′–5′ phosphodiester linkages.

17. Enzymes: Biological catalysts that lower activation energy.

18. Ribozymes: Catalytic RNA molecules.

19. Apoenzyme + cofactor: Holoenzyme.

20. Competitive inhibition: Inhibitor competes for the active site.

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