Biomolecules Comprehensive JEE Study Guide

Fundamentals of Biomolecules

• Biomolecules are defined as organic compounds that form the basis of life.
• They build up the living system and are responsible for its growth and maintenance.
• The major classes of biomolecules essential to biosystems include carbohydrates, proteins, nucleic acids, vitamins, and hormones.

Carbohydrates: Classification and Monosaccharides

• Carbohydrates are defined as optically active polyhydroxy aldehydes or ketones, or compounds that produce such units upon hydrolysis.
• They are also referred to as saccharides.
• According to their behavior on hydrolysis, carbohydrates are classified into three major groups:
  • Monosaccharides:
  • Simplest carbohydrates that cannot be hydrolysed further into simpler compounds.
  • General molecular formula is $(CH_2O)_n$, where $n = 3 - 7$.
  • Examples include glucose, fructose, and ribose.
  • Oligosaccharides:
  • Carbohydrates that yield $2$ to $10$ monosaccharide units upon hydrolysis.
  • Further sub-classified based on unit yield into disaccharides, trisaccharides, tetrasaccharides, etc.
  • Examples include sucrose, lactose, and maltose.
  • Polysaccharides:
  • Carbohydrates that yield a large number of monosaccharide units upon hydrolysis.
  • They are not sweet in taste and are consequently termed non-sugars.
  • Examples include cellulose, starch, and glycogen.
• Classification of Monosaccharides:
  • Classified based on the number of carbon atoms and the present functional group.
  • Aldose: A monosaccharide containing an aldehyde functional group ($-\text{CHO}$).
  • Ketose: A monosaccharide containing a ketone functional group ($-\text{C=O}$).

Glucose and Fructose

• Glucose:
  • Serves as the monomer for many other carbohydrates and is probably the most abundant organic compound on Earth.
  • Occurs in nature both in the free state (present in sweet fruits and honey) and in combined forms.
  • Molecular formula is $C_6H_{12}O_6$.
  • Preparation Methods:
  • From Sucrose: Hydrolysis of sucrose with dilute acid yields equimolar glucose and fructose:       $C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6$
  • From Starch: Industrial preparation by boiling starch or cellulose with dilute acid under pressure:       $(C_6H_{10}O_5)_n + nH_2O \rightarrow nC_6H_{12}O_6$
  • Cyclic Structure of Glucose:
  • Exists in a six-membered pyranose ring related to pyran.
  • Forms two cyclic stereoisomers (anomers) differing in configuration at $C1$: $\alpha\text{-D-(+)-glucopyranose}$ and $\beta\text{-D-(+)-glucopyranose}$.
  • 
• Fructose:
  • Naturally occurring monosaccharide found alongside glucose in fruits, honey, and vegetables.
  • Obtained alongside glucose through the hydrolysis of the disaccharide sucrose.
  • Serves as an important ketohexose with the molecular formula $C_6H_{12}O_6$.
  • Categorized as the sweetest carbohydrate that occurs naturally and is used as a sweetener in pure form.
  • Stereochemistry belongs to the D-series and is levorotatory, written as $\text{D-(-)-fructose}$.
  • Open-Chain Structure:
  • Contains a keto group at $C2$ and primary alcohol groups at $C1$ and $C6$.
  • 
  • Cyclic Structures of Fructose:
  • Forms a five-membered furanose ring through intramolecular hemiacetal formation between $C2$ and $C5$.
  • Crystallizes into two anomeric Haworth forms: $\alpha\text{-D-(-)-fructofuranose}$ and $\beta\text{-D-(-)-fructofuranose}$.
  • 

Disaccharides

• Disaccharides yield two molecules of either identical or different monosaccharides upon hydrolysis with dilute acids or enzymes.
• Glycosidic Linkage:
  • The covalent oxide linkage connecting two monosaccharide units.
  • Formed by the loss of a water molecule ($H_2O$), joining the units through an oxygen bridge.
• Sucrose:
  • Common disaccharide with formula $C_{12}H_{22}O_{11}$.
  • Hydrolysis yields an equimolar mixture of $\text{D-(+)-glucose}$ and $\text{D-(-)-fructose}$:     $C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6$
  • The glycosidic linkage connects $C1$ of $\alpha\text{-D-glucose}$ to $C2$ of $\beta\text{-D-fructose}$.
  • 
• Maltose:
  • Composed of two $\alpha\text{-D-glucose}$ units.
  • Linkage connects $C1$ of the first glucose unit to $C4$ of the second glucose unit via an $\alpha(1 \rightarrow 4)$ glycosidic bond.
  • Exhibits reducing properties because a free aldehyde group can be produced at $C1$ of the second glucose unit in solution, categorizing it as a reducing sugar.
  • 
• Lactose:
  • Commonly known as milk sugar due to its natural occurrence in milk.
  • Composed of $\beta\text{-D-galactose}$ and $\beta\text{-D-glucose}$.
  • Joined via a $\beta(1 \rightarrow 4)$ glycosidic linkage between $C1$ of galactose and $C4$ of glucose.
  • Operates as a reducing sugar due to the presence of a free hemiacetal hydroxyl group at $C1$ of glucose.
  • Broken down into free glucose and galactose during mammalian digestion by the enzyme lactase secreted by intestinal cells.
  • 

Polysaccharides

• Polysaccharides consist of a large number of monosaccharide units joined together by glycosidic linkages, functioning predominantly as structural components or food storage reservoirs.
• Starch:
  • Main storage polysaccharide of plants and a major dietary energy source for human beings.
  • Found in high concentrations in cereals, roots, tubers, and vegetables.
  • Polymer of $\alpha\text{-glucose}$ composed of two distinct components:
  • Amylose: The water-soluble component of starch.
  • Amylopectin: The water-insoluble component of starch.
• Cellulose:
  • Occurs in plants as the most abundant organic substance in the entire plant kingdom.
  • Possesses the empirical formula $(C_6H_{10}O_5)_n$.
  • Linear polysaccharide consisting of several hundred to many thousands of D-glucose units joined by $\beta(1 \rightarrow 4)$ linkages.
• Glycogen:
  • The carbohydrate storage form in the animal body, primarily deposited in the liver and muscles.
  • Commonly referred to as "animal starch" because its structural layout resembles that of amylopectin.

Amino Acids and Protein Architecture

• Proteins:
  • Polymers composed of $\alpha\text{-amino acids}$.
  • Dietary sources include milk, cheese, fish, meat, pulses, and peanuts.
  • Required for the growth, maintenance, and structural functions of living organisms.
• Amino Acids:
  • Organic molecules containing both a carboxyl group ($-\text{COOH}$) and an amino group ($-\text{NH}_2$).
  • Classified as $\alpha$, $\beta$, $\gamma$, $\delta$, etc., based on the relative position of the amino group relative to the carboxyl group.
  • Hydrolysis of naturally occurring proteins yields exclusively $\alpha\text{-amino acids}$.
  • General structure of an $\alpha\text{-amino acid}$ is represented as:     $R\text{-CH(NH}_2\text{)COOH}$
  • 
• Essential vs. Non-Essential Amino Acids:
  • Non-essential Amino Acids: Amino acids that can be synthesized within the body.
  • Essential Amino Acids: Amino acids that cannot be synthesized by the body and must be acquired through the diet.
• Zwitterion Structure:
  • In neutral aqueous solution, the acidic carboxyl group loses a proton ($-\text{COO}^-$) while the basic amino group gains a proton ($-\text{NH}_3^+$).
  • Forms a neutral internal salt carrying both positive and negative charges known as a dipolar ion or Zwitterion:     $R\text{-CH(NH}_2\text{)COOH} \rightleftharpoons R\text{-CH(NH}_3^+\text{)COO}^-$
  • 
• Classification of Proteins by Molecular Shape:
  • Fibrous Proteins:
  • Polypeptide chains run parallel and are held together to form fiber-like structures.
  • Insoluble in water.
  • Examples: Keratin (found in hair, wool, skin) and myosin (present in muscles).
  • Globular Proteins:
  • Polypeptide chains fold around to produce a spherical, globular conformation.
  • Soluble in water.
  • Examples: Insulin and albumin.
• Structural Levels of Proteins:
  • Primary Structure: The specific sequence in which amino acids are linked to each other via peptide bonds along a polypeptide chain.
  • Secondary Structure: The spatial conformation or shape in which a long polypeptide chain can exist (such as $\alpha\text{-helix}$ or $\beta\text{-pleated sheet}$).
  • Tertiary Structure: The overall three-dimensional folding and bending of the polypeptide chains into complex native architecture.
  • Quaternary Structure: The spatial arrangement and assembly of two or more independent polypeptide subunits with respect to one another in multi-subunit protein complexes.
• Denaturation of Proteins:
  • Disturbance of secondary and tertiary structural organization caused by physical or chemical factors (such as variations in temperature or $pH$).
  • Leads to unfolding of native structure and total loss of biological activity while primary structure remains intact.
  • Examples include the coagulation of egg white upon boiling and the curdling of milk upon acidification.
  • When an egg is boiled, the soluble globular proteins (albumin) denature into an insoluble, rubber-like mass that physically traps and retains all internal water by forming hydrogen bonds.
• Biological Importance of Proteins:
  • Hormonal coordination: Peptide hormones such as insulin and glucagon regulate metabolic activities.
  • Tissue development: Direct structural participation in growth and repair mechanisms of tissues.
  • Hemostasis: Fibrinogen acts in the blood clotting cascade to arrest bleeding.
  • Gas transport: Haemoglobin transports oxygen from blood to various peripheral tissues.
  • Cellular framework: Function as fundamental structural components of cells.
  • Immune defense: Specialized proteins termed immunoglobulins act as antibodies to neutralize infections.
• Enzymes:
  • Specialized biocatalysts enabling intricate chemical sequences to occur within living organisms under very mild physiological conditions.
  • Example: Maltase catalyses the hydrolysis of maltose into two glucose molecules.

Vitamins: Classification, Sources, and Deficiencies

• Vitamins are essential organic compounds required in small quantities in the diet to execute specific biological functions for normal maintenance, optimal growth, and general health.
• Solubility Classification:
  • Fat-Soluble Vitamins:
  • Soluble in fats and oils, insoluble in water.
  • Stored in adipose tissue and the liver.
  • Includes Vitamins A, D, E, and K.
  • Water-Soluble Vitamins:
  • Soluble in water.
  • Includes the B-complex vitamins and Vitamin C.
  • Readily excreted in urine and cannot be stored within the body (with the exception of Vitamin $B_{12}$); must be regularly provided through diet.
• Vitamins Reference Table:

Nucleic Acids and Hormones

• Nucleic Acids:
  • Chromosomes located within the cell nucleus are the particles responsible for heredity; they are composed of proteins joined to nucleic acids.
  • Divided into two distinct categories: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).
• Sugar Residues:
  • DNA contains $\beta\text{-D-2-deoxyribose}$.
  • RNA contains $\beta\text{-D-ribose}$.
  • 
• Nitrogenous Bases:
  • DNA possesses four bases: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Thymine ($T$).
  • RNA possesses four bases: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Uracil ($U$).
• Forms of RNA:
  • RNA occurs in three major functional classes: messenger RNA ($\text{mRNA}$), ribosomal RNA ($\text{rRNA}$), and transfer RNA ($\text{tRNA}$).
  • These classes collaboratively execute cellular protein synthesis.
• Comparison Between DNA and RNA:

• Hormones:
  • Intercellular chemical messengers produced by ductless endocrine glands.
  • Secreted directly into the blood stream and transported to target tissues to coordinate and maintain the equilibrium of biological activities.

Solved Examination Problems

• Problem 1:
  • Question: How is glucose related to fructose?
  • 1) Functional group isomerism
  • 2) Rotamers
  • 3) Position isomerism
  • 4) Geometrical isomerism
  • Solution: Glucose and fructose share the identical molecular formula $C_6H_{12}O_6$, but differ in their principal functional groups: glucose contains an aldo group ($-\text{CHO}$) while fructose possesses a keto group ($-\text{C=O}$). They are functional group isomers. Hence, option 1 is correct.
• Problem 2:
  • Question: The sweetest carbohydrate is ________.
  • 1) sucrose
  • 2) glucose
  • 3) fructose
  • 4) lactose
  • Solution: Fructose is the sweetest naturally occurring carbohydrate. Hence, option 3 is correct.
• Problem 3:
  • Question: A carbohydrate is treated with $\alpha\text{-naphthol}$ and concentrated $H_2SO_4$. What colour will be formed at the junction of two liquids?
  • 1) Blood-red
  • 2) Violet
  • 3) Brown
  • 4) Orange
  • Solution: Carbohydrates react with an alcoholic solution of $\alpha\text{-naphthol}$ in the presence of concentrated $H_2SO_4$ to form a purple or violet ring at the liquid junction. This reaction corresponds to Molisch's test for carbohydrates. Hence, option 2 is correct.
• Problem 4:
  • Question: Milk changes after digestion into
  • 1) cellulose
  • 2) fructose
  • 3) glucose
  • 4) lactose
  • Solution: The primary disaccharide present in milk is lactose. Before absorption by intestinal villi, lactose is broken down by the brush-border enzyme lactase into its constituent monosaccharides, glucose and galactose. Hence, option 3 is correct.
• Problem 5:
  • Question: Cellulose is a polymer of
  • 1) glucose
  • 2) fructose
  • 3) ribose
  • 4) sucrose
  • Solution: Cellulose has the empirical formula $(C_6H_{10}O_5)_n$ and consists of an unbranched linear polymer chain of several hundred to thousands of $\beta(1 \rightarrow 4)$ linked D-glucose units. Hence, option 1 is correct.

Practice Problems

• Question 1: Glucose gives silver mirror with ammoniacal silver nitrate because it has ________.
  • 1) aldehyde group
  • 2) ester group
  • 3) ketone group
  • 4) alcoholic silver nitrate
  • Correct Answer: 1) aldehyde group.
• Question 2: Lactose gives ______ on hydrolysis.
  • 1) glucose
  • 2) glucose and galactose
  • 3) fructose
  • 4) glucose and fructose
  • Correct Answer: 2) glucose and galactose.
• Question 3: The two forms of D-glucopyranose obtained from the solution of D-glucose are called ________.
  • 1) isomers
  • 2) anomers
  • 3) epimers
  • 4) enantiomers
  • Correct Answer: 2) anomers.
• Question 4: Water-insoluble component of starch is ________.
  • 1) amylopectin
  • 2) amylose
  • 3) cellulose
  • 4) None of these
  • Correct Answer: 1) amylopectin.
• Question 5: Which of the following enzymes hydrolyses starch to glucose?
  • 1) Amylase
  • 2) Invertase
  • 3) Lactase
  • 4) Maltase
  • Correct Answer: 1) Amylase.

Frequently Asked Questions

• What are monosaccharides?
  • Monosaccharides are the simplest carbohydrate units that cannot be hydrolysed into smaller polyhydroxy aldehydes or ketones.
  • Their empirical general formula is $(CH_2O)_n$, where $n = 3 - 7$.
  • Prominent examples include glucose, fructose, and ribose.
• Why cannot vitamin C be stored in our body?
  • Vitamin C is an intensely water-soluble vitamin.
  • Due to its solubility in aqueous media, it cannot be retained in fatty tissues and is continuously excreted from the body via urine and sweat.
  • It must be continually replenished through regular dietary intake.
• What are the expected products of hydrolysis of lactose?
  • Hydrolytic cleavage of lactose via acid or lactase breaks the $\beta(1 \rightarrow 4)$ glycosidic bond to yield equimolar quantities of $\beta\text{-D-galactose}$ and $\beta\text{-D-glucose}$.
• What is a glycosidic linkage?
  • A glycosidic linkage is an oxide bond formed between two monosaccharide rings via the dehydration loss of a water molecule ($H_2O$).
  • It unites adjacent saccharide units through a covalent oxygen bridge.
• Where does the water present in the egg go after boiling the egg?
  • Boiling an egg subjects its proteins to denaturation, altering tertiary and secondary structures without modifying primary polypeptide sequences.
  • The soluble globular albumin in egg white denatures and aggregates into an insoluble, rubber-like mass.
  • This insoluble protein matrix traps and absorbs all surrounding water by establishing extensive networks of hydrogen bonds.