Comprehensive Study Notes: Biomolecules (Unit 10)

Introduction to Biomolecules

A living system grows, sustains, and reproduces itself, despite being composed entirely of non-living atoms and molecules. The study of the chemical processes occurring within living organisms falls under biochemistry.

Living systems are comprised of complex biomolecules including carbohydrates, proteins, nucleic acids, and lipids. Simple molecules like vitamins and mineral salts also perform critical biological functions. Carbohydrates and proteins are essential components of human food. These biomolecules interact dynamically to form the molecular logic of life processes.

$\text{Biomolecules}$ are the foundational chemical units that drive the harmonious and synchronous progress of chemical reactions within living organisms.

Carbohydrates

Carbohydrates are primarily produced by plants and form a massive group of naturally occurring organic compounds. Common examples include cane sugar, glucose, and starch.

Historically, carbohydrates were defined as hydrates of carbon due to their general formula $C_x(H_2O)_y$. For example, glucose ($C_6H_{12}O_6$) fits into this formula as $C_6(H_2O)_6$. However, this definition is inaccurate:

• Acetic acid ($CH_3COOH$) fits the formula as $C_2(H_2O)_2$, but it is not a carbohydrate.

• Rhamnose ($C_6H_{12}O_5$) is a carbohydrate, but it does not fit the general formula.

Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or polyhydroxy ketones, or compounds that yield such units upon hydrolysis. Carbohydrates that taste sweet are called sugars. The sugar used at home is sucrose, while the sugar found in milk is lactose. Carbohydrates are also termed saccharides, derived from the Greek word sakcharon, meaning sugar.

Classification of Carbohydrates

Carbohydrates are classified based on their behavior upon hydrolysis into three primary categories:

1. Monosaccharides: Carbohydrates that cannot be hydrolyzed further to give simpler units of polyhydroxy aldehyde or ketone. Approximately 20 monosaccharides occur naturally (e.g., glucose, fructose, ribose).

2. Oligosaccharides: Carbohydrates that yield 2 to 10 monosaccharide units upon hydrolysis. They are further divided into:

   • Disaccharides: Yield 2 monosaccharide units (e.g., sucrose yields 1 glucose + 1 fructose; maltose yields 2 glucose molecules).

   • Trisaccharides: Yield 3 monosaccharide units.

   • Tetrasaccharides: Yield 4 monosaccharide units.

3. Polysaccharides: Carbohydrates that yield a large number of monosaccharide units upon hydrolysis (e.g., starch, cellulose, glycogen, gums). Polysaccharides are non-sugars because they are not sweet in taste.

Carbohydrates can also be classified as reducing or non-reducing sugars:

• Reducing Sugars: Carbohydrates that reduce Fehling's solution and Tollens' reagent. All monosaccharides (whether aldoses or ketoses), maltose, and lactose are reducing sugars.

• Non-Reducing Sugars: Carbohydrates in which the functional aldehydic or ketonic groups are involved in glycosidic bonding and cannot reduce these reagents (e.g., sucrose).

Classification of Monosaccharides

Monosaccharides are named based on the number of carbon atoms and the functional group present (Aldehyde $\rightarrow$ Aldose; Ketone $\rightarrow$ Ketose):

• 3 Carbons (Triose): Aldotriose / Ketotriose

• 4 Carbons (Tetrose): Aldotetrose / Ketotetrose

• 5 Carbons (Pentose): Aldopentose / Ketopentose

• 6 Carbons (Hexose): Aldohexose / Ketohexose

• 7 Carbons (Heptose): Aldoheptose / Ketoheptose

Glucose ($C_6H_{12}O_6$)

Glucose is an aldohexose, also known as dextrose. It occurs freely in nature in sweet fruits, honey, and ripe grapes, as well as in combined forms as the monomer of starch and cellulose.

Preparation of Glucose

1. From Sucrose (Cane Sugar): Boiling sucrose with dilute $HCl$ or $H_2SO_4$ in an alcoholic solution yields equimolar amounts of glucose and fructose:

$\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_{12}\text{O}_6 \text{ (Glucose)} + \text{C}_6\text{H}_{12}\text{O}_6 \text{ (Fructose)}$

2. From Starch (Commercial Preparation): Produced commercially by the hydrolysis of starch or cellulose by boiling with dilute $H_2SO_4$ at $393\,K$ under $2-3\,atm$ pressure:

$(\text{C}_6\text{H}_{10}\text{O}_5)_n + n\text{H}_2\text{O} \xrightarrow[393\,K;\, 2-3\text{ atm}]{\text{H}^+} n\text{C}_6\text{H}_{12}\text{O}_6 \text{ (Glucose)}$

Structural Elucidation of Glucose

The structure of glucose was established through the following experimental evidence:

1. Molecular Formula: Molecular analysis established the formula as $C_6H_{12}O_6$

2. Straight Chain of Six Carbons: Prolonged heating with hydrogen iodide ($HI$) yields $n\text{-hexane}$, proving that all six carbon atoms are linked in a straight chain:

$\text{CHO}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{HI},\, \Delta} \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \text{ (n-Hexane)}$

3. Presence of a Carbonyl Group ($>C=O$): Glucose reacts with hydroxylamine ($NH_2OH$) to form an oxime, and adds hydrogen cyanide ($HCN$) to form a cyanohydrin:

$\text{CHO}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{NH}_2\text{OH}} \text{CH=N-OH}-(CHOH)_4-\text{CH}_2\text{OH}$

$\text{CHO}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{HCN}} \text{CH(OH)CN}-(CHOH)_4-\text{CH}_2\text{OH}$

4. Presence of an Aldehyde Group ($-\text{CHO}$): Mild oxidation with bromine water ($Br_2\text{ water}$) converts glucose to gluconic acid (a six-carbon monocarboxylic acid):

$\text{CHO}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{Br}_2\text{ water}} \text{COOH}-(CHOH)_4-\text{CH}_2\text{OH} \text{ (Gluconic Acid)}$

5. Presence of Five $-OH$ Groups: Acetylation of glucose with acetic anhydride yields glucose pentaacetate. Because glucose is a stable compound, the five $-OH$ groups must be attached to five distinct carbon atoms:

$\text{CHO}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{Acetic anhydride}} \text{CHO}-(\text{CH-O-CO-CH}_3)_4-\text{CH}_2\text{-O-CO-CH}_3$

6. Presence of a Primary Alcohol Group ($-\text{CH}_2\text{OH}$): Oxidation of both glucose and gluconic acid with nitric acid ($HNO_3$) yields saccharic acid (a dicarboxylic acid):

$\text{CHO}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{Oxidation}} \text{COOH}-(CHOH)_4-\text{COOH} \text{ (Saccharic Acid)}$

$\text{COOH}-(CHOH)_4-\text{CH}_2\text{OH} \xrightarrow{\text{Oxidation}} \text{COOH}-(CHOH)_4-\text{COOH} \text{ (Saccharic Acid)}$

Spatial Configuration and D/L Notations

Fischer assigned the spatial arrangement of hydroxyl groups. Glucose is correctly designated as $D-(+)\text{-glucose}$.

• The symbol $(+)$ indicates that the compound is dextrorotatory.

• The symbol $D$ denotes the spatial configuration relative to $D-(+)\text{-glyceraldehyde}$.

In Fischer projection formulas, if the $-OH$ group on the lowest asymmetric carbon atom (C-5 in glucose) lies on the right-hand side, it is assigned the D-configuration. If it lies on the left-hand side, it is assigned the L-configuration. The most oxidized carbon (C-1, $-CHO$) is placed at the top.

Cyclic Structure of Glucose

The open-chain aldehyde structure fails to explain several experimental observations:

1. Glucose does not yield Schiff's test, nor does it form an addition product with $NaHSO_3$.

2. Glucose pentaacetate does not react with hydroxylamine ($NH_2OH$), indicating the absence of a free $-CHO$ group.

3. Glucose exists in two distinct crystalline forms:

   • $\alpha$-form: Melting point $419\,K$, obtained by crystallization from a concentrated glucose solution at $303\,K$.

   • $\beta$-form: Melting point $423\,K$, obtained by crystallization from a hot, saturated aqueous solution at $371\,K$.

To account for these facts, glucose forms a cyclic hemiacetal ring structure wherein the $-OH$ group at C-5 adds to the aldehyde carbonyl at C-1, creating a six-membered oxygen-containing ring.

The C-1 carbon atom becomes asymmetric upon cyclization and is termed the anomeric carbon. The resulting stereoisomers ($\alpha$ and $\beta$) are called anomers:

• $\alpha\text{-}D\text{-(+)-glucose}$: The $-OH$ group at C-1 is on the right in Fischer projection.

• $\beta\text{-}D\text{-(+)-glucose}$: The $-OH$ group at C-1 is on the left in Fischer projection.

The six-membered cyclic ring is called a pyranose structure due to its similarity to pyran. The Haworth structures represent these rings in three dimensions.

Fructose ($C_6H_{12}O_6$)

Fructose is a ketohexose obtained alongside glucose by hydrolyzing sucrose. It is a naturally occurring laevorotatory monosaccharide, designated as $D-(-)\text{-fructose}$.

• It contains a ketonic functional group at C-2 and a six-carbon straight chain.

• It forms a five-membered cyclic hemiacetal ring via addition of the C-5 $-OH$ group to the C-2 ketonic carbonyl group.

• The five-membered ring contains one oxygen atom and four carbon atoms, termed a furanose structure by analogy to furan.

• It exists in $\alpha$ and $\beta$ anomeric forms.

Disaccharides

Disaccharides consist of two monosaccharide units joined by an oxide linkage formed by the loss of a water molecule. This linkage through an oxygen atom is called a glycosidic linkage.

1. Sucrose

• Yields an equimolar mixture of $\alpha\text{-}D\text{-glucose}$ and $\beta\text{-}D\text{-fructose}$ on hydrolysis.

• Glycosidic linkage connects C-1 of $\alpha\text{-}D\text{-glucose}$ and C-2 of $\beta\text{-}D\text{-fructose}$.

• Because the reducing groups (C-1 aldehyde of glucose and C-2 ketone of fructose) are involved in glycosidic bonding, sucrose is a non-reducing sugar.

• Invert Sugar: Sucrose is dextrorotatory ($+66.5^\text{o}$). Upon hydrolysis, it produces dextrorotatory glucose ($+52.5^\text{o}$) and laevorotatory fructose ($-92.4^\text{o}$). Because the magnitude of laevorotation of fructose exceeds the dextrorotation of glucose, the overall hydrolysate mixture is laevorotatory ($-$\text{sign}$). Hence, hydrolysis of sucrose is called inversion, and the product is called invert sugar.

2. Maltose

• Composed of two $\alpha\text{-}D\text{-glucose}$ units linked by a C-1 to C-4 glycosidic linkage (C-1 of unit I to C-4 of unit II).

• The C-1 of the second glucose unit can form a free aldehyde group in solution, making maltose a reducing sugar.

3. Lactose (Milk Sugar)

• Composed of $\beta\text{-}D\text{-galactose}$ and $\beta\text{-}D\text{-glucose}$ linked by a C-1 to C-4 glycosidic linkage (C-1 of galactose to C-4 of glucose).

• The C-1 of the glucose unit has a free aldehyde group in solution, making lactose a reducing sugar.

Polysaccharides

Polysaccharides are long-chain polymers of monosaccharides joined by glycosidic linkages. They serve primarily as structural components or energy storage materials.

1. Starch

Starch is the primary storage polysaccharide in plants and a major dietary source for humans. It is a polymer of $\alpha\text{-glucose}$ consisting of two components:

• Amylose: Water-soluble component constituting $15-20\%$ of starch. It is a long, unbranched polymer of 200–1000 $\alpha\text{-}D\text{-(+)-glucose}$ units connected via C-1–C-4 glycosidic linkages.

• Amylopectin: Water-insoluble component constituting $80-85\%$ of starch. It is a highly branched chain polymer where main linear chains are formed by C-1–C-4 glycosidic linkages, and branching occurs via C-1–C-6 glycosidic linkages.

2. Cellulose

Cellulose is the most abundant organic substance in the plant kingdom, forming the primary structural constituent of plant cell walls. It is a straight-chain polysaccharide composed exclusively of $\beta\text{-}D\text{-glucose}$ units joined by C-1–C-4 glycosidic linkages.

3. Glycogen

Glycogen is known as animal starch because its structure resembles highly branched amylopectin. It is stored in the animal liver, muscles, and brain, and is also found in yeast and fungi. When the body requires glucose, enzymes hydrolyze glycogen into glucose units.

Biological Importance of Carbohydrates

• Carbohydrates provide major dietary energy.

• Starch (in plants) and glycogen (in animals) act as storage molecules.

• Cellulose provides structural support in plant cell walls, wood for furniture, and cotton fiber for clothing.

• Serves as raw material for industrial applications including textiles, paper, lacquers, and breweries.

• Two aldopentoses—$D\text{-ribose}$ and $2\text{-deoxy-}D\text{-ribose}$—are essential structural building blocks of nucleic acids.

Proteins

Proteins are the most abundant biomolecules in living organisms, forming the structural and functional basis of life. Chief dietary sources include milk, cheese, pulses, peanuts, fish, and meat. The word protein originates from the Greek word proteios, meaning "primary" or "of prime importance."

All proteins are polymers of $\alpha\text{-amino acids}$.

Amino Acids

Amino acids contain both amino ($-NH_2$) and carboxyl ($-COOH$) functional groups. In $\alpha\text{-amino acids}$, the amino group is attached to the $\alpha\text{-carbon}$ adjacent to the carboxyl group:

$R-\text{CH(NH}_2\text{)-COOH}$

Classification of Amino Acids

1. Based on Chemical Nature:

   • Neutral Amino Acids: Contain equal numbers of $-NH_2$ and $-COOH$ groups (e.g., Glycine, Alanine).

   • Acidic Amino Acids: Contain more $-COOH$ groups than $-NH_2$ groups (e.g., Aspartic acid, Glutamic acid).

   • Basic Amino Acids: Contain more $-NH_2$ groups than $-COOH$ groups (e.g., Lysine, Arginine).

2. Based on Dietary Requirement:

   • Essential Amino Acids: Amino acids that cannot be synthesized by the body and must be supplied through the diet (e.g., Valine, Leucine, Isoleucine, Arginine, Lysine, Threonine, Methionine, Phenylalanine, Tryptophan, Histidine).

   • Non-Essential Amino Acids: Amino acids that can be synthesized internally by the body (e.g., Glycine, Alanine, Serine, Cysteine, Tyrosine, Glutamic acid, Aspartic acid, Glutamine, Asparagine, Proline).

Physical Properties and Zwitter Ion Structure

Amino acids are colorless, crystalline, high-melting solids that are soluble in water. They behave like salts rather than simple amines or carboxylic acids due to internal proton transfer.

In aqueous solution, the carboxyl group loses a proton ($H^+$) and the amino group accepts a proton, forming a neutral dipolar ion called a Zwitter ion:

$H_2N-CH(R)-COOH \rightleftharpoons {}^+H_3N-CH(R)-COO^-$

In its Zwitter ionic form, amino acids exhibit amphoteric behavior, reacting with both acids and bases.

Except for glycine ($H_2N-CH_2-COOH$), all naturally occurring $\alpha\text{-amino acids}$ contain an asymmetric $\alpha\text{-carbon}$ atom and are optically active. Most naturally occurring amino acids have the L-configuration, represented with the $-NH_2$ group on the left side in Fischer projections.

Structure of Proteins

Proteins are formed when $\alpha\text{-amino acids}$ polymerize via peptide linkages (amide bonds). A peptide bond ($-\text{CO-NH}-$) is formed between the $-COOH$ group of one amino acid and the $-NH_2$ group of another, with the elimination of a water molecule:

$\text{H}_2\text{N-CH}_2\text{-COOH} + \text{H}_2\text{N-CH(CH}_3\text{)-COOH} \xrightarrow{-\text{H}_2\text{O}} \text{H}_2\text{N-CH}_2\text{-CO-NH-CH(CH}_3\text{)-COOH}$

• Dipeptides: Contain 2 amino acid residues linked by 1 peptide bond (e.g., glycylalanine, Gly-Ala).

• Tripeptides: Contain 3 amino acid residues linked by 2 peptide bonds.

• Polypeptides: Contain up to 100 amino acid residues.

• Proteins: Polypeptides containing more than 100 amino acid residues with a molecular mass exceeding $10,000\,u$. (Note: Insulin is considered a protein despite having only 51 amino acids because it possesses a well-defined protein conformation).

Classification by Molecular Shape

1. Fibrous Proteins: Polypeptide chains run parallel and are held together by hydrogen and disulfide bonds, forming fiber-like structures. They are insoluble in water (e.g., keratin in hair, wool, silk; myosin in muscles).

2. Globular Proteins: Polypeptide chains coil into spherical shapes. They are soluble in water (e.g., insulin, albumins).

Structural Levels of Organization

1. Primary Structure: The specific linear sequence of amino acids linked along the polypeptide chain. Any alteration in this sequence produces a different protein.

2. Secondary Structure: The spatial conformation assumed by the polypeptide backbone due to regular hydrogen bonding between $>C=O$ and $-NH-$ groups of peptide bonds:

   • $\alpha\text{-Helix}$: Polypeptide chain twists into a right-handed screw (helix) where the $-NH$ group of each amino acid residue forms an hydrogen bond with the $>C=O$ group of an adjacent turn 4 residues ahead.

   • $\beta\text{-Pleated Sheet}$: Polypeptide chains are stretched out to near maximum extension and laid side-by-side, held together by intermolecular hydrogen bonds, resembling pleated folds of drapery.

3. Tertiary Structure: Represents the overall three-dimensional folding of the secondary structures, creating distinct fibrous or globular shapes. It is stabilized by hydrogen bonds, disulfide bonds, van der Waals forces, and electrostatic interactions.

4. Quaternary Structure: The spatial arrangement and association of two or more polypeptide sub-units relative to one another.

Denaturation of Proteins

A protein in its natural biological environment possessing a distinct 3D conformation and biological function is called a native protein.

When a native protein is subjected to physical changes (temperature changes) or chemical changes (pH changes), its hydrogen bonds are disrupted. Globules unfold and helices uncoil, leading to the complete loss of biological activity. This process is called denaturation.

• During denaturation, secondary and tertiary structures are destroyed, but the primary structure remains intact.

• Examples: Coagulation of egg white upon boiling; curdling of milk caused by lactic acid production by lactobacilli bacteria.

Enzymes

Enzymes are specialized biocatalysts that accelerate chemical reactions in biosystems. Chemically, almost all enzymes are globular proteins.

• Enzymes are highly specific for a given substrate and reaction.

• They are named by adding the suffix -ase to the substrate or reaction type (e.g., maltase hydrolyzes maltose into glucose; oxidoreductases catalyze redox reactions).

• Enzymes drastically lower the activation energy ($E_a$) of biological reactions. For example, the activation energy for acid hydrolysis of sucrose is $6.22\,kJ\,mol^{-1}$, whereas hydrolysis catalyzed by the enzyme sucrase requires an activation energy of only $2.15\,kJ\,mol^{-1}$.

Vitamins

Vitamins are essential organic compounds required in tiny dietary amounts to perform specific biological functions for normal growth, health, and maintenance of an organism. Most vitamins cannot be synthesized internally by humans and must be supplied through diet (though gut bacteria synthesize a small portion).

The term originally coined was Vitamine (vital + amine). The letter 'e' was dropped when later research revealed that most vitamins do not contain amino groups.

Classification of Vitamins

1. Fat-Soluble Vitamins: Soluble in fats/oils but insoluble in water. Include Vitamins A, D, E, and K. They are stored in the liver and adipose (fat-storing) tissues.

2. Water-Soluble Vitamins: Soluble in water. Include B-complex vitamins and Vitamin C. They must be supplied regularly in the diet because they are readily excreted in urine and cannot be stored in the body (except Vitamin $B_{12}$, which is stored in the body).

Major Vitamins, Sources, and Deficiency Diseases

1. Vitamin A:

   • Sources: Fish liver oil, carrots, butter, milk

   • Deficiency Diseases: Xerophthalmia (hardening of cornea), Night blindness

2. Vitamin $B_1$ (Thiamine):

   • Sources: Yeast, milk, green vegetables, cereals

   • Deficiency Disease: Beri beri (loss of appetite, retarded growth)

3. Vitamin $B_2$ (Riboflavin):

   • Sources: Milk, egg white, liver, kidney

   • Deficiency Diseases: Cheilosis (fissuring at corners of mouth and lips), digestive disorders, skin burning sensation

4. Vitamin $B_6$ (Pyridoxine):

   • Sources: Yeast, milk, egg yolk, cereals, grams

   • Deficiency Disease: Convulsions

5. Vitamin $B_{12}$:

   • Sources: Meat, fish, egg, curd

   • Deficiency Disease: Pernicious anaemia (RBCs deficient in hemoglobin)

6. Vitamin C (Ascorbic Acid):

   • Sources: Citrus fruits, amla, green leafy vegetables

   • Deficiency Disease: Scurvy (bleeding gums)

7. Vitamin D:

   • Sources: Exposure to sunlight, fish, egg yolk

   • Deficiency Diseases: Rickets (bone deformities in children), Osteomalacia (soft bones and joint pain in adults)

8. Vitamin E:

   • Sources: Vegetable oils (wheat germ oil, sunflower oil)

   • Deficiency Diseases: Increased fragility of RBCs, muscular weakness

9. Vitamin K:

   • Sources: Green leafy vegetables

   • Deficiency Disease: Increased blood clotting time

Nucleic Acids

Hereditary traits are transmitted across generations through chromosomes present in the cell nucleus. Chromosomes consist of proteins and long-chain polymers of nucleotides called nucleic acids (polynucleotides).

There are two main types of nucleic acids:

1. Deoxyribonucleic Acid (DNA)

2. Ribonucleic Acid (RNA)

Important figures in nucleic acid research include James Dewey Watson, Francis Crick, and Maurice Wilkins (awarded the 1962 Nobel Prize for discovering the double helix structure of DNA), as well as Har Gobind Khorana (shared the 1968 Nobel Prize for cracking the genetic code).

Chemical Composition of Nucleic Acids

Complete hydrolysis of nucleic acids yields three distinct chemical components:

1. A Pentose Sugar:

   • In DNA: $\beta\text{-}D\text{-2-deoxyribose}$

   • In RNA: $\beta\text{-}D\text{-ribose}$

2. Phosphoric Acid ($H_3PO_4$)

3. Nitrogenous Heterocyclic Bases:

   • DNA Bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

   • RNA Bases: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).

Nucleosides vs. Nucleotides

• Nucleoside: Formed when a nitrogenous base attaches to the $1'\text{-position}$ of the pentose sugar:

$\text{Nucleoside} = \text{Sugar} + \text{Base}$

• Nucleotide: Formed when a nucleoside is esterified with phosphoric acid at the $5'\text{-position}$ of the pentose sugar:

$\text{Nucleotide} = \text{Sugar} + \text{Base} + \text{Phosphate}$

Nucleotides polymerize via phosphodiester linkages joining the $3'$ carbon of one pentose sugar to the $5'$ carbon of the next pentose sugar.

Structure of Nucleic Acids

• Primary Structure: The specific sequence of nucleotides in the polynucleotide chain.

• Secondary Structure of DNA: James Watson and Francis Crick proposed the double-stranded helix model for DNA. Two polynucleotide chains run in opposite (antiparallel) directions and wind around a common axis. The strands are held together by hydrogen bonds between complementary base pairs:

  • Adenine pairs specifically with Thymine via two hydrogen bonds ($A = T$).

  • Cytosine pairs specifically with Guanine via three hydrogen bonds ($C \equiv G$).

• Secondary Structure of RNA: RNA consists of a single-stranded helix that may fold back onto itself. RNA exists in three functional types:

  1. Messenger RNA (m-RNA)

  2. Ribosomal RNA (r-RNA)

  3. Transfer RNA (t-RNA)

Biological Functions of Nucleic Acids & DNA Fingerprinting

• DNA as Hereditary Material: DNA acts as the genetic reserve and is responsible for preserving species identity over generations. It undergoes self-duplication (replication) during cell division to pass identical strands to daughter cells.

• Protein Synthesis: DNA holds the genetic code for protein synthesis, while RNA molecules perform the physical synthesis of proteins in the cell.

• DNA Fingerprinting: Every individual has a unique sequence of bases in their DNA. This sequence is identical across all body cells and cannot be altered. Applications include:

  1. Forensic identification of criminals.

  2. Paternity testing.

  3. Identification of disaster victims.

  4. Evolutionary and racial group tracking.

Hormones

Hormones are chemical molecules acting as intercellular messengers. They are secreted directly into the bloodstream by endocrine glands and transported to target tissues.

Chemical Classes of Hormones

1. Steroid Hormones: Estrogens, androgens, cortisol, aldosterone.

2. Polypeptide Hormones: Insulin, glucagon, endorphins, growth hormones.

3. Amino Acid Derivatives: Epinephrine, norepinephrine, thyroxine.

Biological Roles of Specific Hormones

• Blood Glucose Regulation: Insulin (secreted by pancreas in response to high blood sugar) lowers blood glucose, whereas glucagon increases blood glucose levels.

• Response to Stimuli: Epinephrine and norepinephrine mediate emergency responses to external stress.

• Thyroxine: An iodinated amino acid derivative of tyrosine produced by the thyroid gland:

  • Hypothyroidism: Caused by low thyroxine levels; characterized by lethargy, obesity, and thyroid enlargement (goiter). Controlled by adding sodium iodide ($NaI$) to commercial table salt ("Iodized salt").

  • Hyperthyroidism: Caused by excessively high levels of thyroxine.

• Adrenal Cortex Hormones:

  • Glucocorticoids: Regulate carbohydrate metabolism, modulate inflammatory reactions, and manage stress responses.

  • Mineralocorticoids: Regulate water and salt excretion by the kidneys.

  • Addison's Disease: Results from adrenal cortex dysfunction; characterized by hypoglycemia, weakness, and stress susceptibility. It is fatal if untreated with corticoid therapy.

• Gonadal Hormones (Sex Hormones):

  • Testosterone: Major male sex hormone; drives development of male secondary sexual characteristics (deep voice, facial hair, physical build).

  • Estradiol: Main female sex hormone; drives female secondary sexual characteristics and regulates the menstrual cycle.

  • Progesterone: Prepares the uterine lining for implantation of the fertilized egg.

Intext Questions & Solutions Reference

• Question 10.1: Why are glucose or sucrose soluble in water, while cyclohexane or benzene are insoluble?

  • Explanation: Glucose and sucrose molecules contain multiple $-OH$ groups capable of forming strong intermolecular hydrogen bonds with water molecules. Cyclohexane and benzene are non-polar hydrocarbons that cannot form hydrogen bonds with water.

• Question 10.2: What are the expected products of hydrolysis of lactose?

  • Explanation: Hydrolysis of lactose yields equimolar amounts of $\beta\text{-}D\text{-galactose}$ and $\beta\text{-}D\text{-glucose}$.

• Question 10.3: How do you explain the absence of an aldehyde group in the pentaacetate of D-glucose?

  • Explanation: Cyclization of glucose involves the C-1 $-CHO$ group reacting with the C-5 $-OH$ group to form a cyclic hemiacetal. In glucose pentaacetate, the C-1 $-OH$ is acetylated, preventing the ring from opening back into the open-chain form containing a free $-CHO$ group.

• Question 10.4: Why are melting points and water solubility of amino acids higher than corresponding halo acids?

  • Explanation: Amino acids exist as dipolar Zwitter ions (${}^+H_3N-CH(R)-COO^-$) held together by strong electrostatic ionic attraction forces, leading to high melting points. The charged zwitter ionic groups also interact strongly with polar water molecules via ion-dipole interactions, conferring high water solubility.

• Question 10.5: Where does the water present in an egg go after boiling?

  • Explanation: During boiling, the proteins in egg white and yolk undergo denaturation and coagulation. The water molecules become entrapped within the denatured protein network via hydrogen bonding and physical adsorption.

• Question 10.6: Why cannot Vitamin C be stored in our body?

  • Explanation: Vitamin C (ascorbic acid) is a water-soluble vitamin. It is readily excreted in urine and must be supplied continuously in the diet.

• Question 10.7: What products are formed when a nucleotide from DNA containing thymine is hydrolysed?

  • Explanation: Complete hydrolysis yields thymine (nitrogenous base), $\beta\text{-}D\text{-2-deoxyribose}$ (pentose sugar), and phosphoric acid ($H_3PO_4$).

• Question 10.8: Hydrolysis of RNA shows no quantitative relationship among the bases obtained. What does this suggest about RNA structure?

  • Explanation: The absence of fixed 1:1 molar ratios between bases (such as $A:U$ or $C:G$) indicates that RNA is a single-stranded molecule, unlike double-stranded DNA which requires complementary base pairing.