Carbohydrates

Carbohydrates are a large class of natural organic compounds primarily produced by plants. Common examples include sugar, glucose, and starch. Historically, they were thought to be hydrates of carbon with the general formula $\textsf{C}_x\text{(H}_2\text{O})_y$. While glucose ($\textsf{C}_6\text{H}_{12}\text{O}_6$) fits this ($\textsf{C}_6\text{(H}_2\text{O)}_6$), some compounds fitting the formula (like acetic acid, $\textsf{C}_2\text{(H}_2\text{O)}_2$) are not carbohydrates, and some carbohydrates (like rhamnose, $\textsf{C}_6\text{H}_{12}\text{O}_5$) do not fit the formula.

Chemically, carbohydrates are defined as **optically active polyhydroxy aldehydes or ketones** or compounds that yield such units upon hydrolysis. Many carbohydrates are sweet and called sugars (saccharides, from Greek 'sakcharon'). Common sugars include sucrose (cane sugar) and lactose (milk sugar).

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Classification Of Carbohydrates

Carbohydrates are classified based on their hydrolysis behaviour:

• (i) Monosaccharides: These are the simplest carbohydrates that cannot be hydrolysed into smaller polyhydroxy aldehyde or ketone units. Approximately 20 monosaccharides occur naturally, including glucose, fructose, and ribose.
• (ii) Oligosaccharides: These yield 2 to 10 monosaccharide units upon hydrolysis. They are further classified as disaccharides (2 units), trisaccharides (3 units), etc. Disaccharides are most common, yielding two monosaccharide units which may be the same (e.g., maltose yields two glucose units) or different (e.g., sucrose yields one glucose and one fructose unit).
• (iii) Polysaccharides: These yield a very large number of monosaccharide units upon hydrolysis. Examples include starch, cellulose, and glycogen. Polysaccharides are typically not sweet and are called non-sugars.

Carbohydrates can also be classified based on their ability to reduce Fehling's solution and Tollens' reagent:

• Reducing Sugars: These reduce Fehling's solution and Tollens' reagent. All monosaccharides (aldoses and ketoses) are reducing sugars. Some disaccharides (like maltose and lactose) are also reducing sugars.
• Non-reducing Sugars: These do not reduce Fehling's solution or Tollens' reagent. Sucrose is a common example.

Monosaccharides are further classified by the number of carbon atoms and the functional group:

• Based on functional group: Aldoses (contain an aldehyde group), Ketoses (contain a ketone group).
• Based on number of carbon atoms: Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C), Heptose (7C), etc.

Combinations are used for naming, e.g., aldotriose (3C aldose), ketohexose (6C ketose).

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Monosaccharides

Important monosaccharides include Glucose and Fructose.

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Glucose

Glucose is a common monosaccharide, an aldohexose ($\textsf{C}_6\text{H}_{12}\text{O}_6$). It exists freely in nature (sweet fruits, honey, ripe grapes) and in combined form (in disaccharides and polysaccharides).

Preparation of Glucose:

• 1. From Sucrose (Cane sugar): Boiling sucrose with dilute $\textsf{HCl}$ or $\textsf{H}_2\text{SO}_4$ yields an equimolar mixture of glucose and fructose.
• 2. From Starch: Industrially, glucose is obtained by the hydrolysis of starch. Starch is boiled with dilute $\textsf{H}_2\text{SO}_4$ at 393 K under pressure.

Structure of Glucose: Glucose is also known as dextrose. It is the monomer unit for starch, cellulose, and glycogen. Its open-chain structure ($\textsf{C}_6\text{H}_{12}\text{O}_6$) was determined through various reactions:

• Molecular formula is $\textsf{C}_6\text{H}_{12}\text{O}_6$.
• Prolonged heating with HI forms n-hexane, indicating a straight chain of six carbon atoms.
• Reacts with hydroxylamine to form an oxime and adds HCN to form a cyanohydrin, confirming the presence of a carbonyl ($>\textsf{C=O}$) group.
• Oxidation with mild oxidising agents like bromine water gives gluconic acid (a six-carbon carboxylic acid), indicating the carbonyl group is an aldehyde (–CHO) at C-1.
• Acetylation with acetic anhydride gives glucose pentaacetate, confirming five –OH groups attached to different carbon atoms.
• Oxidation with strong oxidising agents like nitric acid yields saccharic acid (a dicarboxylic acid) from both glucose and gluconic acid, indicating a primary alcoholic (–CH$_2$OH) group (at C-6) in glucose.

The exact spatial arrangement of the –OH groups (configuration) was determined by Fischer. D-glucose is represented with the –OH on the lowest asymmetric carbon (C-5) on the right side, correlating it to D-(+)-glyceraldehyde. The (+) indicates dextrorotatory nature, which is not directly related to the D/L configuration notation.

Cyclic Structure of Glucose: The open-chain structure did not explain all properties of glucose, such as the absence of Schiff's test, no reaction with $\textsf{NaHSO}_3$ to form an addition product, and the existence of two crystalline forms ($\alpha$ and $\beta$) with different melting points and optical rotations.

This was explained by proposing that glucose exists primarily in cyclic hemiacetal forms formed by the reaction between the aldehyde group (C-1) and the hydroxyl group at C-5. This forms a six-membered ring, analogous to pyran (a cyclic ether with five carbons and one oxygen). The two cyclic forms, $\alpha$ and $\beta$, differ in the configuration at C-1 (the original aldehyde carbon), which is called the anomeric carbon. These two isomers ($\alpha$-anomer and $\beta$-anomer) are called anomers and exist in equilibrium with a small amount of the open-chain form in aqueous solution.

The cyclic structures are more accurately represented by **Haworth structures**.

Mutarotation: The specific rotation of glucose in water changes over time until it reaches an equilibrium value. This phenomenon, called mutarotation, is due to the interconversion between the $\alpha$ and $\beta$ cyclic forms via the open-chain intermediate.

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Fructose

Fructose is an important **ketohexose** ($\textsf{C}_6\text{H}_{12}\text{O}_6$). It is obtained along with glucose from the hydrolysis of sucrose and is found naturally in fruits, honey, and vegetables. It is a laevorotatory compound and is designated as D-(-)-fructose. Its open-chain structure has a ketonic group at C-2 and a straight chain of six carbons.

Fructose also exists in cyclic forms due to the addition of the –OH group at C-5 to the ketone group (C-2). This forms a five-membered ring, analogous to furan, and the cyclic forms are called furanose structures. Like glucose, it exists as $\alpha$ and $\beta$ anomers, differing in configuration at the anomeric carbon (C-2).

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Disaccharides

Disaccharides yield two monosaccharide units upon hydrolysis. The two units are linked by a **glycosidic linkage**, an oxide linkage formed by the loss of a water molecule between the anomeric carbon of one monosaccharide and a hydroxyl group of another. If the anomeric carbons of both monosaccharides are involved in the glycosidic linkage, the disaccharide is non-reducing (e.g., sucrose). If one anomeric carbon is free, it's a reducing sugar (e.g., maltose, lactose).

• (i) Sucrose: Hydrolyses to $\alpha$-D-glucose and $\beta$-D-fructose. The glycosidic linkage is between C1 of $\alpha$-D-glucose and C2 of $\beta$-D-fructose. Both anomeric carbons are involved, making sucrose a **non-reducing sugar**. Sucrose is dextrorotatory, but its hydrolysis product (invert sugar) is laevorotatory because fructose's laevorotation is greater than glucose's dextrorotation.
• (ii) Maltose: Composed of two $\alpha$-D-glucose units linked by an $\alpha$-glycosidic linkage between C1 of one glucose and C4 of the other. The C1 of the second glucose unit is free, allowing it to open to the aldehyde form, making maltose a **reducing sugar**.
• (iii) Lactose: Found in milk, composed of $\beta$-D-galactose and $\beta$-D-glucose. The linkage is a $\beta$-glycosidic linkage between C1 of galactose and C4 of glucose. The C1 of the glucose unit is free, making lactose a **reducing sugar**.

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Polysaccharides

Polysaccharides are polymers of many monosaccharide units joined by glycosidic linkages. They serve primarily as food storage or structural materials.

• (i) Starch: The main storage polysaccharide in plants, found in cereals, roots, and tubers. It is a polymer of $\alpha$-glucose and has two components:
  • Amylose: Water-soluble (15-20%), a long unbranched chain of $\alpha$-D-(+)-glucose units linked by $\textsf{C}1–\textsf{C}4$ glycosidic linkages.
  • Amylopectin: Water-insoluble (80-85%), a branched chain polymer of $\alpha$-D-glucose units. The main chain has $\textsf{C}1–\textsf{C}4$ linkages, while branching occurs via $\textsf{C}1–\textsf{C}6$ glycosidic linkages.
• (ii) Cellulose: Exclusively in plants, the most abundant organic substance in the plant kingdom, a major component of plant cell walls. It is a straight-chain polysaccharide of only $\beta$-D-glucose units joined by $\textsf{C}1–\textsf{C}4$ $\beta$-glycosidic linkages.
• (iii) Glycogen: The storage carbohydrate in animals (animal starch), primarily in liver, muscles, and brain. Its structure is similar to amylopectin but is even more highly branched. It is broken down to glucose when the body needs energy. Also found in yeast and fungi.

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Importance Of Carbohydrates

Carbohydrates are vital for life:

• They are a major portion of our food and the primary source of energy (e.g., glucose, honey).
• Used as storage molecules (starch in plants, glycogen in animals).
• Provide structural support (cellulose in plant cell walls, chitin in fungi and insects - though chitin is a modified carbohydrate).
• Used as raw materials in industries (textiles, paper, lacquers, breweries, furniture from wood/cellulose, clothing from cotton/cellulose).
• Components of nucleic acids (ribose and deoxyribose).
• Found in biosystems combined with proteins and lipids (glycoproteins, glycolipids), involved in cell recognition and signalling.

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Proteins

Proteins are the most abundant biomolecules in living systems, crucial for growth, maintenance, structure, and function. They are polymers of **$\alpha$-amino acids**.

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Amino Acids

Amino acids contain both an **amino group (–NH$_2$)** and a **carboxyl group (–COOH)**. In $\alpha$-amino acids (the building blocks of proteins), both functional groups are attached to the same carbon atom, called the $\alpha$-carbon. The $\alpha$-carbon also carries a hydrogen atom and a variable side chain (R group).

Over 20 different $\alpha$-amino acids are commonly found in proteins. They have trivial names (e.g., Glycine for its sweet taste, Tyrosine first from cheese). They are represented by three-letter (e.g., Gly, Ala) or one-letter (e.g., G, A) symbols.

Examples of common natural amino acids:

Name	Side Chain (R)	Three-letter symbol	One-letter code
Glycine	H–	Gly	G
Alanine	$\textsf{CH}_3\text{–}$	Ala	A
Valine*	$\textsf{(CH}_3)_2\text{CH–}$	Val	V
Leucine*	$\textsf{(CH}_3)_2\text{CH–CH}_2\text{–}$	Leu	L
Isoleucine*	$\textsf{CH}_3\text{–CH}_2\text{–CH(CH}_3)\text{–}$	Ile	I
Serine	$\textsf{HO–CH}_2\text{–}$	Ser	S
Threonine*	$\textsf{CH}_3\text{–CH(OH)–}$	Thr	T
Cysteine	$\textsf{HS–CH}_2\text{–}$	Cys	C
Methionine*	$\textsf{CH}_3\text{–S–CH}_2\text{–CH}_2\text{–}$	Met	M
Aspartic acid	$\textsf{HOOC–CH}_2\text{–}$	Asp	D
Glutamic acid	$\textsf{HOOC–CH}_2\text{–CH}_2\text{–}$	Glu	E
Asparagine	$\textsf{H}_2\text{N–CO–CH}_2\text{–}$	Asn	N
Glutamine	$\textsf{H}_2\text{N–CO–CH}_2\text{–CH}_2\text{–}$	Gln	Q
Lysine*	$\textsf{H}_2\text{N–(CH}_2)_4\text{–}$	Lys	K
Arginine*	$\textsf{HN=C(NH}_2)\text{–NH–(CH}_2)_3\text{–}$	Arg	R
Histidine*		His	H
Phenylalanine*	$\textsf{C}_6\text{H}_5\text{–CH}_2\text{–}$	Phe	F
Tyrosine	$\textsf{(p)HO–C}_6\text{H}_4\text{–CH}_2\text{–}$	Tyr	Y
Tryptophan*		Trp	W
Proline	(cyclic)	Pro	P

* denotes essential amino acids.

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Classification Of Amino Acids

Amino acids are classified based on the net charge of their side chain (R group) or the ratio of amino to carboxyl groups:

• Neutral Amino Acids: Have an equal number of amino and carboxyl groups (e.g., Glycine, Alanine).
• Acidic Amino Acids: Have more carboxyl groups than amino groups (due to an acidic group in the side chain, e.g., Aspartic acid, Glutamic acid).
• Basic Amino Acids: Have more amino groups than carboxyl groups (due to a basic group in the side chain, e.g., Lysine, Arginine).

Based on nutritional requirements:

• Essential Amino Acids: Cannot be synthesised by the body and must be obtained from the diet (e.g., Valine, Leucine, Isoleucine, Arginine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine).
• Non-essential Amino Acids: Can be synthesised by the body.

Amino acids are typically colourless, crystalline, water-soluble solids with high melting points, behaving like salts rather than simple amines or carboxylic acids. This is due to the formation of a **zwitter ion** (dipolar ion) in aqueous solution. The carboxyl group loses a proton, which is accepted by the amino group, resulting in a molecule with both positive and negative charges but overall neutral.

Zwitter ions exhibit **amphoteric behaviour**, reacting with both acids (accepting a proton at the carboxylate group) and bases (donating a proton from the ammonium group).

Except for glycine (where R=H), all other $\alpha$-amino acids have an asymmetric $\alpha$-carbon atom and are therefore **optically active**, existing as enantiomers (D and L forms). Most naturally occurring amino acids have the **L-configuration**, represented with the –NH$_2$ group on the left in a Fischer projection.

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Structure Of Proteins

Proteins are polymers of $\alpha$-amino acids linked by **peptide bonds** (or peptide linkages). A peptide bond is an amide linkage formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of a water molecule.

Linking two amino acids forms a **dipeptide**. Three amino acids linked by two peptide bonds form a **tripeptide**, and so on. When many amino acids (more than ten) are linked, the product is called a **polypeptide**. A protein is generally defined as a polypeptide with more than 100 amino acid residues and a molecular mass above 10,000 u, although this distinction isn't always strict (e.g., insulin has 51 amino acids but is a protein). Amino acid residues in a polypeptide chain are linked sequentially, starting from the N-terminus (free amino group) to the C-terminus (free carboxyl group).

Proteins are classified by molecular shape:

• (a) Fibrous Proteins: Polypeptide chains run parallel and are held together by hydrogen bonds and disulfide bonds, forming fibre-like structures. They are typically insoluble in water (e.g., keratin in hair/wool, myosin in muscles).
• (b) Globular Proteins: Polypeptide chains coil into a spherical shape. These are usually soluble in water (e.g., insulin, albumins).

Protein structure is described at four levels of complexity:

• (i) Primary Structure: The specific, linear sequence of amino acids in each polypeptide chain of a protein. This sequence is genetically determined and fundamental to the protein's identity and function.
• (ii) Secondary Structure: Refers to the regular folding patterns of the polypeptide backbone due to hydrogen bonding between the –NH– and $>\textsf{C=O}$ groups of the peptide bonds. Common secondary structures include the **$\alpha$-helix** (right-handed coil with hydrogen bonds parallel to the helix axis) and the **$\beta$-pleated sheet** (polypeptide chains laid side-by-side held by intermolecular hydrogen bonds).
• (iii) Tertiary Structure: The overall three-dimensional folding of the polypeptide chain, resulting from further folding of the secondary structure. This gives the protein its specific globular or fibrous shape. Stabilised by various interactions between amino acid side chains, including hydrogen bonds, disulfide linkages, ionic interactions (electrostatic forces), and van der Waals forces.
• (iv) Quaternary Structure: Exists in proteins composed of two or more polypeptide chains (subunits). It describes the spatial arrangement of these subunits relative to each other and how they associate to form the complete protein complex.

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Denaturation Of Proteins

A protein in its biologically active, three-dimensional structure is called a **native protein**. Denaturation occurs when a protein's native structure is disrupted by physical (heat, radiation) or chemical (acids, bases, organic solvents) agents.

Denaturation primarily affects the **secondary, tertiary, and quaternary structures** (if present) by breaking the hydrogen bonds, ionic bonds, and van der Waals forces that stabilise them. The polypeptide chains unfold, and the helical/folded conformations are lost. The **primary structure (amino acid sequence) remains intact**.

Upon denaturation, the protein loses its specific three-dimensional shape and, consequently, its biological activity. Common examples include the coagulation of egg white upon boiling (albumins denature) and the curdling of milk due to lactic acid (casein denatures).

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Enzymes

Enzymes are **biocatalysts** that significantly speed up chemical reactions within living organisms under mild conditions (physiological temperature and pH). They are essential for processes like digestion, energy production, and synthesis of molecules.

Almost all enzymes are **globular proteins**. They are highly **specific** for the reaction they catalyse and the substrate they act upon. Enzymes are typically named after the substrate they act on or the type of reaction they catalyse, with the ending '-ase' (e.g., maltase catalyses maltose hydrolysis, oxidoreductase catalyses oxidation-reduction reactions).

Enzymes work by **reducing the activation energy** of a reaction, similar to chemical catalysts. For example, the activation energy for sucrose hydrolysis is much lower when catalysed by sucrase than by acid.

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Mechanism Of Enzyme Action

The mechanism of enzyme action involves the enzyme binding to its specific substrate(s) at a region called the **active site**, forming an enzyme-substrate complex. The active site has a specific shape that fits the substrate (like a lock and key or induced fit model). Within the enzyme-substrate complex, the enzyme facilitates the chemical transformation of the substrate into product(s). The product(s) are then released from the active site, and the enzyme is free to catalyse the reaction again.

Key aspects of the mechanism:

• Binding of enzyme (E) to substrate (S) at the active site: $\textsf{E + S} \rightarrow \textsf{ES}$ (Enzyme-Substrate complex)
• Conversion of substrate to product (P) within the complex: $\textsf{ES} \rightarrow \textsf{EP}$ (Enzyme-Product complex)
• Release of product(s) from the enzyme: $\textsf{EP} \rightarrow \textsf{E + P}$

Enzymes achieve catalysis through various means, including orienting substrates correctly, providing a favourable microenvironment, straining substrate bonds, and participating directly in the reaction via amino acid side chains at the active site.

Only small quantities of enzymes are needed because they are not consumed in the reaction.

Vitamins

Vitamins are organic compounds required in the diet in small amounts for normal growth, health, and maintenance of biological functions. They are considered **essential food factors** because the body generally cannot synthesise them (though some gut bacteria produce certain vitamins). Deficiency of vitamins causes specific diseases.

The term "Vitamine" was originally coined from 'vital' and 'amine' because early identified vitamins contained amino groups, but this is not true for all vitamins. The final 'e' was dropped, resulting in the term 'vitamin'.

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Classification Of Vitamins

Vitamins are classified based on their solubility:

• (i) Fat-soluble vitamins: Soluble in fat and oils, insoluble in water. These include **Vitamins A, D, E, and K**. They are stored in the liver and adipose (fat) tissues.
• (ii) Water-soluble vitamins: Soluble in water. These include **B-group vitamins (B$_1$, B$_2$, B$_6$, B$_{12}$, etc.) and Vitamin C**. Water-soluble vitamins are readily excreted in urine (except Vitamin B$_{12}$) and are not stored in the body to a significant extent, so they need to be supplied regularly in the diet.

Examples of important vitamins, their sources, and deficiency diseases:

Sl. No.	Name of Vitamin	Sources	Deficiency diseases
1.	Vitamin A	Fish liver oil, carrots, butter, milk	Xerophthalmia (eye cornea hardening), Night blindness
2.	Vitamin B$_1$ (Thiamine)	Yeast, milk, green vegetables, cereals	Beri beri (loss of appetite, retarded growth)
3.	Vitamin B$_2$ (Riboflavin)	Milk, egg white, liver, kidney	Cheilosis (fissuring at mouth corners), digestive disorders, skin burning sensation
4.	Vitamin B$_6$ (Pyridoxine)	Yeast, milk, egg yolk, cereals, grams	Convulsions
5.	Vitamin B$_{12}$	Meat, fish, egg, curd	Pernicious anaemia (haemoglobin deficient RBCs)
6.	Vitamin C (Ascorbic acid)	Citrus fruits, amla, green leafy vegetables	Scurvy (bleeding gums)
7.	Vitamin D	Sunlight exposure, fish, egg yolk	Rickets (bone deformities in children), Osteomalacia (soft bones, joint pain in adults)
8.	Vitamin E	Vegetable oils (wheat germ oil, sunflower oil)	Increased RBC fragility, muscular weakness
9.	Vitamin K	Green leafy vegetables	Increased blood clotting time

Excess vitamin intake can also be harmful; supplementation should be done under medical advice.

Nucleic Acids

Nucleic acids are biomolecules found in the nucleus of living cells, responsible for transmitting inherent characters (heredity) from one generation to the next. They are long chain polymers of repeating units called **nucleotides**, and are hence called **polynucleotides**. The two main types are **deoxyribonucleic acid (DNA)** and **ribonucleic acid (RNA)**.

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Chemical Composition Of Nucleic Acids

Complete hydrolysis of DNA or RNA yields three components:

1. A pentose sugar: A five-carbon sugar. In DNA, it is $\beta$-D-2-deoxyribose. In RNA, it is $\beta$-D-ribose.

2. Phosphoric acid: $\textsf{H}_3\text{PO}_4$.

3. Nitrogen-containing heterocyclic compounds (Bases):

• DNA contains four bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). A and G are purines (two rings), C and T are pyrimidines (one ring).
• RNA also contains four bases: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U). Uracil (U) is a pyrimidine that replaces Thymine (T) in RNA. A and G are purines, C and U are pyrimidines.

Note: DNA contains A, G, C, T. RNA contains A, G, C, U.

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Structure Of Nucleic Acids

A **nucleoside** is formed by the attachment of a base to the 1' position of the pentose sugar. The carbons in the sugar ring are numbered 1', 2', 3', 4', 5' to distinguish them from the atoms in the base.

A **nucleotide** is formed when a nucleoside is linked to a phosphoric acid molecule at the 5' position of the sugar moiety (via a phosphodiester bond). It consists of a base, a sugar, and a phosphate group.

Nucleotides link together to form the polynucleotide chain of nucleic acids. They are joined by **phosphodiester linkages** between the 5' carbon of one nucleotide's sugar and the 3' carbon of the next nucleotide's sugar.

The sequence of nucleotides in the nucleic acid chain constitutes its **primary structure**. Nucleic acids also have secondary structures.

DNA Secondary Structure: James Watson and Francis Crick proposed the **double helix** structure for DNA. Two polynucleotide strands coil around each other, held together by hydrogen bonds between specific pairs of bases on opposite strands. The strands are **complementary**: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds (A=T), and Cytosine (C) always pairs with Guanine (G) via three hydrogen bonds (C$\equiv$G). The sugar-phosphate backbone forms the outside of the helix, and the bases are stacked inside. The two strands run in opposite directions (antiparallel).

RNA Secondary Structure: RNA is typically a **single-stranded** molecule. However, it can fold back on itself to form regions of double helix within the single strand, stabilised by complementary base pairing (A=U, C$\equiv$G). This folding creates complex 3D structures essential for RNA function.

There are three main types of RNA with different functions:

• Messenger RNA (mRNA): Carries genetic information from DNA in the nucleus to the ribosomes in the cytoplasm for protein synthesis.
• Ribosomal RNA (rRNA): A structural component of ribosomes, the cellular machinery for protein synthesis.
• Transfer RNA (tRNA): Carries specific amino acids to the ribosome during protein synthesis, matching them to the mRNA sequence.

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Biological Functions Of Nucleic Acids

Nucleic acids are fundamental for life processes:

• DNA: The chemical basis of heredity and the repository of genetic information.
  • **Replication:** DNA can self-duplicate during cell division, ensuring identical genetic information is passed to daughter cells.
  • **Transcription:** DNA serves as a template for the synthesis of RNA molecules, carrying the genetic code.
  • **Controlling Protein Synthesis:** DNA contains the coded message (genes) that specifies the sequence of amino acids in proteins.
• RNA: Primarily involved in protein synthesis (translation).
  • mRNA carries the genetic code from DNA.
  • rRNA is the site of protein synthesis.
  • tRNA brings the correct amino acids to the ribosome based on the mRNA sequence.

**DNA fingerprinting** is a technique that uses the unique sequence of bases in an individual's DNA for identification purposes in forensics, paternity testing, identifying remains, and studying evolutionary relationships. It is highly reliable as DNA sequence is unique and stable.

Hormones

Hormones are signaling molecules that act as **intercellular messengers** in the body. They are produced by endocrine glands and transported via the bloodstream to target tissues where they exert specific effects. Hormones help to maintain **homeostasis** (balance of biological activities) and regulate various physiological processes.

Chemically, hormones are diverse:

• Some are **steroids**, derived from cholesterol (e.g., estrogens, androgens, corticosteroids like cortisol, aldosterone).
• Some are **polypeptides or proteins** (e.g., insulin, glucagon, growth hormone, endorphins).
• Some are **amino acid derivatives** (e.g., epinephrine/adrenaline, norepinephrine/noradrenaline, thyroxine).

Functions of hormones include:

• Regulating metabolism (e.g., insulin and glucagon regulating blood glucose levels, thyroxine regulating metabolism).
• Mediating stress responses (e.g., epinephrine, norepinephrine, glucocorticoids).
• Controlling growth and development (e.g., growth hormone, sex hormones).
• Regulating reproduction and development of secondary sex characteristics (e.g., testosterone, estradiol, progesterone).
• Controlling fluid and electrolyte balance (e.g., mineralocorticoids).

Dysfunction of endocrine glands or hormone production can lead to various diseases (e.g., hypothyroidism/hyperthyroidism from thyroid issues, Addison's disease from adrenal cortex dysfunction).

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Summary

Biomolecules are the complex organic molecules that constitute living systems. Key classes discussed are carbohydrates, proteins, nucleic acids, and vitamins, along with hormones.

• Carbohydrates are optically active polyhydroxy aldehydes/ketones or their hydrolysable forms. Classified as monosaccharides (glucose, fructose), oligosaccharides (disaccharides like sucrose, maltose, lactose), and polysaccharides (starch, cellulose, glycogen). Monosaccharides and some oligosaccharides are reducing sugars if they have a free anomeric carbon capable of opening to an aldehyde/ketone. Structure involves straight chains and cyclic hemiacetal/acetal forms. Monosaccharide units are linked by glycosidic bonds. Important for energy storage and structure.
• Proteins are polymers of $\alpha$-amino acids linked by peptide bonds. Amino acids are zwitterionic and amphoteric. Essential amino acids must be from diet. Protein structure exists at primary (sequence), secondary ($\alpha$-helix, $\beta$-sheet), tertiary (overall 3D folding), and quaternary (subunit arrangement) levels. Fibrous proteins are insoluble structural proteins; globular proteins are soluble and functional. Denaturation is the loss of secondary/tertiary/quaternary structure and biological activity upon heating or chemical treatment; primary structure remains intact.
• Enzymes are mostly globular proteins that act as highly specific biocatalysts, lowering activation energy for reactions. Their action involves binding substrates at an active site.
• Vitamins are essential accessory food factors required in small amounts. Classified by solubility: fat-soluble (A, D, E, K) stored in the body, and water-soluble (B group, C) generally not stored and readily excreted. Deficiencies cause specific diseases.
• Nucleic Acids (DNA, RNA) are polynucleotides, polymers of nucleotides. A nucleotide = base + pentose sugar + phosphate. DNA sugar is deoxyribose, RNA sugar is ribose. DNA bases: A, G, C, T; RNA bases: A, G, C, U. Nucleotides linked by 5'–3' phosphodiester bonds.
• DNA has a double helix secondary structure with complementary base pairing (A-T, C-G) between antiparallel strands, holding genetic information. RNA is single-stranded, involved in protein synthesis (mRNA, rRNA, tRNA).
• DNA is the basis of heredity (replication) and contains the code for proteins (transcription). RNA carries out protein synthesis (translation).
• Hormones are intercellular messengers produced by endocrine glands. Chemically diverse (steroids, polypeptides, amino acid derivatives). Regulate biological activities (metabolism, growth, reproduction, stress response).

Exercises

Questions covering definitions, classification, structures, preparation, properties, functions, and differences between various biomolecules discussed in the chapter.

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