BiomoleculesClass 12 Chemistry Notes

Carbohydrates

Carbohydrates are a large group of naturally occurring organic compounds, primarily produced by plants. Common examples include cane sugar, glucose, and starch. The name "carbohydrate" comes from the fact that many of these compounds have a general formula of $\mathrm{C}_{\mathrm{x}}\left(\mathrm{H}_{2}\mathrm{O}\right)_{\mathrm{y}}$, making them appear as "hydrates of carbon."

For instance, glucose ($\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6}$) fits the formula $\mathrm{C}_{6}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}$. However, this formula is not a strict definition.

• Acetic acid ($\mathrm{CH}_{3}\mathrm{COOH}$) fits the formula $\mathrm{C}_{2}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2}$ but is not a carbohydrate.
• Rhamnose ($\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{5}$) is a carbohydrate but does not fit the general formula.

A more accurate chemical definition is: Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce these units upon hydrolysis.

Carbohydrates that are sweet in taste are often called sugars. The common table sugar is sucrose, and the sugar in milk is lactose. Carbohydrates are also known as saccharides, from the Greek word sakcharon, meaning sugar.

Classification of Carbohydrates

Carbohydrates are classified based on how they behave when they undergo hydrolysis (breaking down with water).

• (i) Monosaccharides: These are the simplest carbohydrates and cannot be hydrolyzed further into smaller units. About 20 are known to exist in nature.

  • Examples: Glucose, fructose, ribose.

• (ii) Oligosaccharides: These carbohydrates yield two to ten monosaccharide units upon hydrolysis.

  • Disaccharides are the most common, yielding two monosaccharide units. These units can be the same or different. For example, sucrose hydrolysis gives one glucose and one fructose molecule, while maltose hydrolysis gives two glucose molecules.
  • Others include trisaccharides (3 units) and tetrasaccharides (4 units).

• (iii) Polysaccharides: These yield a large number of monosaccharide units upon hydrolysis. They are not sweet and are sometimes called non-sugars.

  • Examples: Starch, cellulose, glycogen.

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

• Reducing sugars are carbohydrates that can reduce Fehling's solution and Tollens' reagent. All monosaccharides (both aldoses and ketoses) are reducing sugars.

Monosaccharides

Monosaccharides are further classified based on two features:

1. Number of carbon atoms: Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C).
2. Functional group: Aldose (contains an aldehyde group) or Ketose (contains a keto group).

This leads to combined names like aldohexose (a six-carbon sugar with an aldehyde group, like glucose) or ketohexose (a six-carbon sugar with a keto group, like fructose).

Carbon atoms	General term	Aldehyde	Ketone
3	Triose	Aldotriose	Ketotriose
4	Tetrose	Aldotetrose	Ketotetrose
5	Pentose	Aldopentose	Ketopentose
6	Hexose	Aldohexose	Ketohexose
7	Heptose	Aldoheptose	Ketoheptose

Glucose

Glucose ($\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6}$) is an aldohexose, also known as dextrose. It's found in sweet fruits and honey and is the monomer for large carbohydrates like starch and cellulose.

Preparation of Glucose:

1. From Sucrose: Boiling sucrose with dilute $\mathrm{HCl}$ or $\mathrm{H}_{2}\mathrm{SO}_{4}$ yields glucose and fructose in equal amounts. $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11} + \mathrm{H}_{2} \mathrm{O} \xrightarrow{\mathrm{H}^{+}} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} + \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}$ $\text{Sucrose} \qquad \qquad \qquad \text{Glucose} \quad \text{Fructose}$
2. From Starch: Commercially, glucose is produced by hydrolyzing starch with dilute $\mathrm{H}_{2}\mathrm{SO}_{4}$ at high temperature ($393$ K) and pressure. $\left(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}_{5}\right)_{\mathrm{n}} + \mathrm{nH}_{2} \mathrm{O} \xrightarrow[\text{393 K; 2-3 atm}]{\mathrm{H}^{+}} \mathrm{nC}_{6} \mathrm{H}_{12} \mathrm{O}_{6}$ $\text{Starch or cellulose} \qquad \qquad \qquad \qquad \text{Glucose}$

Structure of Glucose Evidence for the straight-chain structure of glucose:

1. Molecular Formula: Found to be $\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6}$.
2. Straight Chain: Heating with HI makes it form n-hexane, showing all six carbons are in a straight chain.
3. Carbonyl Group: It reacts with hydroxylamine and hydrogen cyanide, confirming a carbonyl group ($>\mathrm{C}=\mathrm{O}$).
4. Aldehyde Group: Mild oxidation with bromine water converts it to gluconic acid (a six-carbon carboxylic acid), indicating the carbonyl group is an aldehyde.
5. Five -OH Groups: Acetylation with acetic anhydride forms glucose pentaacetate, confirming five hydroxyl (-OH) groups, each on a different carbon atom.
6. Primary Alcohol Group: Oxidation with nitric acid produces a dicarboxylic acid (saccharic acid), indicating the presence of a primary alcohol (-OH) group.

Configuration of Glucose Glucose is correctly named D(+)-glucose.

• D refers to its relative configuration. The -OH group on the lowest asymmetric carbon (C-5) is on the right, similar to D-glyceraldehyde. This is a convention for drawing the structure and has no direct relation to optical activity.
• (+) indicates that it is dextrorotatory, meaning it rotates plane-polarized light to the right.

Cyclic Structure of Glucose The straight-chain structure couldn't explain some observations:

1. Glucose doesn't give a positive Schiff's test, which is typical for aldehydes.
2. Glucose pentaacetate doesn't react with hydroxylamine, suggesting the absence of a free -CHO group.
3. Glucose exists in two different crystalline forms, $\alpha$-glucose and $\beta$-glucose, with different melting points.

This led to the proposal of a cyclic structure. The -OH group at C-5 adds to the -CHO group at C-1, forming a six-membered cyclic hemiacetal ring. This new structure is called a pyranose structure, named after the six-membered ring compound pyran.

The C-1 carbon, which was the aldehyde carbon, is now called the anomeric carbon. The $\alpha$ and $\beta$ forms are isomers that differ only in the configuration of the -OH group at this anomeric carbon. They are called anomers.

• $\alpha$-D-(+)-Glucopyranose
• $\beta$-D-(+)-Glucopyranose

These two forms exist in equilibrium with the open-chain structure in solution.

Fructose

Fructose is an important ketohexose found in fruits, honey, and vegetables. It is obtained with glucose from the hydrolysis of sucrose.

Structure of Fructose Fructose has the same molecular formula as glucose, $\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6}$, but it contains a ketone group at C-2 and a six-carbon straight chain. It belongs to the D-series and is laevorotatory (rotates plane-polarized light to the left), so it is named D-(-)-fructose.

Like glucose, fructose also exists in a cyclic form. The -OH group at C-5 adds to the keto group at C-2, forming a five-membered ring. This structure is called a furanose structure, named after the five-membered ring compound furan. Fructose also has $\alpha$ and $\beta$ anomers.

• $\alpha$-D-(-)-Fructofuranose
• $\beta$-D-(-)-Fructofuranose

Disaccharides

Disaccharides are formed when two monosaccharides are joined by an oxide linkage, created by the loss of a water molecule. This bond is called a glycosidic linkage.

• Non-reducing sugars: If the reducing groups (aldehydic or ketonic groups) of both monosaccharides are involved in the glycosidic bond, the resulting sugar is non-reducing. Example: Sucrose.
• Reducing sugars: If one of the functional groups is free, the sugar is reducing. Example: Maltose and Lactose.

(i) Sucrose

• Composition: On hydrolysis, sucrose gives one molecule of D-(+)-glucose and one molecule of D-(-)-fructose.
• Linkage: The bond is between C1 of $\alpha$-D-glucose and C2 of $\beta$-D-fructose.
• Property: It is a non-reducing sugar because both anomeric carbons are involved in the linkage.
• Invert Sugar: Sucrose is dextrorotatory, but its hydrolysis product is laevorotatory because the laevorotation of fructose ($-92.4^{\circ}$) is greater than the dextrorotation of glucose ($+52.5^{\circ}$). This change in rotation from (+) to (-) is called inversion, and the product mixture is called invert sugar.

(ii) Maltose

• Composition: Composed of two $\alpha$-D-glucose units.
• Linkage: The glycosidic linkage is between C1 of one glucose unit and C4 of the other.
• Property: It is a reducing sugar because the aldehyde group at C1 of the second glucose unit is free.

(iii) Lactose

• Common Name: Milk sugar.
• Composition: Composed of $\beta$-D-galactose and $\beta$-D-glucose.
• Linkage: The linkage is between C1 of galactose and C4 of glucose.
• Property: It is a reducing sugar because the aldehyde group at C1 of the glucose unit can be freed.

Polysaccharides

Polysaccharides are polymers containing a large number of monosaccharide units linked by glycosidic bonds. They primarily serve as food storage or structural materials.

(i) Starch

• Function: The main storage polysaccharide in plants. A major dietary source for humans (found in cereals, roots, tubers).
• Composition: A polymer of $\alpha$-glucose. It consists of two components:
  • Amylose: A water-soluble, long, unbranched chain of 200-1000 $\alpha$-D-glucose units linked by C1-C4 glycosidic bonds. It makes up 15-20% of starch.
  • Amylopectin: A water-insoluble, branched-chain polymer of $\alpha$-D-glucose. The main chain has C1-C4 linkages, and branching occurs via C1-C6 linkages. It makes up 80-85% of starch.

(ii) Cellulose

• Function: The most abundant organic substance in the plant kingdom, forming the main constituent of plant cell walls.
• Composition: A straight-chain polysaccharide composed only of $\beta$-D-glucose units.
• Linkage: The units are joined by glycosidic linkages between C1 of one glucose unit and C4 of the next.

(iii) Glycogen

• Function: The main storage polysaccharide in animals, hence it is also called animal starch. It is stored in the liver, muscles, and brain.
• Structure: Its structure is similar to amylopectin but is more highly branched.
• Role: When the body needs glucose, enzymes break down glycogen to release it.

Importance of Carbohydrates

• Food Source: They form a major part of our diet, providing energy.
• Energy Storage: Stored as starch in plants and glycogen in animals.
• Structural Material: Cellulose forms the cell walls of plants and bacteria. We use cellulose as wood for furniture and cotton for clothing.
• Industrial Raw Materials: Used in industries like textiles, paper, and breweries.
• Components of Nucleic Acids: The sugars D-ribose and 2-deoxy-D-ribose are essential components of RNA and DNA.

Proteins

Proteins are the most abundant biomolecules in living systems. They are fundamental to the structure and function of life, required for growth and maintenance. The word "protein" comes from the Greek proteios, meaning "of prime importance." All proteins are polymers of $\alpha$-amino acids.

Amino Acids

Amino acids are organic compounds containing both an amino ($-\mathrm{NH}_{2}$) group and a carboxyl ($-\mathrm{COOH}$) group. The amino acids obtained from protein hydrolysis are all $\alpha$-amino acids, meaning the amino group is attached to the alpha-carbon (the carbon atom next to the -COOH group).

Structure and Properties:

• Amino acids are generally represented by a three-letter or one-letter symbol (e.g., Glycine is Gly or G).
• In aqueous solution, the -COOH group can lose a proton and the $-\mathrm{NH}_{2}$ group can accept one, forming a dipolar ion called a zwitter ion.
• This zwitterionic form gives amino acids salt-like properties: they are colorless, crystalline, water-soluble solids with high melting points.
• Because they have both acidic and basic groups, amino acids are amphoteric, meaning they can react with both acids and bases.
• Except for glycine, all $\alpha$-amino acids are optically active because the $\alpha$-carbon is asymmetric. Most naturally occurring amino acids have L-configuration.

Classification of Amino Acids

Amino acids are classified based on the relative number of amino and carboxyl groups:

• Neutral: Equal number of $-\mathrm{NH}_{2}$ and $-\mathrm{COOH}$ groups.
• Acidic: More $-\mathrm{COOH}$ groups than $-\mathrm{NH}_{2}$ groups.
• Basic: More $-\mathrm{NH}_{2}$ groups than $-\mathrm{COOH}$ groups.

They are also classified based on whether the body can synthesize them:

• Essential amino acids: Cannot be synthesized by the body and must be obtained through diet.
• Non-essential amino acids: Can be synthesized by the body.

Structure of Proteins

Amino acids are linked together by a peptide bond (or peptide linkage), which is an amide bond ($-\mathrm{CO}-\mathrm{NH}-$) formed between the -COOH group of one amino acid and the $-\mathrm{NH}_{2}$ group of another, with the elimination of a water molecule.

• Dipeptide: Formed from two amino acids.
• Tripeptide: Formed from three amino acids.
• Polypeptide: Formed from more than ten amino acids.
• Protein: A polypeptide with more than 100 amino acid residues and a molecular mass greater than 10,000u.

Types of Proteins based on Molecular Shape:

1. Fibrous Proteins: Polypeptide chains run parallel, forming fiber-like structures held together by hydrogen and disulfide bonds. They are generally insoluble in water.
   • Examples: Keratin (in hair, wool), myosin (in muscles).
2. Globular Proteins: Polypeptide chains coil around to form a spherical shape. They are usually soluble in water.
   • Examples: Insulin, albumins.

Levels of Protein Structure: The structure of proteins can be described at four levels of complexity.

• (i) Primary Structure: This is the specific sequence in which amino acids are linked in a polypeptide chain. Any change in this sequence creates a different protein.

• (ii) Secondary Structure: This refers to the shape in which a long polypeptide chain can exist due to regular folding of its backbone. It is stabilized by hydrogen bonds between the $>\mathrm{C}=\mathrm{O}$ and $-\mathrm{NH}-$ groups of the peptide bonds.

  • $\alpha$-Helix: The polypeptide chain twists into a right-handed screw (helix).
  • $\beta$-Pleated Sheet: Polypeptide chains are stretched out and laid side-by-side, held together by intermolecular hydrogen bonds.

• (iii) Tertiary Structure: This describes the overall three-dimensional folding of the polypeptide chain, which includes the folding of the secondary structures. This folding gives rise to the fibrous or globular shapes. The main forces stabilizing this structure are hydrogen bonds, disulfide linkages, van der Waals forces, and electrostatic forces.

• (iv) Quaternary Structure: Some proteins are composed of two or more polypeptide chains, called sub-units. The quaternary structure describes the spatial arrangement of these sub-units with respect to each other.

Denaturation of Protein

A protein in its natural biological state, with its unique 3D structure and activity, is called a native protein.

Denaturation is the process where a protein loses its biological activity when subjected to physical changes (like temperature) or chemical changes (like pH).

• During denaturation, the hydrogen bonds are disturbed, causing the globular proteins to unfold and helices to uncoil.
• The secondary and tertiary structures are destroyed, but the primary structure (the amino acid sequence) remains intact.

• The coagulation of egg white when boiled is a common example of denaturation.
• The curdling of milk is caused by the formation of lactic acid, which changes the pH and denatures the milk proteins.

Enzymes

Enzymes are biocatalysts that speed up chemical reactions in living organisms under mild conditions. Almost all enzymes are globular proteins.

Characteristics of Enzymes:

• Specificity: They are highly specific for a particular reaction and a particular substrate (the molecule they act upon).
• Nomenclature: They are often named after the substrate they act on, with the ending "-ase". For example, maltase catalyzes the hydrolysis of maltose.
• Efficiency: They are needed only in small quantities and work by reducing the activation energy of a reaction. For example, the enzyme sucrase significantly lowers the activation energy for sucrose hydrolysis compared to acid hydrolysis.

Vitamins

Vitamins are organic compounds required in small amounts in the diet to perform specific biological functions for normal growth, health, and maintenance of the organism. Their deficiency causes specific diseases.

Most vitamins cannot be synthesized in the body and are considered essential food factors. The term originated from "vital amine," but the 'e' was dropped when it was discovered that not all of them contain amino groups.

Classification of Vitamins

Vitamins are classified based on their solubility:

• (i) Fat-Soluble Vitamins: Soluble in fat and oils but not in water. They can be stored in the liver and adipose (fat-storing) tissues.

  • Includes: Vitamins A, D, E, and K.

• (ii) Water-Soluble Vitamins: Soluble in water. They must be supplied regularly in the diet because they are easily excreted in urine and cannot be stored in the body (except for vitamin B₁₂).

  • Includes: B group vitamins and Vitamin C.

Important Vitamins, Sources, and Deficiency Diseases

Name of Vitamin	Sources	Deficiency Diseases
Vitamin A	Fish liver oil, carrots, butter, milk	Xerophthalmia (hardening of cornea), Night blindness
Vitamin B₁ (Thiamine)	Yeast, milk, green vegetables, cereals	Beri-beri (loss of appetite, retarded growth)
Vitamin B₂ (Riboflavin)	Milk, egg white, liver, kidney	Cheilosis (fissuring at corners of mouth), digestive disorders
Vitamin B₆ (Pyridoxine)	Yeast, milk, egg yolk, cereals, grams	Convulsions
Vitamin B₁₂	Meat, fish, egg, curd	Pernicious anaemia (RBC deficient in haemoglobin)
Vitamin C (Ascorbic acid)	Citrus fruits, amla, green leafy vegetables	Scurvy (bleeding gums)
Vitamin D	Exposure to sunlight, fish, egg yolk	Rickets (bone deformities in children), Osteomalacia (soft bones in adults)
Vitamin E	Vegetable oils (wheat germ, sunflower)	Increased fragility of RBCs, muscular weakness
Vitamin K	Green leafy vegetables	Increased blood clotting time

Nucleic Acids

Nucleic acids are biomolecules responsible for the transmission of hereditary characters from one generation to the next. They are found in the nucleus of a cell within structures called chromosomes.

There are two main types of nucleic acids:

1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)

Nucleic acids are long-chain polymers of nucleotides, so they are also called polynucleotides.

Chemical Composition of Nucleic Acids

Complete hydrolysis of DNA or RNA yields three components:

1. A Pentose Sugar:
   • In DNA: $\beta$-D-2-deoxyribose
   • In RNA: $\beta$-D-ribose
2. Phosphoric Acid ($\mathrm{H}_{3}\mathrm{PO}_{4}$)
3. Nitrogen-Containing Heterocyclic Bases:
   • In DNA: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
   • In RNA: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).

Structure of Nucleic Acids

• Nucleoside: A unit formed by attaching a base to the 1' position of the sugar.
• Nucleotide: A unit formed when a nucleoside is linked to phosphoric acid at the 5' position of the sugar moiety.

Nucleotides are joined together by a phosphodiester linkage between the 5' carbon of one sugar and the 3' carbon of the next sugar, forming a polynucleotide chain.

Primary Structure: The sequence of nucleotides in the chain of a nucleic acid.

Secondary Structure:

• DNA: James Watson and Francis Crick proposed the double helix structure for DNA. Two nucleic acid chains are wound around each other and held together by hydrogen bonds between specific pairs of bases. The two strands are complementary.
  • Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.
  • Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.
• RNA: RNA is typically a single-stranded helix that can sometimes fold back on itself. There are three types of RNA, each with a different function: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).

DNA Fingerprinting

Just as every individual has unique fingerprints, the sequence of bases in their DNA is also unique. This information is called a DNA fingerprint. It is the same in every cell of a person's body and cannot be altered.

Applications:

• Forensics: Identifying criminals.
• Paternity Testing: Determining the father of an individual.
• Identification: Identifying bodies after accidents.
• Evolutionary Studies: Identifying racial groups to study biological evolution.

Biological Functions of Nucleic Acids

1. Heredity: DNA is the chemical basis of heredity. It carries the genetic information and is responsible for maintaining the identity of species over millions of years. DNA can self-duplicate during cell division, ensuring identical copies are passed to daughter cells.
2. Protein Synthesis: The synthesis of proteins in a cell is a key function of nucleic acids. The message for synthesizing a specific protein is stored in DNA. Various RNA molecules then carry out the actual synthesis process in the cell.

Hormones

Hormones are molecules that act as intercellular messengers. They are produced by endocrine glands and are released directly into the bloodstream, which transports them to their site of action.

Chemical Nature of Hormones:

• Steroids: e.g., estrogens, androgens.
• Polypeptides: e.g., insulin, endorphins.
• Amino Acid Derivatives: e.g., epinephrine, norepinephrine.

Functions of Hormones:

• Maintain Biological Balance: Insulin and glucagon work together to regulate blood glucose levels.
• Respond to Stimuli: Epinephrine and norepinephrine mediate the body's response to external stimuli (the "fight or flight" response).
• Growth and Development: Growth hormones and sex hormones (like testosterone in males and estradiol in females) control growth and the development of secondary sex characteristics.
• Metabolism: Thyroxine, an iodinated amino acid derivative from the thyroid gland, controls metabolism. Low iodine levels can lead to hypothyroidism and an enlarged thyroid gland (goiter).
• Control Body Functions: Hormones from the adrenal cortex (glucocorticoids and mineralocorticoids) control carbohydrate metabolism, inflammatory reactions, stress responses, and the excretion of water and salt by the kidneys.