Class 12 Chemistry Chapter 14 Biomolecules Notes | YoLearn.ai

Key Concepts to Master in Biomolecules

• Carbohydrates: Polyhydroxy aldehydes or ketones, or compounds that produce them on hydrolysis. Classified by number of monosaccharide units.
• Proteins: Polymers of α-amino acids linked by peptide bonds. Essential for structure, function, and regulation of body tissues and organs.
• Amino Acids: Building blocks of proteins; contain both amino (-NH₂) and carboxyl (-COOH) groups. Can exist as zwitterions.
• Enzymes: Biocatalysts, mostly proteins, highly specific, accelerate biochemical reactions by lowering activation energy.
• Nucleic Acids (DNA & RNA): Genetic material. Polymers of nucleotides (base + sugar + phosphate). DNA stores genetic info, RNA helps express it.
• Vitamins: Organic compounds required in small amounts for normal growth and metabolic functions. Classified as fat-soluble (A, D, E, K) or water-soluble (B-complex, C).
• Denaturation of Proteins: Loss of native 3D structure of protein due to physical or chemical changes (e.g., heat, pH) leading to loss of biological activity.
• Glycosidic Linkage: The ether linkage formed between two monosaccharide units by loss of a water molecule.

Essential Biomolecules Terminology

Monosaccharides

Simple sugars that cannot be hydrolyzed further into smaller units (e.g., glucose, fructose, ribose).

Disaccharides

Carbohydrates formed by the condensation of two monosaccharide units (e.g., sucrose, maltose, lactose).

Polysaccharides

Complex carbohydrates formed by joining many monosaccharide units (e.g., starch, cellulose, glycogen).

Peptide Bond

An amide linkage (-CO-NH-) formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of a water molecule.

Zwitterion

A dipolar ion formed when an amino acid protonates its amino group and deprotonates its carboxyl group, resulting in both positive and negative charges within the same molecule.

Nucleotide

The basic building block of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.

Coenzyme

A non-protein organic molecule that binds to an enzyme to assist in catalysis, often derived from vitamins.

Anomers

Stereoisomers of cyclic saccharides that differ in configuration only at the anomeric carbon (the carbonyl carbon of the open-chain form).

Carbohydrates: The Energy Providers

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds which produce such units on hydrolysis. They are broadly classified based on their hydrolysis products:

• Monosaccharides: These are the simplest carbohydrates and cannot be hydrolyzed further into smaller units. Examples include glucose (an aldohexose), fructose (a ketohexose), and ribose. They are often classified by the number of carbon atoms (e.g., trioses, tetroses, pentoses, hexoses) and the functional group (aldoses for aldehydes, ketoses for ketones). Monosaccharides typically exist in cyclic hemiacetal or hemiketal forms in aqueous solutions, forming five-membered (furanose) or six-membered (pyranose) rings. The carbon atom at the carbonyl group becomes a new chiral center, called the anomeric carbon, leading to α and β anomers.

• Disaccharides: These yield two monosaccharide units upon hydrolysis. Common examples include sucrose (glucose + fructose, linked by an α-1,2-glycosidic bond), maltose (glucose + glucose, linked by an α-1,4-glycosidic bond), and lactose (glucose + galactose, linked by a β-1,4-glycosidic bond). Sucrose is a non-reducing sugar because its anomeric carbons are involved in the glycosidic bond, preventing ring opening to the aldehyde form. Maltose and lactose are reducing sugars as they possess a free anomeric carbon.

• Polysaccharides: These are polymers formed by many monosaccharide units. They are typically tasteless and insoluble in water. Important examples include starch (a storage polysaccharide in plants, composed of amylose and amylopectin), cellulose (structural polysaccharide in plants, unbranched β-D-glucose units), and glycogen (animal starch, highly branched structure similar to amylopectin).

DNA vs. RNA: The Nucleic Acid Showdown

Levels of Protein Structure

1. Primary Structure — The specific linear sequence of amino acids in the polypeptide chain. Determined by peptide bonds. Dictates all subsequent levels of structure.
2. Secondary Structure — Local folding of the polypeptide chain into regular, recurring structures. Common forms are α-helix (coiled structure stabilized by intramolecular H-bonds between C=O and N-H of amino acids four residues apart) and β-pleated sheet (extended structure stabilized by intermolecular H-bonds between adjacent polypeptide segments). Folds are due to hydrogen bonding between the backbone atoms.
3. Tertiary Structure — The overall 3D folding of a single polypeptide chain, resulting from interactions between the side chains (R groups) of amino acids. Stabilized by various forces: hydrogen bonds, disulfide bridges (-S-S-), ionic bonds (salt bridges), and hydrophobic interactions.
4. Quaternary Structure — The arrangement of multiple polypeptide subunits (each with its own tertiary structure) to form a functional protein complex. Not all proteins possess quaternary structure (e.g., hemoglobin has four subunits). Stabilized by the same forces as tertiary structure.

Worked Examples: Biomolecules Concepts

• {"title":"1. Identifying Reducing Sugar","bodyMarkdown":"Question: Is sucrose a reducing sugar? Justify your answer.\n\nSolution: No, sucrose is not a reducing sugar. It is formed by a glycosidic linkage between C1 of α-glucose and C2 of β-fructose. Both the anomeric carbons (C1 of glucose and C2 of fructose) are involved in forming the glycosidic bond. This prevents the ring structures from opening up to form the free aldehyde or ketone groups necessary to reduce Tollens' or Fehling's reagents."}
• {"title":"2. Peptide Bond Formation","bodyMarkdown":"Question: Draw the dipeptide formed from Alanine (Ala) and Glycine (Gly) when Ala is the N-terminal amino acid.\n\nSolution:\nAlanine: CH₃-CH(NH₂)-COOH\nGlycine: H-CH(NH₂)-COOH\n\nWhen Alanine is N-terminal, its carboxyl group reacts with the amino group of Glycine:\n\nCH₃-CH(NH₂)-CO-NH-CH₂-COOH (Alanyl-Glycine)\n\nThis shows the peptide bond (-CO-NH-) linking the two amino acids."}

Exam Tip: Avoiding Common Traps in Biomolecules

Pay close attention to reducing vs. non-reducing sugars – this is a frequent source of confusion. Remember that any monosaccharide is a reducing sugar. Disaccharides like maltose and lactose are reducing because one anomeric carbon is free; sucrose is non-reducing because both anomeric carbons are involved in the glycosidic linkage. For proteins, clearly differentiate between the forces stabilizing each level of structure (primary: peptide-bonds; secondary: H-bonds between backbone; tertiary/quaternary: various interactions between R-groups including disulfide). Also, know the deficiency diseases of key vitamins (e.g., Vitamin A - night blindness, Vitamin D - rickets, Vitamin C - scurvy).

Practice Questions with Solutions

• Q1: Name the sugar unit present in DNA and RNA. How do they differ? A1: DNA contains deoxyribose, while RNA contains ribose. Deoxyribose lacks an oxygen atom at the 2' carbon position compared to ribose.
• Q2: What is the significance of essential amino acids? A2: Essential amino acids cannot be synthesized by the human body and must be obtained from the diet. They are crucial for protein synthesis and various metabolic functions.
• Q3: Define denaturation of protein. Give an example. A3: Denaturation is the process where a protein loses its native 3D structure (secondary, tertiary, quaternary) due to physical or chemical changes (heat, pH, salts), leading to loss of its biological activity. Example: Coagulation of egg white upon boiling.
• Q4: Which type of vitamins are stored in the body and which are excreted? A4: Fat-soluble vitamins (A, D, E, K) are stored in the body's fatty tissues and liver. Water-soluble vitamins (B-complex, C) are generally not stored and are excreted through urine.

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