Biomolecules | CBSE - Wyatt's Notes

sources:

• text: Standard textbook reference

Biomolecules are organic molecules essential for life. This topic covers carbohydrates, proteins, nucleic acids, and vitamins.

• Carbohydrates: polyhydroxy aldehydes or ketones ($C_n(H_2O)_n$)
• Monosaccharides: glucose, fructose, galactose (cannot be hydrolyzed)
• Disaccharides: sucrose, maltose, lactose (two monosaccharides linked)
• Polysaccharides: starch, cellulose, glycogen (many monosaccharides)
• Glycosidic bond: links monosaccharides (loss of water)
• Proteins: polymers of amino acids linked by peptide bonds
• Amino acids: contain $-\text{NH}_2$ and $-\text{COOH}$ groups
• Peptide bond: $-\text{CO}-\text{NH}-$ (condensation of $-\text{COOH}$ and $-\text{NH}_2$)
• Nucleic acids: DNA and RNA (polynucleotides)
• DNA: deoxyribonucleic acid (double helix, bases A, T, G, C)
• RNA: ribonucleic acid (single strand, bases A, U, G, C)
• Enzymes: protein catalysts with high specificity

Problem: Classify glucose, sucrose, and starch as mono-, di-, or polysaccharide.

Solution:

• Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$): Monosaccharide. Cannot be hydrolyzed into simpler sugars. It is an aldose (contains an aldehyde group).

• Sucrose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$): Disaccharide. Hydrolyzes to glucose + fructose: $\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{acid}} \text{C}_6\text{H}_{12}\text{O}_6 \text{ (glucose)} + \text{C}_6\text{H}_{12}\text{O}_6 \text{ (fructose)}$

• Starch ($(C_6H_{10}O_5)_n$): Polysaccharide. Contains amylose (linear) and amylopectin (branched). Hydrolyzes to glucose.

Common mistake: Confusing molecular formula with classification. Two different monosaccharides can have the same molecular formula (glucose and fructose are isomers).

Problem: Write the formation of a dipeptide from glycine and alanine.

Solution:

Glycine: $\text{NH}_2\text{CH}_2\text{COOH}$ (R = H) Alanine: $\text{NH}_2\text{CH(CH}_3)\text{COOH}$ (R = CH$_3$)

Peptide bond formation (condensation): $\text{NH}_2\text{CH}_2\text{COOH} + \text{NH}_2\text{CH(CH}_3)\text{COOH} \rightarrow \text{NH}_2\text{CH}_2\text{CONHCH(CH}_3)\text{COOH} + \text{H}_2\text{O}$

The peptide bond ($-\text{CO}-\text{NH}-$) links the carboxyl group of glycine to the amino group of alanine.

Product: glycylalanine (Gly-Ala)

Common mistake: Writing the peptide bond as $-\text{NH}-\text{CO}-$ instead of $-\text{CO}-\text{NH}-$. The carbonyl carbon is bonded to the nitrogen.

Problem: Compare the structure and function of DNA and RNA.

Solution:

Key difference: DNA has thymine (T) while RNA has uracil (U). The double-stranded structure of DNA makes it more stable for long-term genetic storage.

Common mistake: Thinking RNA is always single-stranded. Some viruses have double-stranded RNA.

1. What is the product of hydrolysis of maltose?
2. Draw the structure of the peptide bond.
3. Why is DNA more stable than RNA for storing genetic information?

Biomolecules are the basis of biochemistry, molecular biology, and medicine. Understanding their structure and function is essential for drug design, genetic engineering, and understanding diseases.

The molecules of life are just chemistry in action: Think of carbohydrates as your body’s quick-energy fuel (like gasoline for a car), proteins as the workers and structural materials (like bricks and workers in a building), and DNA as the instruction manual (like a blueprint). The beauty is that all three use just a handful of simple building blocks — simple sugars, 20 amino acids, and 4 nucleotide bases — arranged in different sequences to create the staggering complexity of life.

Why it matters: Understanding biomolecules is the foundation of medicine (how drugs interact with proteins), nutrition (what your body actually does with food), and biotechnology (how we engineer organisms to produce insulin, biofuels, and more). Every disease has a molecular basis, and every treatment targets a biomolecule.

The key insight: The glycosidic bond in carbohydrates, the peptide bond in proteins, and the phosphodiester bond in nucleic acids are all formed by the same principle — condensation reactions that join monomers while releasing water. Life is built by a single chemical strategy repeated in different contexts.

• Monosaccharides give positive Tollens’ and Fehling’s tests (reducing sugars)
• Sucrose is a non-reducing sugar (glycosidic bond involves both anomeric carbons)
• Proteins give biuret test (violet color with Cu$^{2+}$)
• DNA denaturation: breaking of hydrogen bonds (heat, pH change)
• Enzyme specificity: lock-and-key model

Writing the peptide bond as -NH-CO- instead of -CO-NH-. The peptide bond forms between the carboxyl group of one amino acid and the amino group of the next. The correct structure is -CO-NH- where the carbonyl carbon is bonded to the nitrogen. Reversing this gives an incorrect connectivity that does not represent actual peptide chemistry.

Assuming all carbohydrates follow the general formula Cn(H2O)n. While many carbohydrates fit this formula, exceptions exist. Deoxyribose (C5H10O4) and rhamnose (C6H12O5) do not follow the general formula. The definition is based on polyhydroxy aldehyde or ketone structure, not the empirical formula.

Confusing the anomeric carbon in disaccharides. In sucrose, the glycosidic bond links C1 of glucose to C2 of fructose, involving both anomeric carbons. This is why sucrose is a non-reducing sugar. Students often assume all disaccharides are reducing sugars, but this depends on whether a free anomeric carbon remains.

• Polymers: Biomolecules are natural polymers — proteins are amino acid polymers, polysaccharides are sugar polymers, and nucleic acids are nucleotide polymers.
• Coordination Compounds: Metal ions coordinate to biomolecules — hemoglobin (Fe²⁺ with heme), chlorophyll (Mg²⁺ with chlorin), and enzyme active sites often contain coordinated metals.
• Solutions: Biomolecules dissolve in aqueous solutions, and their osmotic pressure is critical for biological function.
• Surface Chemistry: Enzyme-substrate interactions involve surface adsorption, and cell membranes are organized by amphiphilic biomolecules.