Learning Objectives
- Understand the chemical composition of living organisms
- Learn the structure and functions of carbohydrates, proteins, lipids, and nucleic acids
- Study the nature and properties of enzymes
- Understand the concept of metabolic pathways
Key Concepts
Chemical Composition of Living Organisms
Living organisms contain both organic (carbohydrates, proteins, lipids, nucleic acids) and inorganic (water, minerals) compounds. Elements: C, H, O, N are most abundant. Dry weight analysis reveals proteins as the most abundant organic molecules in cells.
Carbohydrates
General formula: Cn(H2O)n. Monosaccharides: Simplest sugars โ Trioses (glyceraldehyde, dihydroxyacetone), Pentoses (ribose in RNA, deoxyribose in DNA), Hexoses (glucose, fructose, galactose). Disaccharides: Sucrose (glucose + fructose), Maltose (glucose + glucose), Lactose (glucose + galactose). Formed by glycosidic bond. Polysaccharides: Starch (storage in plants), Glycogen (storage in animals), Cellulose (structural in plant cell wall โ most abundant organic molecule on Earth), Chitin (in fungal cell wall and arthropod exoskeleton), Inulin (polymer of fructose).
Proteins
Polymers of amino acids linked by peptide bonds. 20 standard amino acids. Essential amino acids cannot be synthesized by the body. Structure levels: Primary (linear sequence), Secondary (alpha-helix or beta-pleated sheet โ H bonds), Tertiary (3D folding โ disulfide bonds, hydrophobic interactions), Quaternary (multiple polypeptide chains โ e.g., haemoglobin has 4 subunits). Proteins function as enzymes, structural components (collagen, keratin), transport (haemoglobin), defense (antibodies), hormones (insulin).
Lipids
Insoluble in water, soluble in organic solvents. Simple lipids: Fats and oils โ glycerol + 3 fatty acids (ester bond). Saturated fats (no double bonds โ solid at room temperature, e.g., butter) vs Unsaturated fats (double bonds present โ liquid, e.g., vegetable oil). Compound lipids: Phospholipids (in cell membrane), Glycolipids. Steroids: Cholesterol, hormones (testosterone, estrogen, cortisol).
Nucleic Acids
Polymers of nucleotides. Each nucleotide = nitrogenous base + pentose sugar + phosphate group. DNA: Deoxyribose sugar, bases A-T (2 H-bonds), G-C (3 H-bonds), double-stranded helix (Watson & Crick model). RNA: Ribose sugar, bases A-U, G-C, usually single-stranded. Types: mRNA, tRNA, rRNA.
Enzymes
Biological catalysts, mostly proteins (except ribozymes โ RNA enzymes). Properties: Highly specific, not consumed in reaction, lower activation energy, work best at optimum temperature and pH. Cofactors: Non-protein components needed for enzyme activity โ Metal ions (Zn, Mn, Co), Coenzymes (organic โ NAD, FAD, CoA), Prosthetic groups (tightly bound โ haem in haemoglobin). Holoenzyme = Apoenzyme (protein part) + Cofactor.
Inhibition: Competitive inhibition (substrate analog binds active site โ reversed by increasing substrate concentration) vs Non-competitive inhibition (inhibitor binds at allosteric site โ not reversed by substrate).
Summary
Biomolecules include carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates serve as energy sources and structural components. Proteins have diverse functions and four structural levels. Lipids form membranes and store energy. Nucleic acids (DNA and RNA) carry genetic information. Enzymes are biological catalysts that lower activation energy.
Important Terms
- Peptide bond: Covalent bond between amino group and carboxyl group of amino acids
- Glycosidic bond: Bond between monosaccharides in carbohydrates
- Activation energy: Energy required to start a chemical reaction; enzymes lower it
- Cofactor: Non-protein component needed for enzyme function
- Denaturation: Loss of protein structure due to heat, pH change, etc.
- Ribozyme: RNA molecule with catalytic activity
- Holoenzyme: Complete enzyme = apoenzyme + cofactor
- Nucleotide: Monomer of nucleic acid = base + sugar + phosphate
Quick Revision
- Most abundant organic molecule on Earth: Cellulose
- Sucrose = glucose + fructose; Lactose = glucose + galactose
- Protein structure: Primary โ Secondary โ Tertiary โ Quaternary
- DNA: A=T (2 H-bonds), GโกC (3 H-bonds)
- Enzymes: specific, lower activation energy, not consumed
- Competitive inhibition: reversed by excess substrate
- Ribozymes = RNA enzymes (non-protein catalysts)
- Glycogen = animal starch; stored in liver and muscles