Biological Molecules
Subject: Biology
Topic: 3
Cambridge Code: 0610 / 0970 / 5090
Carbohydrates
Carbohydrates - Molecules containing C, H, O in ratio 1:2:1
Monosaccharides (Simple Sugars)
Glucose:
- Hexose sugar (6 carbons)
- Used in respiration
- Forms structure with oxygen (aldehyde group)
Fructose: Same formula as glucose (isomer)
- Sweetest sugar
- Found in fruits
Ribose and Deoxyribose:
- Pentose sugars (5 carbons)
- In RNA and DNA
Disaccharides (Double Sugars)
Formed by condensation of two monosaccharides
- Maltose: Glucose + Glucose
- Sucrose: Glucose + Fructose
- Lactose: Glucose + Galactose
Condensation reaction: Two sugars join, releasing water
Polysaccharides
Polysaccharides - Chains of monosaccharides
Starch
- Energy storage in plants
- Insoluble in water
- Mixture of: amylose (unbranched) and amylopectin (branched)
- Broken down to glucose
Glycogen
- Energy storage in animals
- More branched than starch
- Stored in liver and muscles
- Quickly mobilized
Cellulose
- Structural support in plants
- Insoluble, fibrous
- Main component of cell walls
- Humans cannot digest (no cellulase enzyme)
Proteins
Proteins - Polymers of amino acids
Amino Acids
- Contain: amino group (NH₂), carboxyl group (COOH), R group
- 20 different amino acids in proteins
- R group determines properties
Peptide Bonds
Condensation between amino acids forming peptide bond
Protein Structure
Primary: Sequence of amino acids (determined by DNA)
Secondary:
- Alpha helix (coiled)
- Beta pleated sheet (folded)
- Stabilized by hydrogen bonds
Tertiary: 3D folded shape
- Stabilized by disulfide bonds, hydrogen bonds, ionic bonds
- R groups interact
Quaternary: Multiple polypeptide chains (e.g., hemoglobin)
Denaturation
Breaking of bonds in protein causing loss of function
- Heat, pH extremes, heavy metals
- Process usually irreversible
Lipids
Lipids - Non-polar organic molecules, insoluble in water
Triglycerides
Formed by: Glycerol + 3 Fatty Acids
Saturated fat: All C-C single bonds
- Solid at room temperature
- Examples: butter, animal fat
Unsaturated fat: Contains C=C double bonds
- Liquid at room temperature
- Examples: olive oil, vegetable oil
Phospholipids
- Glycerol + 2 Fatty Acids + Phosphate group
- Main component of cell membranes
- Amphipathic (hydrophobic tail, hydrophilic head)
Cholesterol
- Sterol lipid
- Component of cell membranes
- Precursor for steroid hormones
Nucleic Acids
Nucleic Acids - Polymers of nucleotides
Nucleotide Structure
- Sugar (ribose or deoxyribose)
- Phosphate group
- Nitrogenous base
DNA
- Deoxyribose sugar
- Double helix structure
- Bases: A, T, G, C
- Base pairing: A-T (2 bonds), G-C (3 bonds)
- Stores genetic information
RNA
- Ribose sugar
- Usually single-stranded
- Bases: A, U, G, C (uracil instead of thymine)
- Types: mRNA, tRNA, rRNA
Testing for Biological Molecules
| Molecule | Test | Positive Result |
|---|---|---|
| Reducing sugar | Benedict's test | Brick-red precipitate |
| Starch | Iodine | Blue-black color |
| Protein | Biuret test | Purple color |
| Lipid | Sudan III test | Orange-red color |
Key Points
- Carbohydrates: energy and structure
- Proteins: enzymes, antibodies, structure
- Lipids: energy storage, membranes
- Nucleic acids: genetic information
- Condensation and hydrolysis reactions
- Know structure-function relationships
Practice Questions
- Compare starch and glycogen storage
- Draw dipeptide formation
- Explain protein denaturation
- Distinguish saturated/unsaturated fats
- Describe DNA structure
- Predict test results for molecules
Revision Tips
- Know all monomer units
- Understand condensation reactions
- Learn protein structure levels
- Distinguish between lipid types
- Know DNA vs RNA differences
- Practice tests for molecules