Comprehensive Summary of Biological Molecules

Water Properties and Molecular Basics

Ever wondered why life exists in water? Water's unique properties make it perfect for supporting life. Its ability to flow creates habitats and transport systems, whilst its high density keeps aquatic environments stable and insulated from temperature changes.

Water molecules are polar, meaning they have both positive and negative charges. The oxygen atom carries a slight negative charge, whilst hydrogen atoms are slightly positive. This creates hydrogen bonds between water molecules - weak attractions that give water its special properties like high surface tension (think insects walking on water) and excellent solvent abilities.

You'll need to master some key definitions here. Condensation reactions join molecules by removing water, whilst hydrolysis reactions break them apart by adding water. Monomers are single units that link together to form polymers - like building blocks creating larger structures.

Key Insight: Remember that carbohydrates are made from monosaccharides, proteins from amino acids, and nucleic acids from nucleotides.

Carbohydrates: Sugars and Starches

Carbohydrates are your body's main energy source, containing carbon, hydrogen, and oxygen. Think of them as fuel for your cells - from the glucose in your blood to the starch stored in potatoes.

Monosaccharides like glucose are single sugar units. When two join together through condensation reactions, they form disaccharides. Maltose comes from two glucose molecules, sucrose (table sugar) from glucose and fructose, and lactose (milk sugar) from glucose and galactose.

Polysaccharides are long chains of sugar units serving different purposes. Plants store energy as starch (made of amylose and amylopectin), whilst animals use glycogen for energy storage. The branching structure of glycogen makes it more compact - perfect for quick energy release when you need it.

Ribose and deoxyribose are special five-carbon sugars. Ribose appears in RNA, whilst deoxyribose (with one less oxygen atom) forms part of DNA structure.

Memory Tip: Remember that condensation removes water (like condensation on windows disappearing), whilst hydrolysis adds water back.

Cellulose and Structural Carbohydrates

Cellulose might be impossible for humans to digest, but it's crucial for plant structure. Made from beta glucose units, cellulose forms straight chains rather than the coiled shapes of starch and glycogen.

The key difference lies in the bonding. Beta glucose molecules alternate their orientation, creating strong, straight chains held together by 1-4 glycosidic bonds. These chains bundle together to form microfibrils (up to 70 chains) and macrofibrils (up to 400 chains).

This bundling creates incredible strength in plant cell walls. The crosslinks between chains provide structural stability, whilst spaces between bundles allow water and minerals to pass through freely. Additional substances like cutin add extra reinforcement.

Other organisms use different structural molecules. Bacterial cell walls contain peptidoglycan, whilst some animals have chitin in their exoskeletons. All follow the same principle: long chains crosslinked for maximum strength.

Real-World Connection: This is why paper (made from cellulose) is so strong, and why dietary fibre helps your digestion despite being indigestible.

Lipids: Fats and Membranes

Lipids are mostly carbon and hydrogen with little oxygen, making them hydrophobic (water-repelling). This property makes them perfect for energy storage and waterproofing.

Triglycerides form when glycerol (a three-carbon alcohol) bonds with three fatty acids through ester bonds. These molecules serve multiple functions: energy storage (more concentrated than carbohydrates), insulation, buoyancy for aquatic animals, and protection around organs.

Fatty acids can be saturated (all single bonds) or unsaturated (containing double bonds). The structure affects properties - saturated fats are typically solid at room temperature, whilst unsaturated fats remain liquid.

Phospholipids contain two fatty acids and a phosphate group attached to glycerol. The phosphate head is hydrophilic (water-loving) whilst fatty acid tails are hydrophobic. In cell membranes, they arrange as a bilayer with heads facing outward and tails pointing inward, creating a selective barrier.

Practical Point: Understanding lipid structure explains why oil and water don't mix, and how cell membranes control what enters and exits cells.

Proteins: Structure and Function

Proteins are incredibly versatile molecules made from amino acids. Each amino acid contains an amino group, a carboxyl group, and a variable R group that determines its properties.

Amino acids join through condensation reactions, forming peptide bonds and releasing water. Two amino acids create a dipeptide, whilst long chains form polypeptides and proteins.

Protein structure has four levels. Primary structure is the amino acid sequence. Secondary structure involves folding into alpha helixes or beta sheets using hydrogen bonds. Tertiary structure creates 3D shapes through various bonds (hydrogen, ionic, disulphide). Quaternary structure combines multiple polypeptide chains.

Fibrous proteins like collagen, keratin, and elastin provide structural support. Collagen gives strength to tendons and bones. Keratin (rich in disulphide bridges) forms hair and nails. Elastin stretches in skin and blood vessels.

Globular proteins are spherical and water-soluble. Insulin regulates blood glucose levels, whilst pepsin digests proteins in your stomach. Their specific 3D shapes determine their functions.

Study Strategy: Focus on how structure determines function - a protein's shape directly relates to its job in the body.