Biological Molecules - Summary

Water molecules are held together by hydrogen bonds, which form due to the attraction between the partially positive hydrogen atom of one molecule and the partially negative oxygen atom of another. This creates cohesion between molecules.

Water is polar, meaning it has regions of partial positive and negative charge. This allows it to dissolve many ionic and polar substances, enabling metabolic reactions to occur in solution within living organisms.

Water has a high specific heat capacity, meaning it absorbs a lot of heat before its temperature changes. This stabilizes environments and maintains enzyme activity at optimum temperatures. It also has a high latent heat of vaporization, allowing it to cool surfaces during sweating.

Ice has a lower density because its molecules form a lattice with hydrogen bonds that push them further apart. This causes ice to float, insulating the water below and providing a stable habitat for aquatic life.

A monomer is a small basic molecular subunit, while a polymer is a large, complex molecule made up of repeating monomer subunits joined by covalent bonds through condensation reactions.

In condensation, two monomers join by removing a water molecule. A covalent bond forms when an OH group from one monomer and an H from another are removed, producing water as a byproduct.

Hydrolysis splits a polymer into monomers through the addition of water. A covalent bond is broken as an OH group and an H atom are added to the resulting molecules.

Carbohydrates are composed of carbon (C), hydrogen (H), and oxygen (O).

Examples include glucose (energy source for cells), fructose (fruit sugar), ribose (in RNA), and galactose. Glucose is a hexose sugar (6 carbons), while ribose is a pentose sugar (5 carbons).

A disaccharide is formed when two monosaccharides join via a glycosidic bond in a condensation reaction, releasing water. Examples include maltose (glucose + glucose), sucrose (glucose + fructose), and lactose (glucose + galactose).

Starch is a polysaccharide made of α-glucose with glycosidic bonds. It includes amylose (unbranched 1,4 bonds, helical) and amylopectin (branched 1,4 and 1,6 bonds). It serves as plant energy storage, is compact, insoluble, and easily hydrolysed to release glucose.

Glycogen, also made of α-glucose, is more highly branched than starch. This allows faster hydrolysis for glucose release during high energy demand. It is found in animals, insoluble, and highly coiled for compact storage.

Cellulose is made of β-glucose molecules linked by 1,4 glycosidic bonds. Alternate glucose molecules flip 180°, allowing hydrogen bonds between chains, forming strong microfibrils. It gives plant cell walls high tensile strength and rigidity.

It detects reducing sugars. Add Benedict’s reagent to a sample and heat in a boiling water bath. A positive result forms a colored precipitate (green to brick red depending on sugar concentration).

First, hydrolyse with dilute HCl and heat. Neutralise with sodium hydrogen carbonate, then perform the Benedict’s test. A brick-red precipitate indicates a non-reducing sugar like sucrose.

It tests for starch. Add iodine in potassium iodide solution. A color change from brown/orange to blue-black (amylose) or purple-black (amylopectin) indicates a positive result.

Lipids contain carbon, hydrogen, and oxygen. They are nonpolar and hydrophobic, making them insoluble in water. A triglyceride consists of one glycerol and three fatty acids joined by ester bonds.

Saturated fatty acids have no double bonds, are straight, tightly packed, solid at room temperature, and usually from animal fats. Unsaturated fatty acids have one or more double bonds, are bent, loosely packed, liquid at room temperature, and usually from plants.

At high temperatures, cholesterol stabilises membranes by reducing fluidity and increasing melting point. At low temperatures, it prevents tight packing and stops freezing by maintaining fluidity.

Use the emulsion test. Mix the sample with ethanol, shake, then add water and shake again. A milky/cloudy white emulsion indicates lipids are present.

Proteins contain C, H, O, N (and sometimes S). Their monomers are amino acids. Functions include metabolism, structure, transport, and immunity.

It detects peptide bonds in proteins. Add Biuret reagent (copper sulfate + sodium hydroxide). A purple color indicates a positive result.

Primary: amino acid sequence. Secondary: α-helix and β-sheet (hydrogen bonds). Tertiary: 3D folding (ionic, hydrogen, disulfide, hydrophobic bonds). Quaternary: multiple polypeptides (e.g., haemoglobin).

Fibrous: long, insoluble, structural (e.g., collagen, keratin, elastin). Globular: compact, soluble, functional (e.g., enzymes, antibodies, hormones, haemoglobin).