Biological Molecules

The One Thing

The structure of a biological molecule determines its properties and therefore its function in living organisms. Condensation reactions build larger biological molecules, whereas hydrolysis uses water to break them down; food tests identify these molecules through characteristic chemical changes.

Reactions, Processes and Experiments

Key Terms

• Carbohydrate: An organic molecule made mainly of carbon, hydrogen and oxygen; it includes sugars and starch.
• Glucose: A simple sugar used in respiration to release energy.
• Monosaccharide: A single sugar unit, such as glucose.
• Disaccharide: A sugar made from two monosaccharides joined together, such as maltose.
• Polysaccharide: A large carbohydrate made from many sugar units, such as starch, glycogen or cellulose.
• Starch: A compact, insoluble storage carbohydrate found mainly in plants.
• Glycogen: A storage carbohydrate found mainly in animals and fungi; it is highly branched for rapid glucose release.
• Cellulose: A structural polysaccharide in plant cell walls; its fibres provide strength.
• Condensation reaction: A reaction that joins molecules together and produces water.
• Hydrolysis: A reaction that uses water to break a large molecule into smaller molecules.
• Glycosidic bond: The bond formed when two sugar molecules join in a condensation reaction.
• Lipid: A group of water-insoluble molecules including fats and oils, used for energy storage and other functions.
• Glycerol: An alcohol molecule that forms part of a triglyceride.
• Fatty acid: A long hydrocarbon chain with an acidic end; fatty acids combine with glycerol to form lipids.
• Triglyceride: A lipid formed from one glycerol molecule and three fatty acid molecules.
• Ester bond: The bond formed between glycerol and a fatty acid during a condensation reaction.
• Saturated fat: A fat whose fatty acid chains contain no carbon-carbon double bonds.
• Unsaturated fat: A fat whose fatty acid chains contain one or more carbon-carbon double bonds.
• Protein: A large molecule made from amino acids and folded into a specific shape.
• Amino acid: A building block of proteins containing an amino group, a carboxyl group and a variable R group.
• Peptide bond: The bond formed between two amino acids in a condensation reaction.
• Polypeptide: A chain of many amino acids joined by peptide bonds.
• Denaturation: A permanent change in a protein’s shape caused by factors such as high temperature or extreme pH, usually stopping its function.
• Enzyme: A protein that acts as a biological catalyst by speeding up chemical reactions.
• Water: An inorganic molecule that acts as a solvent, transport medium, reactant and temperature regulator.
• Reducing sugar test: A test using Benedict’s solution and heat to detect reducing sugars such as glucose.
• Iodine test: A test for starch in which iodine solution changes from orange-brown to blue-black if starch is present.
• Biuret test: A test for protein in which biuret reagent changes from blue to lilac or purple.
• Emulsion test: A test for lipids in which ethanol is added to the food sample, followed by water; a cloudy white emulsion indicates lipid.

Easily Confused

• Monosaccharide, disaccharide and polysaccharide: A monosaccharide is one sugar unit, a disaccharide contains two joined monosaccharides, and a polysaccharide contains many sugar units.
• Starch and glycogen: Starch is mainly a plant storage carbohydrate, whereas glycogen is mainly found in animals and fungi and is highly branched.
• Starch and cellulose: Starch is a storage carbohydrate, whereas cellulose is a structural polysaccharide forming strong fibres in plant cell walls.
• Condensation and hydrolysis: Condensation joins molecules and produces water; hydrolysis uses water to split molecules.
• Glycosidic, ester and peptide bonds: Glycosidic bonds join sugar molecules, ester bonds join glycerol to fatty acids, and peptide bonds join amino acids.
• Saturated and unsaturated fats: Saturated fats contain no carbon-carbon double bonds in their fatty acid chains; unsaturated fats contain one or more.
• Benedict’s test and iodine test: Benedict’s test identifies reducing sugars and requires heating; iodine identifies starch and changes to blue-black when starch is present.
• Biuret test and emulsion test: The Biuret test identifies protein through a lilac or purple colour; the emulsion test identifies lipid through a cloudy white suspension.
• Positive and negative controls: A known sample acts as a positive control, whereas distilled water acts as a negative control.
• Water bath and naked flame: Benedict’s test should be heated in a water bath; a naked flame is unsuitable because the procedure involves liquid and ethanol used in lipid testing may be flammable.

What Gets Asked

• Structural identification: Questions may ask how the structures of starch, glycogen, cellulose, lipids or proteins suit their functions. Marks are lost by failing to link structure to function, such as starch’s insolubility, glycogen’s branching or cellulose’s strong fibres.
• Chemical equations: Questions may require the equations for photosynthesis, 6CO2 + 6H2O -> C6H12O6 + 6O2, and respiration, C6H12O6 + 6O2 -> 6CO2 + 6H2O. Marks are lost by omitting reactants or products.
• Condensation and hydrolysis: Questions may ask how monosaccharides, triglycerides or amino acids are joined or broken apart. The key distinction is that condensation produces water, whereas hydrolysis uses water.
• Food-test identification: Questions may present colour changes and require identification of the nutrient. The specific results are: Benedict’s positive results progress towards brick-red; iodine becomes blue-black for starch; Biuret becomes lilac or purple for protein; and the emulsion test produces a cloudy white suspension for lipid.
• Experimental method and controls: Questions may ask for the correct procedure, including heating Benedict’s test in a hot water bath, or the use of positive controls and distilled-water negative controls. Marks are lost by using a naked flame or omitting suitable controls.
• Water’s biological importance: Questions may require explanations based on polarity, solvent properties, high specific heat capacity and participation in hydrolysis and photosynthesis. Answers should connect these properties to transport, temperature regulation and chemical reactions.