Key concepts in biology study guide

This core practical involves planning and carrying out investigations to identify the presence of starch, reducing sugars, proteins, and fats in unknown food samples using specific chemical reagents. Each reagent produces a characteristic colour change in the presence of the target molecule. For starch, iodine solution turns from yellow-brown to blue-black. Reducing sugars are detected with Benedict's solution, which, upon heating, changes from blue to green, yellow, orange, or brick-red depending on the sugar concentration. Proteins are identified using the Biuret test, where sodium hydroxide and copper(II) sulfate solution produce a lilac/purple colour from an initial blue. Fats and oils are detected by the emulsion test, where ethanol is added to the sample, then water, resulting in a cloudy white emulsion if fat is present. Control experiments using distilled water or known pure substances are crucial for accurate comparison. The independent variable is the type of food sample being tested, while the dependent variable is the observed colour change. Important control variables include reagent concentration, sample volume, heating time (for Benedict's test), and temperature. Repeat trials improve the reliability of results. Careful observation and recording of colour changes are essential for accurate conclusions.

Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution). When studying osmosis in living tissues, such as potato cylinders, we often measure changes in mass. If the tissue gains mass, it has absorbed water via osmosis. If it loses mass, it has lost water. To quantify these changes, we calculate the percentage change in mass. This involves finding the difference between the final and initial masses, dividing by the initial mass, and then multiplying by 100. This percentage allows for a standardised comparison between experiments, regardless of the initial mass of the sample.