Pearson Edexcel International GCSE Science Single Award Practical Skills 2: Biology observations and investigations

This note maps the explicitly labelled Biology practical points in Pearson Science Single Award 4SS0: food tests, enzyme temperature, photosynthesis evidence, quadrat sampling and yeast respiration. It is a method-and-evidence guide for written assessment, not a separate laboratory paper.

Food tests

Use separate clean samples and controls:

• iodine solution changes orange-brown to blue-black with starch;
• heated Benedict's solution changes from blue toward green, yellow, orange or brick-red with reducing sugar;
• biuret reagent changes blue to lilac or purple with protein;
• ethanol followed by water produces a cloudy white emulsion with lipid.

Record starting and final observations. A known positive confirms that reagents can produce the expected result, while a negative control shows the baseline. Use a water bath for Benedict's test rather than heating a sealed tube directly. Ethanol is flammable, so keep it away from flames.

Enzyme activity and temperature

A suitable investigation can time starch disappearance when amylase acts. Prepare equal amylase and starch volumes at controlled pH, equilibrate them in a water bath, mix and sample at fixed intervals onto iodine wells. The endpoint is the first sample that no longer turns blue-black.

Temperature is independent and time to endpoint is measured. Rate may be represented by $1/t$. Control pH, concentrations, volumes, sampling interval and endpoint judgement. Very high temperature can denature the enzyme, while low temperature slows collisions.

Colour judgement is subjective. Use the same observer and lighting, a reference colour or a colorimeter where appropriate. Repeat with fresh mixtures because continuing to sample one mixture is not an independent repeat.

Photosynthesis evidence

Pearson requires practical understanding of oxygen production, starch formation and the need for light, carbon dioxide and chlorophyll.

For oxygen production, an aquatic plant can release bubbles or gas under controlled light. Gas volume is stronger than bubble count because bubble sizes vary. Control plant species and length, temperature, carbon dioxide availability, time and lamp distance.

For starch evidence, destarch a plant in darkness, expose it under the chosen condition, then test a leaf. Heat the leaf in water, use hot ethanol in a water bath to remove chlorophyll, rinse and add iodine. A covered leaf region tests the need for light. Carbon dioxide can be removed in a sealed setup with a suitable absorber and compared with a control. A variegated leaf compares green and non-green regions for chlorophyll.