Using a light microscope

To view cells with a light microscope:

• Prepare a thin sample on a slide, add a stain (e.g. iodine) to make structures visible, and lower a coverslip to avoid air bubbles.
• Start on the lowest-power objective; focus using the coarse then fine focus.
• Total magnification = eyepiece × objective lens.

Draw a clear labelled diagram with a sharp pencil, no shading, and always record the magnification used.

Your turn — total magnification

Your turn — real size

Food tests

Standard tests identify the biological molecules in a food:

• Starch — add iodine solution: orange/brown → blue-black.
• Reducing sugars (e.g. glucose) — add Benedict's solution and heat in a water bath: blue → green → yellow → brick-red (more sugar = redder).
• Protein — add Biuret solution: blue → purple/lilac.
• Lipids (fats) — the emulsion test with ethanol gives a milky-white emulsion.

Remember: Benedict's must be heated; iodine and Biuret work at room temperature. A colour change shows a positive result.

Testing for starch

Testing for reducing sugar

Match each food test to its result

Tap a test, then its positive result.

Aseptic technique & sampling

Aseptic technique keeps microbial cultures free from contamination:

• Sterilise the inoculating loop in a Bunsen flame before and after use.
• Work near the flame; lift the Petri dish lid as little as possible.
• Tape the lid (not fully sealed) and incubate at 25 °C in schools to limit growth of human pathogens.

Sampling: use random quadrats to estimate populations and transects to study change with distance. Scale up the mean per quadrat to the whole area.

Why 25 °C? incubating below body temperature reduces the risk of growing pathogens that thrive at 37 °C.

Why sterilise the loop?

Variables & fair testing

Every good experiment controls its variables:

• Independent variable — the one you deliberately change.
• Dependent variable — the one you measure.
• Control variables — kept the same to make it a fair test.

Good data is repeatable (same person, same method), reproducible (others get the same), accurate (close to the true value) and precise (values close together).

Fair test: only the independent variable should change — control everything else, or you can't be sure what caused the result.

Which is the dependent variable?

Evaluating experiments

A good evaluation judges how much you can trust your results:

• Identify anomalous results (that don't fit the pattern) and consider ignoring them when calculating a mean.
• Suggest sources of error and how to improve accuracy (better equipment) and reliability (repeat and take a mean).
• Comment on whether the evidence supports the conclusion.

Reliability tip: repeating readings and calculating a mean reduces the effect of random error and makes results more reliable.

Sort each into its group

Tap each item, then tap which type of variable it is.

The big ideas to know

Press Finish to see your score.