Microscopy and Food Tests

Microscopy is all about getting a clear view of tiny specimens like onion cells. Cut your specimen thin, add a drop of iodine stain, and carefully lower the coverslip to avoid air bubbles. Always start with the lowest magnification and work your way up whilst adjusting the focus knobs.

Food tests reveal what nutrients are hiding in your samples. You'll need to grind your food with distilled water, filter it, then test for different molecules. Benedict's solution turns green-yellow-red for reducing sugars, iodine goes blue-black for starch, Biuret reagent turns purple for proteins, and Sudan III creates a red layer for lipids.

Safety is crucial - wear goggles for all chemical tests and handle ethanol carefully since it's highly flammable. The emulsion test using ethanol will show a cloudy mixture if lipids are present.

Top Tip: Remember the colour changes - they're often the key to exam success!

Culturing Microorganisms and Osmosis

Culturing bacteria requires sterile technique to prevent contamination. Sterilise your inoculation loop until red-hot, flame the flask neck, and keep the Petri dish lid barely open. Use zig-zag streaks to spread bacteria and add antiseptic discs to test their effectiveness.

Store your plates upside down at maximum 25°C to prevent dangerous bacteria growing. After 48 hours, measure the zone of inhibition around each disc - larger clear areas mean stronger antiseptics.

Osmosis experiments use potato cylinders in different sugar concentrations. Cut identical cylinders, measure their mass and length precisely, then leave them overnight in solutions. Calculate percentage changes to see how water moves in and out of cells.

Plot graphs showing mass change against sugar concentration to visualise osmosis in action. Higher sugar concentrations will draw water out, making potatoes lighter and shorter.

Remember: Sterile technique prevents contamination - flame everything that touches your culture!

Photosynthesis and Enzyme Experiments

Photosynthesis can be measured by counting oxygen bubbles from pondweed. Place your setup 10cm from an LED light source and add sodium hydrogen carbonate for extra carbon dioxide. Count bubbles for one minute, repeat three times, then test different distances.

Use the inverse square law to calculate light intensity and plot it against photosynthesis rate. Closer light sources provide more energy for faster photosynthesis, producing more oxygen bubbles per minute.

Enzyme activity tests show how pH affects amylase breaking down starch. Mix amylase with buffer solution and starch, then test drops every 10 seconds with iodine solution. When iodine stays orange-brown instead of turning blue-black, all the starch has been digested.

The quicker this colour change happens, the better the enzyme works at that pH. Test different pH values to find the optimum conditions where amylase works fastest.

Key Point: Enzymes work best at specific pH levels - find the sweet spot where starch breaks down quickest!