Looking at Cells and Investigating Osmosis

Observing plant cells is dead easy once you get the hang of it. You'll peel off a thin layer of onion skin, pop it on a slide with some water, and add iodine solution to make the cells visible. The trick is using the lowest magnification first to find your cells, then switching to higher powers for detail.

When drawing what you see, use continuous lines with no gaps and always include the magnification. Safety-wise, keep iodine solution away from your skin and eyes - it's not dangerous but it stains everything!

Osmosis experiments use potato cylinders in different sugar concentrations. Cut three identical cylinders, weigh them, then leave overnight in solutions ranging from pure water to concentrated sugar. The potatoes will gain or lose mass depending on water movement.

Top tip: Always remove excess water with a paper towel before weighing - you want the potato's mass, not the solution clinging to it!

Understanding Osmosis Results and Food Tests

The osmosis results make perfect sense when you think about it. Sugar molecules are too chunky to pass through cell membranes, but water molecules slip through easily. In concentrated sugar solutions, water leaves the potato (making it lighter), whilst in pure water, it moves in (making it heavier).

Always remove potato skin because it acts as an extra barrier, and use identical cylinder sizes so the surface area for osmosis stays constant.

Food tests are your chemical detectives for identifying nutrients. Iodine turns blue-black for starch, Benedict's solution changes from blue to brick red for sugars (needs heating in a water bath), Biuret reagent goes purple for proteins, and the ethanol test makes lipids go cloudy.

Remember: Benedict's test is quantitative - different sugar concentrations give different colours from green (low) to brick red (high).

Enzyme Activity and Growing Microorganisms

Testing pH effects on amylase shows how enzymes work best at specific conditions. You'll mix starch, amylase, and different pH buffers, then test drops with iodine every 30 seconds. When the iodine stops turning blue-black, all the starch has been broken down.

The optimum pH might fall between your test values, so don't be surprised if your results aren't perfectly clear-cut. Timing every 30 seconds gives approximate results, and colour changes can be gradual.

Growing microorganisms requires aseptic technique to prevent contamination. Sterilise everything with flame, tape petri dishes so they can't fall open, and incubate upside down at 25°C to stop harmful bacteria thriving.

Safety first: 25°C prevents dangerous pathogens from growing, whilst higher temperatures in schools could create health risks.

Antibiotics and Photosynthesis Experiments

Testing antibiotic effectiveness uses the same sterile techniques but adds filter paper discs soaked in different antibiotics. The zone of inhibition (clear area where bacteria can't grow) shows which antibiotic works best. Calculate the area using πr² to compare effectiveness properly.

Photosynthesis experiments measure oxygen bubble production from pondweed at different light distances. Use LED lights to avoid heat affecting your results, and sodium hydrogen carbonate solution to provide plenty of CO₂.

Count bubbles produced in one minute, but be aware this method has limitations - bubbles come too fast and aren't uniform sizes. Better results come from measuring oxygen volume with a gas syringe.

Key insight: When you double the distance from the light, bubble production drops by four times - this demonstrates the inverse square law of light intensity.

Understanding Light Intensity and Distance

The relationship between distance and light intensity follows a predictable pattern that's crucial for your exams. As distance increases, light intensity decreases dramatically - specifically following the inverse square law.

This means if you move your pondweed twice as far from the light source, it receives four times less light energy. Consequently, photosynthesis rate drops by the same factor, producing far fewer oxygen bubbles.

Understanding this relationship helps explain why plants grow differently at various distances from light sources, and why measuring precise distances matters in your experiments.