practicals Flashcards
(9 cards)
microscopy
Peel a thin layer of onion epidermis and place it flat on a clean slide.
Add a drop of iodine solution (stains the nucleus and cytoplasm).
Carefully place a coverslip on top using a needle to avoid air bubbles.
Start observing under low power lens (e.g. ×4), then switch to higher magnification (×10 or ×40) if needed.
Focus using coarse then fine adjustment knobs.
Draw and label visible structures (cell wall, nucleus, cytoplasm).
Controls: Use same stain, same size tissue.
Allows you to measure cells and identify basic structures.
microbiology
Sterilise petri dishes, agar, and inoculating loop using heat (e.g. autoclave or flaming loop).
Pour sterile nutrient agar into petri dishes and allow it to set.
Dip sterile loop into bacterial culture, streak onto agar in zigzag pattern.
Seal the lid with tape (not fully – allow oxygen in).
Incubate at 25°C for 48 hours (higher temps increase risk of growing harmful pathogens).
For antibiotics/disinfectants: soak paper discs and place on agar; measure zone of inhibition (clear area = bacteria killed).
IV = type/concentration of antibiotic; DV = size of inhibition zone; controls = same bacteria, same temp/time.
Aseptic technique prevents contamination of sample and surroundings.
Safety: wear gloves, clean surfaces, dispose of cultures safely
osmosis on potatoes
Cut equal-sized potato cylinders (same length & diameter).
Measure and record initial mass of each.
Place each cylinder in different sucrose concentrations (e.g. 0.0, 0.2, 0.4, 0.6, 0.8 mol/dm³) for at least 30 minutes.
Remove, blot dry, reweigh.
Calculate percentage change in mass
IV = concentration of solution; DV = change in mass; controls = temperature, time, volume of solution, surface area.
Osmosis: water moves from dilute to concentrated solutions through a partially permeable membrane.
Cylinders in high sucrose lose mass (water leaves), in pure water gain mass.
enzymes and temperature pH
Add equal volumes of amylase, starch, and pH buffer to a test tube.
Place in a water bath at a set temperature (e.g. 20°C).
Every 30 seconds, take a sample and add to iodine in a spotting tile.
Record time taken for iodine to stop turning blue-black (starch broken down).
Repeat for different temperatures (e.g. 20, 30, 40, 60°C).
IV = temperature, DV = time taken for starch to be digested; controls = volume, pH, concentration.
Shows enzyme has an optimum temp (~37°C); too hot = enzyme denatures and stops working.
food tests
Starch: Add iodine – turns blue-black if present.
Sugar: Add Benedict’s reagent, heat in water bath – turns green → yellow → brick red depending on amount.
Protein: Add Biuret solution – turns purple if proteins present.
Lipids: Add ethanol, then water – forms cloudy white emulsion if fats present.
Must use known quantities and repeat tests.
Qualitative tests: detect presence, not exact amount.
Ensure safety with boiling tubes (e.g. goggles, water bath, avoid naked flames).
photosynthesis
Set up pondweed (e.g. Elodea) in water with sodium hydrogencarbonate (CO₂ source).
Place light source at different distances (e.g. 10, 20, 30 cm).
Count oxygen bubbles released in 1 minute or measure gas volume with a syringe.
Repeat and take mean for each distance.
IV = distance (light intensity), DV = rate of photosynthesis; controls = temp, CO₂, same plant.
Can use inverse square law: light ∝ 1/distance².
More light = faster photosynthesis up to a point (limiting factors apply).
human reaction time
Person A sits with arm on table; person B drops ruler without warning.
Measure the distance caught – convert to reaction time using a chart.
Repeat and average 3+ times.
Test different conditions (e.g. after caffeine, with distractions).
IV = factor affecting reaction (e.g. caffeine), DV = distance or time; controls = same ruler, same person, same height.
Simple, reproducible test; improves accuracy by repeating.
Safety: use non-glass rulers if testing many people.
plant hormones
Place cress seeds on damp cotton wool in Petri dishes.
Shine light from one direction only (e.g. lamp on side).
Leave for 2–3 days.
Measure angle or direction of shoot growth
IV = light direction, DV = growth angle or length; controls = temp, seed type, water.
Plants show phototropism – shoot bends toward light due to uneven auxin distribution.
Supports role of hormones in growth response.
decay and temperature
Mix milk with pH indicator (e.g. cresol red) and lipase enzyme.
Place in water baths at different temperatures.
Time how long it takes for colour to change (acid from decay = lower pH).
IV = temperature, DV = time for indicator to change; controls = milk volume, enzyme volume, pH.
Decay happens faster at higher temps (up to ~40°C); slows if too hot (enzymes denature).
Application: food storage, composting.
