Introduction to Biology Required Practicals
In your OxfordAQA Biology (9201) course, there are five mandatory practicals that you must study. Even though you won't take a practical exam in a lab, your written papers (especially Paper 2) will ask you detailed questions about how these experiments are designed, what the results mean, and how to keep them safe.
Think of these practicals as the "scripts" of science. If you understand the how and the why behind each one, you will be able to answer exam questions with confidence. Let’s dive in!
Note: For more details on choosing equipment or analyzing results, see the other chapters in the "Practices of Science" section.
1. Transport in Cells (Osmosis) [B]
This practical investigates how different concentrations of solutions affect the movement of water across a partially permeable membrane.
The Experiment
Most students use potato cylinders placed in different concentrations of sugar or salt solutions. Since potato cell membranes are partially permeable, water will move in or out depending on the concentration gradient.
Key Steps:
1. Cut potato cylinders to the same length.
2. Measure the initial mass or length of each cylinder.
3. Place cylinders in solutions ranging from pure water (0 mol/dm\(^{3}\)) to very concentrated sugar solution.
4. Leave them for a set time, then remove, pat dry, and re-measure.
What to look out for:
Mass Increase: If the solution is hypotonic (less concentrated than the cell), water enters the potato by osmosis. The cells become turgor (firm).
Mass Decrease: If the solution is hypertonic (more concentrated than the cell), water leaves the potato. The cells may undergo plasmolysis.
No Change: The point where the line on your graph crosses the x-axis (0% change) is the isotonic point—this is the concentration inside the potato cell!
Quick Tip: Always pat the potato dry before the final weighing. If you don't, the extra water on the surface will make your mass measurement inaccurate!
2. Photosynthesis
In this practical, you investigate how different variables (like light intensity) affect the rate of photosynthesis.
The Experiment
The most common method uses pondweed (like Elodea) placed in a beaker of water. When the plant photosynthesizes, it releases oxygen (\(O_{2}\)) bubbles.
Key Steps:
1. Place a piece of pondweed in a test tube of water (adding sodium hydrogencarbonate helps provide \(CO_{2}\)).
2. Place a lamp at a specific distance (e.g., 10 cm).
3. Count the number of bubbles produced in one minute, or collect the gas in a syringe to measure the volume.
4. Repeat at different distances (e.g., 20 cm, 30 cm).
Variables:
Independent Variable: The light intensity (changed by moving the lamp distance).
Dependent Variable: The rate of photosynthesis (measured by bubbles per minute).
Control Variables: Temperature (use a glass shield or LED lamp to prevent heat transfer) and \(CO_{2}\) concentration.
Did you know? Photosynthesis is often limited by limiting factors. If you move the lamp very close but the bubbles don't speed up anymore, something else (like temperature or \(CO_{2}\) levels) has become the limiting factor.
3. Digestion (Enzymes) [B]
This practical looks at how temperature and pH affect the rate of enzyme activity. We usually use amylase (which breaks down starch into sugars).
The Experiment
1. Mix amylase and starch solution in a test tube kept at a specific temperature or pH.
2. Every 30 seconds, take a drop of the mixture and add it to a drop of iodine on a spotting tile.
3. Iodine turns blue-black if starch is present. When the iodine stays orange/brown, all the starch has been digested.
4. Repeat at different temperatures or different pH buffers.
Key Takeaway:
Enzymes have an optimum temperature and pH where they work fastest. If the temperature is too high, the active site changes shape and the enzyme is denatured. It will no longer fit the starch molecule, and the reaction will stop.
4. Respiration (Exercise)
This practical investigates how the human body responds to the increased energy demands of physical activity.
The Experiment
1. Measure a person's resting heart rate and breathing rate.
2. Have the person perform a set amount of exercise (e.g., 2 minutes of star jumps).
3. Immediately measure heart rate and breathing rate again.
4. Record how long it takes for the rates to return to "resting" levels.
What’s happening inside?
During exercise, the muscles need more energy from aerobic respiration. The heart and lungs work harder to supply \(O_{2}\) and glucose. If there isn't enough oxygen, anaerobic respiration occurs, producing lactic acid and creating an oxygen debt.
Don't forget: In your exam, you might be asked to design this. Always mention using a large group of people to get an average and ensure the exercise is the same for everyone (standardized).
5. Infection and Response
This practical tests how effective different antibiotics or disinfectants are at killing bacteria.
The Experiment
1. Use aseptic techniques to spread a bacterial culture onto an agar plate (ensuring it is uncontaminated).
2. Place paper discs soaked in different antibiotics or disinfectants onto the agar.
3. Incubate the plate (at a maximum of 25\(^{\circ}\)C in schools to prevent growing dangerous human pathogens).
4. Measure the zone of inhibition (the clear area where bacteria didn't grow) around the discs.
Safety First!
Hazards: Bacteria can cause illness.
Risk Minimization: Use sterile equipment, flame the neck of culture bottles, and lightly tape the lid of the Petri dish (don't seal it completely, as bacteria need oxygen to prevent the growth of harmful anaerobic strains).
Key Concept: The larger the zone of inhibition, the more effective the antibiotic is at killing that specific strain of bacteria.
Quick Summary for Revision
1. Osmosis: Potato in sugar water. Watch for mass changes.
2. Photosynthesis: Pondweed and a lamp. Count the bubbles.
3. Enzymes: Amylase + Starch. Use iodine to see when starch disappears.
4. Exercise: Measure pulse and breathing. Link to energy demand and lactic acid.
5. Microbes: Agar plates and antibiotic discs. Measure the clear "kill zones."
Common Mistake to Avoid: In the exam, don't just say "the experiment was better." Use scientific words like repeatable (you get the same result twice), reproducible (someone else gets the same result), or valid (the experiment actually tested what it was supposed to).