Welcome to Biology Practical Skills!
Success in International AS Level Biology isn't just about what you know in a textbook—it's about what you can do in a laboratory. Unit 3 (Practical Skills in Biology I) is worth 20% of your total IAS grade. In this chapter, we will explore the essential tools, clever techniques, and vital safety rules you need to master the nine Core Practicals. Don't worry if you feel a bit "clumsy" in the lab at first; these notes are designed to turn you into a confident scientist!
1. The Biologist's Toolkit: Choosing the Right Apparatus
Choosing the right tool is the first step toward accurate results. If you use a bucket to measure a teaspoon of water, your data will be messy! Here is how to pick your "weapons" for the Core Practicals:
Measuring Volume
In experiments like Core Practical 4 (enzymes), you need to measure liquids precisely.
• Graduated Pipettes or Syringes: Use these for small, specific volumes (e.g., \(5 \text{ cm}^3\)). They are much more accurate than measuring cylinders.
• Burettes: These are used in Core Practical 2 (Vitamin C content) because they allow you to add liquid drop by drop until the DCPIP changes color.
Controlling Temperature
Temperature affects almost everything in biology, from membrane permeability (Core Practical 3) to enzyme rates.
• Thermostatically Controlled Water Baths: These are better than Bunsen burners because they keep the temperature constant (\(\pm 0.5^\circ\text{C}\)).
• Thermometers: Always check the temperature of the actual reaction mixture, not just the water bath!
Measuring Mass and Force
• Digital Balances: Use these to weigh plant tissue or chemicals. Always "tare" (zero) the balance first.
• Masses and Hangers: In Core Practical 8 (tensile strength), you add weights to plant fibers until they snap. Make sure to add small increments (e.g., \(10\text{g}\) or \(50\text{g}\)) to find the exact breaking point.
Quick Tip: Always choose the smallest measuring tool that can hold your required volume. Measuring \(2 \text{ cm}^3\) in a \(100 \text{ cm}^3\) cylinder leads to a high percentage error!
2. Essential Techniques: Doing Science Right
A "technique" is just a standard way of doing something so that everyone gets the same results. Here are the big ones for your exam:
Microscopy (Core Practicals 5, 6, and 7)
To see cells, you need to master the light microscope.
1. Start Low: Always start with the lowest power objective lens.
2. Focus: Use the coarse adjustment knob first, then the fine adjustment knob for "crisp" edges.
3. Staining: Many cells are transparent. We use stains like acetic orcein (for Core Practical 6) to make chromosomes visible during mitosis.
4. Measurement: We use an eyepiece graticule (a tiny ruler inside the lens). Because the "size" of the graticule changes with magnification, we calibrate it using a stage micrometer.
Aseptic Technique (Core Practical 9)
When working with bacteria, "aseptic" means "without contamination." You want to grow only the bacteria you intended to, and you don't want to get sick!
• Flame the Loop: Heat wire loops until they glow red to kill any stray microbes.
• The 45-Degree Rule: Only open the Petri dish lid slightly (at a \(45^\circ\) angle) to prevent dust and spores from falling in.
• Work near a Bunsen flame: The upward current of hot air carries microbes away from your workspace.
Color Standards vs. Colorimetry
In Core Practical 1 (reducing sugars), you might use color standards. This means comparing your result to a series of tubes with known concentrations to "guess" the value. For more accuracy in Core Practical 3 (beetroot membranes), we use a colorimeter, which gives a digital reading of how much light passes through a liquid.
3. Health and Safety: Keeping the Lab Safe
In Unit 3, you are often asked to identify hazards and risks. They aren't the same thing!
• Hazard: Something with the potential to cause harm (e.g., "Ethanol is flammable").
• Risk: The likelihood of that harm happening (e.g., "Ethanol catching fire if used near a Bunsen burner").
• Precaution: What you do to stop it (e.g., "Keep ethanol away from open flames").
Common Hazards in IAS Biology:
• Chemicals: Benedict’s reagent (irritant), Hydrochloric acid (corrosive), DCPIP (stain). Precaution: Wear safety goggles and gloves.
• Biohazards: Bacteria or fresh plant/animal tissue. Precaution: Disinfect surfaces and wash hands thoroughly.
• Sharps: Scalpels used for cutting plant stems (Core Practical 7) or root tips. Precaution: Always cut away from your body on a white tile.
• Heat: Boiling water for Benedict's test. Precaution: Use a pair of tongs to handle hot tubes.
Did you know? In Core Practical 9, we never seal the Petri dish completely with tape. We use two pieces of tape to allow oxygen to enter. This prevents the growth of dangerous anaerobic bacteria!
4. Fair Testing: Standardising Variables
To make an experiment valid, you must only change the independent variable. Everything else must be "standardised" (kept the same).
If you are testing the effect of temperature on enzymes (Core Practical 4), you must keep the following constant:
• pH: Use a buffer solution.
• Enzyme Concentration: Use the same volume and same percentage of enzyme.
• Substrate Concentration: Use the same volume and same percentage of reactant.
Note: For more details on variables, see the "Planning an Experiment" chapter.
Key Takeaways Summary
1. Accuracy: Use the right tool (e.g., burettes for titrations, graticules for cell size).
2. Safety: Match the precaution to the specific hazard (e.g., goggles for acids, flaming for microbes).
3. Aseptic Technique: Vital for Core Practical 9 to prevent contamination.
4. Calibration: Always calibrate your equipment (like a microscope or a balance) to ensure the data is reliable.
5. Observations: Use diagrams (Core Practical 7) and stains (Core Practical 6) to record what you actually see, not what you think you should see!
Next Steps: To see how these techniques apply to specific experiments, head over to the "Core Practicals 1-9 in Context" chapter!