Welcome to AS 1: Biology Skills & Experimental Techniques
Welcome to the Biology Skills module of Unit AS 1: Experimental Techniques! This unit is internally assessed through a portfolio of practical reports (moderated by CCEA) and makes up 33.3% of your AS Level (or 13.3% of your overall A Level). Practical work can sometimes feel overwhelming with all the equipment, numbers, and strict guidelines, but don't worry if this seems tricky at first. Once you learn the core techniques and the standard report structure, you will be able to approach every practical with confidence.
In this guide, we will break down the four essential biological techniques, the rules for writing top-tier lab reports, and key tips to avoid common examiner pitfalls.
Part 1: The Four Core Biological Techniques
1. Microscopy
Microscopy allows us to observe cells and structures that are invisible to the naked eye. In your portfolio work, you will learn how to prepare slides (such as onion skin cells) and use a light microscope accurately.
Key Steps in Slide Preparation and Viewing:
1. Mounting: Place a thin specimen (e.g., a single layer of onion epidermal tissue) onto a clean glass slide.
2. Staining: Add a stain to highlight cellular structures.
3. Coverslip: Lower a coverslip gently at an angle using a mounted needle to prevent trapping air bubbles.
4. Focusing: Always start with the lowest power objective lens, use the coarse focus knob to bring the stage close, and then use the fine focus knob to make the image sharp.
Measuring Cells (Graticule & Stage Micrometer):
To measure real cell dimensions, an eyepiece graticule (a tiny transparent ruler fitted inside the eyepiece) is calibrated using a stage micrometer (a slide with a known, precise scale). Once calibrated, you can measure the true size of your specimen.
The Magnification Formula:
Always remember the classic I = AM relationship:
\(\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}\)
\(\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}}\)
\(\text{Image size} = \text{Actual size} \times \text{Magnification}\)
Memory Trick: Put this in a formula triangle with Image size (I) at the top, and Actual size (A) and Magnification (M) at the bottom. Make sure both Image size and Actual size are converted to the same measurement units before calculating!
2. Colorimetry
A colorimeter measures the amount of light absorbed or transmitted by a coloured liquid solution. It is commonly used to measure the rate of an enzyme-controlled reaction or to determine the concentration of biological molecules (like glucose) using a calibration curve.
Key Principle: The Beer-Lambert Law
The Beer-Lambert Law states that the absorbance of light is directly proportional to the concentration of the absorbing substance in the solution. In simple terms: the darker the colour / higher the concentration, the more light is absorbed.
Using a Calibration Curve:
1. Measure the absorbance of solutions with known concentrations.
2. Plot a graph of Absorbance (y-axis) against Concentration (x-axis).
3. Measure the absorbance of your unknown sample and use your calibration curve to read off its exact concentration.
3. Chromatography
Chromatography is used to separate individual components from a mixture based on their solubility in a solvent and their attraction to the stationary phase. You will use paper chromatography or Thin Layer Chromatography (TLC) to separate biological mixtures, such as leaf pigments.
Calculating the Retention Factor (\(Rf\)):
Each separated pigment has a characteristic \(Rf\) value under specific conditions:
\(Rf = \frac{\text{Distance moved by substance}}{\text{Distance moved by solvent front}}\)
Golden Rule: Because the substance can never travel further than the solvent front, your \(Rf\) value must always be less than or equal to 1.0 (\(Rf \le 1.0\)). If your calculation gives a number greater than 1, you have accidentally divided the numbers the wrong way around!
4. Aseptic Technique
When culturing microorganisms, you must maintain aseptic technique to prevent contamination of your culture with unwanted environmental microbes and to protect yourself from exposure.
Standard Aseptic Practices:
• Bunsen Burner "Cone of Heat": Work close to a roaring blue Bunsen flame. The rising hot air creates an updraft that prevents airborne microbes from settling on your workspace.
• Flaming Loops: Pass inoculating loops through the hottest part of the Bunsen flame until red hot before and after transferring bacteria to sterilise them.
• Petri Dish Handling: Open petri dish lids only slightly at an angle (like a clamshell) rather than removing them completely.
• Sealing Dishes: Tape petri dishes securely using adhesive tape (without completely sealing around the rim, allowing aerobic conditions) before incubation.
Key Takeaway for Section 1: Master the four core techniques—microscopy (measuring with graticules), colorimetry (absorbance vs. concentration), chromatography (\(Rf\) values \(\le 1.0\)), and aseptic technique (working within the Bunsen heat cone).
Part 2: Scientific Reporting Standards for Your Portfolio
Every practical write-up in your AS 1 portfolio must follow a strict, professional format to meet CCEA assessment criteria:
1. Title and Aim
A concise statement outlining the exact biological question or purpose of the experiment.
2. Risk Assessment
Must clearly outline three distinct elements:
• Hazard: The object or chemical that can cause harm (e.g., glass coverslips, chemical irritants, Bunsen flame).
• Risk: How the hazard could cause injury (e.g., broken glass cutting skin, hot flame causing burns).
• Control Measure: What you will do to reduce the risk (e.g., handle glass with care in the centre of the bench, keep hands away from flame, wear safety goggles).
3. Apparatus and Method
A comprehensive list of apparatus and a clear, chronological, step-by-step procedure that another scientist could follow to replicate your results.
4. Results
• Table Design: All tables must have clear column headers with units included in the header (e.g., Time / s or Concentration / mol dm\(^{-3}\)). Do not write units next to raw numbers inside the data cells.
• Decimal Places: Raw data must be recorded consistently to the correct number of decimal places, matching the precision of the measuring instrument.
5. Data Processing
• Means: Calculate mean values using only concordant results (see pitfalls below), discarding any clear anomalous outliers.
• Standard Deviation: Calculate standard deviations where appropriate to show the spread of data around the mean.
6. Analysis & Conclusion
Explain your patterns, trends, and results using established biological principles and theory.
7. Evaluation
Critically assess your experimental design using two key scientific benchmarks:
• Reliability: The extent to which repeating the experiment yields consistent, reproducible results.
• Validity: Whether the experimental design actually investigates what it was intended to investigate (e.g., were all control variables kept constant?).
Key Takeaway for Section 2: Structure is everything! Always include a 3-part risk assessment, units in table headers only, consistent decimal places, and clear discussions of reliability and validity.
Part 3: Core Measurement Concepts
Precision vs. Accuracy
These two terms sound similar in everyday speech, but in scientific investigations, they mean very different things:
• Precision: How close independent repeated measurements are to each other (closeness of agreement). A method that gives the exact same result five times in a row has high precision.
• Accuracy: How close a measured value is to the true, correct value.
Analogy: Imagine playing darts. If all three darts land tightly clustered together in the number 1 double, your throws are precise but not accurate. If all three hit the bullseye, they are both precise and accurate!
Part 4: Common Pitfalls & How to Avoid Them
CCEA examiners frequently identify simple mistakes that cost students marks. Keep these in mind when completing your AS 1 write-ups:
1. Handling Concordant Readings
When calculating average values (such as titration titres or repeated timer runs), students often mistakenly average all recorded trials. You must identify and use only concordant readings (readings within a narrow specified range, such as within \(0.10\text{ cm}^3\)) and discard clear anomalies before taking a mean.
2. Significant Figures
Do not quote your final calculated answers to more significant figures than your raw measurements justify. If your raw data was recorded to 2 significant figures, a final calculated mean of \(4.333333\) must be rounded appropriately.
3. Weak Evaluations ("Human Error")
Never write "human error" as a limitation in your evaluation. Examiners reject this vague phrase! Instead, identify a specific technical source of error (e.g., "parallax error when reading the meniscus of the liquid in the measuring cylinder" or "difficulty in judging the exact colour endpoint by eye") and propose an objective, technical improvement.
4. Graph Plotting Rules
• Always label axes with quantities and appropriate units (e.g., Absorbance / arbitrary units or Distance / mm).
• Plot points precisely with small crosses (\(\times\)).
• Draw an appropriate line of best fit—either a clear straight line or a smooth curve that follows the trend without forcing it through every single point.
Key Takeaway for Section 4: Avoid vague terms like "human error", calculate means using only concordant readings, respect significant figures, and draw clean lines of best fit with fully labelled axes.