Welcome to the "Doing" Part of Biology!

You’ve planned your experiment, and you know what you’re looking for. Now comes the exciting part: Implementing and Recording. This chapter is all about how we actually carry out our practical work and, crucially, how we write down what happens so that other scientists (and your examiners!) can understand exactly what you found.

Think of this as the "bridge" between your ideas and your results. Whether you are using a microscope to look at cells or a potometer to measure transpiration, the way you implement the method and record the data is what makes your work "scientific."

1. Implementing: Using Apparatus and Techniques

In the OCR Biology A course, you will complete various Practical Activity Groups (PAGs). Implementing means using the right tool for the right job safely and accurately. Here are some of the key techniques you will use:

Using Laboratory Glassware

When measuring volumes, you might use a measuring cylinder, a syringe, or a pipette. To get the most accurate reading, always read the volume from the bottom of the meniscus (the curve of the liquid) at eye level. This helps avoid parallax error.

Microscopy (PAG 1)

Using a light microscope involves more than just looking through a lens. You must be able to:

  • Use high and low power lenses correctly.
  • Calibrate an eyepiece graticule using a stage micrometer. This allows you to measure the actual size of the cells you are seeing.
  • Apply staining techniques (like iodine for starch or methylene blue for DNA) to make clear structures visible.

Dilutions and Reagents

You will often need to perform serial dilutions (a step-by-step dilution of a substance in solution) to change the concentration of a variable, like an enzyme or a sugar. You will also use qualitative reagents (like Benedict’s solution for reducing sugars) to identify biological molecules (PAG 9).

Quick Tip: Always keep your workspace tidy! A cluttered desk leads to "implementing" errors, like knocking over your test tubes or mixing up your pipettes.

2. Recording Quantitative Data

Quantitative data is anything involving numbers. How we record these numbers is very important for your marks.

Tables for Results

When you record data in a table, follow these "golden rules":

  • The Independent Variable (the one you change) goes in the first column.
  • The Dependent Variable (the one you measure) goes in the columns to the right.
  • Headings must include both the quantity and the unit, separated by a forward slash. For example: \( \text{Time / s} \) or \( \text{Concentration / mol dm}^{-3} \).
  • Never write units inside the cells of the table—only in the headings!

Significant Figures (\( \text{SF} \))

Your data should be recorded to a consistent level of precision. Don't worry if this seems tricky at first! A good rule of thumb is that your recorded data should match the precision of the instrument you used. For example, if your thermometer measures to \( 0.5 \text{ } ^{\circ}\text{C} \), don't write down \( 25 \text{ } ^{\circ}\text{C} \); write \( 25.0 \text{ } ^{\circ}\text{C} \).

Common Mistake: Changing the number of decimal places halfway down a column. If your first reading is \( 1.20 \), your next reading should be \( 1.40 \), not just \( 1.4 \).

3. Recording Qualitative Data

Qualitative data is descriptive. In Biology, this often takes the form of scientific drawings (PAG 1) or observations of colour changes.

Biological Drawings

A biological drawing is not an "art project"—it is a scientific record. Follow these rules:

  • Use a sharp HB pencil.
  • Use clear, continuous lines (no "shading" or "sketchy" lines).
  • Ensure the drawing is large (it should take up at least half the space provided).
  • Include annotations (labels that describe what you are seeing, not just the name of the part).
  • Use a ruler for label lines, and ensure the lines touch the structure they are labeling without crossing over each other.

Example: Instead of just labeling "Cell Wall," an annotation might say "Thick cellulose cell wall to provide structural support."

4. Accuracy, Precision, and Errors

While you are implementing your experiment, you need to be aware of how "good" your data is.

Understanding the Terms

  • Accuracy: How close your measurement is to the "true" value.
  • Precision: How close your repeated measurements are to each other.
  • Margin of Error: The uncertainty in a measurement. For example, a ruler might have a margin of error of \( \pm 0.5 \text{ mm} \).

Calculating Percentage Error

You may be asked to calculate the percentage error of your equipment. The formula is:

\( \text{Percentage Error} = \frac{\text{Uncertainty}}{\text{Measured Value}} \times 100 \)

Example: If you measure \( 20.0 \text{ cm}^3 \) of liquid and the uncertainty is \( 0.5 \text{ cm}^3 \):

\( \frac{0.5}{20.0} \times 100 = 2.5\% \)

5. Safety and Ethics

Part of "implementing" is doing things the right way. This includes:

  • Safe use of instruments: Especially during dissections (PAG 2) or when using heat.
  • Aseptic techniques: When working with microorganisms (PAG 7) to prevent contamination of yourself and your samples.
  • Ethical considerations: Treating living organisms with respect and minimizing harm during fieldwork or experiments.

Did you know? In the OCR A Level, your teacher will assess your ability to follow these practical instructions through the Practical Endorsement (CPAC). This is a "Pass/Fail" component that shows you have mastered these essential lab skills!

Chapter Summary Checklist

Key Takeaways:

  • Precision is key: Use instruments correctly and record data to a consistent number of decimal places.
  • Tables: Independent variable on the left, dependent on the right, units only in headings.
  • Drawings: No shading, use clear lines, and add descriptive annotations.
  • Maths: Be ready to use \( \text{SI units} \), standard form, and calculate percentage errors.
  • Cross-reference: For help on what to do with your data after you've recorded it, see the "Analysis of experimental results" chapter.