Welcome to Practical Skills in Biology!

In Biology, it isn't enough to just know the facts; we have to prove them through experiments! Unit 3 (Practical Skills in Biology I) focuses on how we actually carry out investigations, take measurements, and present our data accurately. This chapter covers Implementation (doing the experiment), Measurement (collecting the data), and Significant Figures (making sure our numbers make sense).

Don't worry if you find the "maths side" of Biology a bit intimidating. By the end of these notes, you’ll have a clear toolkit to handle any data question the exam throws at you!

1. Implementation: Doing the Science

Implementation is all about the decisions you make while standing at the lab bench. The exam will often ask you to criticise or suggest improvements to a method.

Number and Range of Readings

To get reliable results, you need to think about how many measurements you take.

  • The Range: This is the spread of the independent variable (the thing you change). For example, if you are testing the effect of temperature on enzymes, a range of \( 10^{\circ}\text{C} \) to \( 50^{\circ}\text{C} \) is better than just testing \( 20^{\circ}\text{C} \) and \( 30^{\circ}\text{C} \).
  • The Number of Intervals: You should aim for at least five different values for your independent variable to see a clear trend.
  • Repeats: We usually repeat each measurement at least three times. This allows us to calculate a mean and identify "odd" results.

Spotting Inconsistent Readings (Anomalies)

An inconsistent reading (or anomaly) is a result that does not fit the pattern of the others.
Example: If you measure the time for an enzyme reaction three times and get \( 12\text{s} \), \( 13\text{s} \), and \( 45\text{s} \), the \( 45\text{s} \) is clearly inconsistent.

What do you do? In an exam, if you spot an anomaly:

  1. Identify it clearly.
  2. Suggest repeating that specific measurement if possible.
  3. Do not include it in your calculation of the mean.

Quick Review: Improving Implementation

If a question asks how to improve an experiment, think: "Should they use more temperatures? Should they repeat it more? Should they use a more precise piece of equipment?"

2. Measurement: Precision and Units

Biology is a quantitative science. We need to measure things accurately using the right tools.

Instrument Limits

Every piece of equipment has a limit to how accurate it can be.
- A standard ruler measures to the nearest \( 1\text{ mm} \).
- A digital balance might measure to \( 0.01\text{ g} \).
Rule: Your results should be reported only to the limits of the least accurate measurement you took.

Standard SI Units

You must always include units in your tables and answers. Common units in Unit 3 include:

  • Concentration: \( \text{mol dm}^{-3} \) or \( \text{g dm}^{-3} \)
  • Volume: \( \text{cm}^3 \) or \( \text{dm}^3 \)
  • Rate: \( \text{s}^{-1} \) or \( \text{cm}^3 \text{ s}^{-1} \)

Did you know? You might need to convert units, such as \( \text{mm}^3 \) to \( \text{cm}^3 \). Remember that because it is "cubed," the conversion factor is also cubed!
\( 1\text{ cm} = 10\text{ mm} \), so \( 1\text{ cm}^3 = 10 \times 10 \times 10 = 1000\text{ mm}^3 \).

3. Significant Figures and Rounding

Significant figures (SF) show how "sure" we are about a number. Using too many or too few can actually make your data less accurate!

The Golden Rules of Significant Figures

  1. Non-zero digits are always significant. \( 245 \) has \( 3 \) SF.
  2. Zeros between non-zero digits are significant. \( 205 \) has \( 3 \) SF.
  3. Leading zeros (at the start) are NOT significant. \( 0.005 \) has only \( 1 \) SF.
  4. Trailing zeros after a decimal point ARE significant. \( 5.00 \) has \( 3 \) SF. This is because those zeros show the measurement was exactly "point zero zero," not just a guess!

Calculating to Correct Significant Figures

When you multiply or divide numbers (e.g., calculating magnification), your final answer should have the same number of significant figures as the measurement with the fewest significant figures used in the calculation.

Example: If you divide \( 10.51 \) (\( 4 \) SF) by \( 2.2 \) (\( 2 \) SF), your calculator gives \( 4.77727... \)
You must round this to \( 2 \) SF: \( 4.8 \).

Standard Form

Sometimes numbers are very small or very large. We use standard form to keep things tidy while retaining significant figures.
\( 0.0050 \text{ mol dm}^{-3} \) is written as \( 5.0 \times 10^{-3} \text{ mol dm}^{-3} \).
Notice we kept the \( .0 \) to show it is \( 2 \) significant figures!

4. Mastering Exam Command Words

In Unit 3, the examiners use specific words to tell you exactly how to handle numbers:

  • Calculate: You must show your working and include a unit.
    \( \text{Magnification} = \frac{\text{size of image}}{\text{size of real object}} \).
  • Determine: This means you need to find a quantitative value, usually by doing a small calculation from data provided in a graph or table.
  • Show that: They give you the answer, and you have to prove it. Pro tip: Your final working should show one more significant figure than the answer they gave you to prove you did it accurately!
  • Comment on: Look at the data provided. Is the range big enough? Are there anomalies? Are the significant figures consistent?

Key Takeaways for Success

1. Consistency is Key: If you record one measurement as \( 12.0 \), record the others in that column as \( 13.0, 14.0 \), etc. Don't mix \( 12 \) and \( 12.0 \).
2. Rounding: Only round your final answer. If you round numbers during the middle of a multi-step calculation, you will end up with a "rounding error."
3. Practical Context: Always link your math back to the biology. If you are calculating the rate of an enzyme, make sure the units are \( \text{s}^{-1} \) or similar.

For more details on specific experiments, check out the chapter on IAS Core Practicals 1-9. For help with drawing the data, see Processing Results, Graphs and Uncertainties.