Question 1

How do I calibrate an eyepiece graticule?

Accepted Answer

Place a stage micrometer (a slide with a scale of known length, usually 1 mm divided into 100 divisions) on the microscope stage. Focus on the scale at the desired magnification. Rotate the eyepiece graticule so its scale aligns with the stage micrometer. Count how many divisions of the eyepiece graticule correspond to a known length on the stage micrometer. For example, if 100 eyepiece divisions equal 1 mm, then each eyepiece division = 0.01 mm (10 μm). Repeat for each magnification used.

Question 2

What is the difference between accuracy and precision in biology practicals?

Accepted Answer

Accuracy refers to how close a measurement is to the true value. For example, using a calibrated pipette gives accurate volumes. Precision refers to how consistent repeated measurements are, regardless of whether they are close to the true value. For instance, if you measure the same sample three times and get 2.1, 2.2, and 2.1 cm³, your results are precise but may not be accurate if the pipette is miscalibrated. In exams, you need to comment on both.

Question 3

How do I use a colorimeter to find the concentration of an unknown solution?

Accepted Answer

First, prepare a series of standard solutions of known concentration (e.g., 0.1, 0.2, 0.3 M) of the coloured substance. Set the colorimeter to the appropriate wavelength (usually the colour complementary to the solution's colour). Zero the colorimeter with a blank (distilled water). Measure the absorbance of each standard and plot a calibration curve of absorbance vs concentration. Then measure the absorbance of your unknown solution and read off its concentration from the curve.

Question 4

What is aseptic technique and why is it important?

Accepted Answer

Aseptic technique is a set of procedures used to prevent contamination of cultures by unwanted microorganisms. It includes sterilising equipment (e.g., by autoclaving or flaming), working near a Bunsen burner to create an updraft that carries airborne microbes away, and using sterile loops and pipettes. It is crucial for obtaining reliable results in microbiology experiments, as contamination can lead to false conclusions about growth or antibiotic sensitivity.

Question 5

How do I calculate magnification from a microscope image?

Accepted Answer

Magnification = size of image ÷ actual size of specimen. If you have a scale bar on the image, measure the length of the scale bar in mm (image size) and note the actual length it represents (e.g., 10 μm). Convert both to the same units (e.g., mm: 10 μm = 0.01 mm). Then divide image size by actual size. Alternatively, if you know the eyepiece and objective lens magnifications, multiply them (e.g., 10x eyepiece × 40x objective = 400x magnification).

Question 6

What statistical test should I use for comparing two means in biology?

Accepted Answer

The Student's t-test is used to compare the means of two groups (e.g., control vs treatment) to see if the difference is statistically significant. It assumes the data are normally distributed and have similar variances. For categorical data (e.g., counts of phenotypes), use the chi-squared test. In exams, you may be asked to calculate a t-value and compare it to a critical value at a given significance level (usually p=0.05) to determine if the null hypothesis can be rejected.

Use of apparatus and techniques

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