Study Guide
A Guide To A Level Biology Practicals
by Muhammad Haris | 15 Oct 2025
After only having seen microscopes in textbooks all your life, now with your A Levels having kicked off, you find yourself actually sitting before one in a lab. It’s safe to say you’re rather nervous to use one, and that’s fine! There’s no denying that practicals can seem intimidating for a lot of newcoming AS students, but lucky for you, we’ve got you all sorted! Presenting, Mojza’s carefully crafted guide to A Level Biology practicals!
Paper Pattern
Your practical component for Biology is your paper 3; it’s worth 40 marks and allotted a time of 2 hours, with the paper consisting of 2 questions. One question is always based on using a light microscope, and the other on conducting an experiment yourself.
Microscopes
First off, microscopes. One of your two questions on the paper will always be based on using a light microscope. Therefore, getting the hang of them is absolutely crucial for a good grade. You can learn more about actually operating one right here.
Plan Diagrams
A compulsory part of the same microscope-based question, would be to draw a ‘low power plan diagram’ of the specimen you just focused onto on your microscope. Think of a plan diagram as a minimalistic overview of your specimen; its purpose is to strictly indicate the rough position and shape of tissues, while excluding any details like individual cells and their colors. Here are some top tips to score well on these questions:
- Use a sharp pencil.
- Draw with solid, confident strokes; no overlapping lines.
- Avoid erasing and redrawing, as it makes your final diagram hard to see and may cost you marks.
- No colouring, no shading.
- When drawing a plant specimen, such as a leaf tissue, make sure to include the cell membrane, AND cell wall.
- Use up more than half of the space given.
- NEVER draw individual cells.
When asked to label your plan diagram, always draw label lines using a ruler, writing the label horizontally, with no arrowheads.
Eyepiece Graticules and Stage Micrometers
These are instruments used to measure the lengths of parts of a specimen being viewed under a microscope. Don’t worry though; you won’t be asked to actually use these instruments. What you will be asked to do, however, is to use given diagrams of an eyepiece graticule and stage micrometer to calculate lengths in that diagram.
As you can see in the diagram, the larger scale is the Stage Micrometer and the smaller one is the Eyepiece Graticule. One division on the SM scale is equal to 0.1mm, and is also equal to 40 EPG divisions. We can use this information and perform some simple proportional calculations to figure out the length of one EPG division.
Try this Past-paper question below to see if you get it.
The correct answer is 10 micrometers, option B. Here’s how we got it:
In the question it says that the divisions on the stage micrometer have a space of 0.1 mm or just 0.1 x 1000 = 100 micrometers.
So in these 100 micrometers, how many eyepiece graticule divisions fit? Well, it’s 40 divisions.
We want the length just for one division:
40 divisions : 100 micrometers
1 division : x micrometers
x = 2.5 micrometers
We are now done with calibration.
Now looking at the second figure:
To find the width of the chloroplast, choose any chloroplast width, measure each small line and multiply that by 2.5 micrometers. You should be getting around 10 micrometers.
It is important to note, when choosing a chloroplast to measure, don’t measure an anomaly (like the big one on the right). Measure the average chloroplast.
Magnification
You can also be asked to calculate the magnification or the actual length of a specimen given in a diagram. For this, you simply need to know your Magnification formula that you studied in O levels, that is:
Magnification = Image length / Actual length
Simple and Serial Dilutions
A crude definition of diluting a substance is to make it less concentrated by adding in more water. You will often be asked to perform dilutions in question 1 of your practical exam. There are two types of dilutions you can be asked to carry out: simple dilution (AKA proportional dilution) and serial dilution.
Serial Dilution
Just like the name states, this type of dilution is done in series, one after another. From the original solution provided to you (AKA the stock solution), take out a part and add it to another beaker, filling the rest of it with water till its concentration is reduced by the factor specified in the question. This process is repeated with your new concentration and so on, until you reach your desired number of concentrations as demanded in the question. It’s important to note that the dilution factor remains the same for every dilution. The most common factors are:
- Reducing the concentration by half, in which you add equal volumes of the previous concentration’s solution, and of distilled water.
- Reducing by a factor of 10, in which, for each concentration the ratio of the volume of the previous concentration, to total required volume for each concentration, should be 1:10. That is, if the total volume of the solution you’re preparing is required to be 10 cm3, add 1 cm3 of the previous concentration and 9 cm3 of distilled water, and repeat this for each concentration
Simple Dilution
be provided with a stock solution and asked to prepare multiple different concentrations as a percentage of the stock solution’s concentration.
For every concentration you make, you first have to decide the volume of stock solution to add, as well as the volume of water to add. You can calculate those values using this formula:
C1V1 = C2V2
C1 here represents the concentration of the stock solution, and V1 the volume of the stock solution required to form your new concentration.
C2 is your required concentration’s percentage, and V2 represents the final volume of the solution you are preparing, which is always equal to the volume of your stock solution.
If this is hard to remember, you can also rearrange this formula into a different form:
Simply reading all this might not make a lot of sense, so here’s a short video explaining both types of dilutions.
Food Tests
As a part of question 1, your knowledge of food tests and their results can also be tested in different ways, such as asking you to briefly describe the experimental procedure for testing for the presence of any one of the three main food groups i.e Proteins, Carbohydrates, and Fats. Such parts are relatively straightforward, and are based on your prior O Levels knowledge. Here’s a brief table to help ring a bell:
Food Group | Test Procedure | Positive result |
Reducing Sugars | Add Benedict’s reagent to sample → heat in a water bath (~80°C) for 2–5 min. | Brick-red precipitate (colour scale from green → yellow → orange → brick red depending on sugar concentration). |
Non-reducing sugars | After negative reducing sugars test: add dilute HCl, boil, cool, neutralise with NaOH → repeat Benedict’s test. | Brick-red precipitate (colour scale from green → yellow → orange → brick red depending on sugar concentration). |
Starch | Add iodine solution (iodine in potassium iodide) to sample. | Blue-black colour. |
Proteins | Add Biuret reagent (or NaOH then dilute CuSO₄ solution without shaking). | Lilac/purple colour. |
Lipids | Add ethanol, shake → add water, shake gently. | Milky-white emulsion. |
General Tips
Time management is crucial in a practical exam. It’s easy to get caught up in repeating a step or rechecking your results, but remember that every minute counts. Plan your work in stages: set a target for when you should finish setting up, when to start taking results, and when to move on to calculations or graph plotting. This will help you avoid a last-minute rush and ensure you have enough time to check your answers before handing in your paper.
Always place your exam paper or any written material on a clean, dry part of your bench, well away from chemicals, water, or flame sources. Spilled liquids or stains can damage your paper, making it unreadable for examiners, and in worst cases, you could lose marks. Keeping your workspace organized will also reduce confusion, minimize accidents, and help you focus better.
Glassware, sharp instruments, and electrical equipment are delicate and can be hazardous if mishandled. Always set up your apparatus steadily, avoid unnecessary movements that could knock things over, and check that everything is clamped securely before starting.
One of the most common mistakes in practical exams is forgetting to include units in your tables. Every column should have both the quantity and its unit clearly labeled, for example: Time / s or Volume of gas / cm³. Omitting these steps cost you arguably the easiest marks on the paper.
All the nerd-talk aside, in the end, you also have to accept that practicals are quite prone to mishaps and accidents; you may spill some chemical all over your question paper, or your microscope slide may end up breaking. Thankfully, paper 3 only holds a weightage of 11.5% on your final A level grade, so calm yourself down if you mess something up on the practical, and be at ease about your results if you did well on the theoretical components, since those hold the most weightage.
Good luck with your A levels, people! You’ve got this!
Mojza
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