Required practicals and working scientifically in biology – GCSE Combined Science Biology

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Biology is learned by doing experiments. The core practicals are the experiments every student should have done, and exam questions often describe one and ask you to explain a step, spot a problem or work out a result.

This matters because you can pick up marks by knowing why each step is done. If you remember the reason, you can answer a question about a practical you have not seen.

By the end you will know the method, the variables and the calculation for each of the main biology practicals.

Planning and results

The independent variable is the one you change. The dependent variable is the one you measure. Control variables are the other things you keep the same. A control experiment is one in which the independent variable is left out so you can show it is the cause of any change.

Repeat the experiment and calculate the mean, leaving out any anomalous result. Quantitative data is measured with numbers. Qualitative data is a description, such as a colour change. A line graph is used when both variables are continuous, and a bar chart when one variable is a category.

Percentage change = (final value − initial value) ÷ initial value × 100. A rate is the change in an amount divided by the time taken. When you write a conclusion, say what the data show, quote some numbers, and do not claim more than the data support.

Microscopes and drawings

To prepare a slide of onion skin, peel a thin layer from inside the bulb and lay it flat on a drop of water on a slide. Add a drop of iodine stain to make the cells easier to see. Lower the coverslip at an angle with a mounted needle, so that no air bubbles are trapped.

Start on the lowest power objective lens. Use the coarse focus to find the cells and the fine focus to sharpen the image. Then move to a higher power if you need to. Never use the coarse focus on a high power lens, because the lens can hit the slide.

Draw what you see with a sharp pencil. Use clear single lines, no shading and no colouring. Draw a large, clear diagram. Label with straight lines that do not cross, give the drawing a title and write the magnification. Magnification = image size ÷ real size.

Food tests

To test for starch, add a few drops of iodine solution. A positive result is a colour change from orange-brown to blue-black. To test for sugars (reducing sugars such as glucose), add Benedict's solution and heat in a hot water bath. A positive result is a change from blue to green, yellow, orange or brick-red, depending on how much sugar there is.

To test for protein, add Biuret reagent. A positive result is blue changing to purple (lilac). To test for lipids, shake the food with ethanol and then pour the mixture into water. A cloudy white emulsion shows lipids. Alternatively add Sudan III stain solution, which stains the lipid layer red.

Crush or grind a solid food and mix it with distilled water first. Take care with hot water baths and with ethanol, which is flammable, and wear eye protection.

Enzymes and pH

This practical investigates the effect of pH on the enzyme amylase, which breaks down starch. Put a drop of iodine solution in each well of a spotting tile. Mix amylase, starch solution and a buffer solution of a known pH in a test tube, in a water bath at a fixed temperature. Start the timer.

Every 30 seconds take a drop of the mixture and add it to a new well of iodine. Blue-black means starch is still there. When the iodine stays orange-brown, all of the starch has gone. Record the time. Repeat with buffers of different pH.

The rate is calculated as rate = 1000 ÷ time (s). A shorter time means a faster rate. Keep the temperature, the volume and the concentration of the starch and the amylase the same. The enzyme works fastest at its optimum pH, and rates fall on either side of it.

Osmosis in potatoes

Cut potato cylinders of the same size using a cork borer. Blot each one dry and weigh it. Put one cylinder in each of several beakers of sucrose solution of different concentrations, from distilled water to a strong solution. Leave them for a set time, such as 30 minutes.

Take the cylinders out, blot them dry and weigh them again. Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100. A cylinder in a dilute solution gains mass because water moves into it by osmosis. A cylinder in a strong solution loses mass because water moves out of it.

Plot the percentage change against the concentration. Where the line crosses zero, there is no net movement of water. That concentration matches the concentration of the potato cells. Keep the size of the cylinders, the volume of solution, the temperature and the time the same.

Light, respiration, reaction time and DNA

Photosynthesis: put a piece of pondweed in water containing sodium hydrogencarbonate, which supplies carbon dioxide. Place a lamp at a measured distance and count the bubbles of oxygen given off in a minute. Move the lamp and repeat. A water bath between the lamp and the beaker stops the temperature rising. Light intensity falls with the distance squared, so it is proportional to 1 ÷ d².

Respiration: in a respirometer, germinating seeds in a tube use oxygen and give out carbon dioxide, which is absorbed by soda lime. The gas volume falls, so coloured liquid in a capillary tube moves towards the seeds. Measure how far it moves in a set time. A control tube uses dead (boiled) seeds. Respiration is exothermic.

Reaction time: one person drops a ruler and the other catches it. Record the distance it falls through the fingers, repeat and find the mean. A shorter distance means a faster reaction time. Compare with the effect of practice, distraction or caffeine, keeping other things the same.

DNA extraction: mash a fruit such as a strawberry. Add washing-up liquid and salt solution, which break open the cell membranes and help the DNA to clump. Filter the mixture. Slowly pour ice-cold ethanol down the side of the tube. The DNA forms white, stringy strands where the layers meet.

Recap

Independent variable: change. Dependent variable: measure. Control variables: keep the same. Repeat and find a mean without anomalies.

Food tests: iodine (starch), Benedict's and heat (sugar), Biuret (protein), ethanol emulsion (lipid). Enzyme practical: time until iodine stays orange-brown, rate = 1000 ÷ time.

Osmosis: percentage change in mass and a graph. Photosynthesis: bubbles and the inverse square law. Respiration: respirometer with soda lime. DNA: detergent, salt, filter, cold ethanol.