Introduction to Biology Required Practicals

Welcome! In your AQA GCSE Combined Science: Trilogy course, you don't just learn about science from a book—you do it! There are 21 required practicals in total. This chapter focuses on the first seven, which are the Biology practicals. These are essential because questions about their methods, equipment, and results will appear on your exam papers.

Don't worry if you find the technical details a bit overwhelming at first. We will break each one down into what you do, what you measure, and what you need to watch out for. Think of these as "science recipes" that help us understand how life works!

RP 1: Using a Light Microscope

In this practical, you learn how to look at things that are too small for the naked eye. You will observe, draw, and label plant and animal cells.

The Method

1. Peel a thin, transparent layer of cells (like onion epidermis) and place it on a glass slide.
2. Add a drop of stain (like iodine) so the parts of the cell are easier to see.
3. Place a cover slip on top carefully to avoid air bubbles.
4. Use the lowest power objective lens first to find the cells, then switch to a higher power to see more detail.
5. Use the coarse adjustment knob to get the cells into focus, then the fine adjustment knob for a clear image.

Key Skills: Drawings and Magnification

When drawing cells, use a sharp pencil and clear, unbroken lines. Do not shade! You must also include a magnification scale. Use this formula to calculate magnification:

\( \text{Magnification} = \frac{\text{size of image}}{\text{size of real object}} \)

Quick Tip: Always start with the lowest power lens. It gives you a wider "field of view," making it much easier to find what you are looking for.

RP 2: Osmosis in Plant Tissue

This practical investigates how the concentration of a solution (salt or sugar) affects the mass of plant tissue, like a potato.

The Method

1. Cut several cylinders of potato to the same length.
2. Measure and record the initial mass of each cylinder using a balance.
3. Place each cylinder into a different concentration of sugar or salt solution (from \( 0.0 \, \text{mol/dm}^3 \) or pure water, up to a high concentration).
4. Leave them for a set amount of time (e.g., overnight or 24 hours).
5. Remove the cylinders, pat them dry with a paper towel, and record the final mass.

Calculating Change

To see how much water moved, calculate the percentage change in mass:

\( \% \, \text{Change} = \frac{\text{change in mass}}{\text{original mass}} \times 100 \)

Common Mistake: Forgetting to dry the potato. If you leave excess liquid on the outside, your mass reading will be too high and your results won't be accurate!

RP 3: Food Tests

Scientists use qualitative reagents (liquids that change color) to find out which nutrients are in a food sample.

The Tests You Must Know

1. Sugars: Add Benedict's solution and heat in a water bath. If sugar is present, it turns from blue to green, yellow, or brick-red (depending on how much sugar there is).
2. Starch: Add Iodine solution. It turns from orange/brown to blue-black.
3. Proteins: Add Biuret reagent. It turns from blue to purple/lilac.
4. Lipids (Fats): You can use Sudan III, which creates a red-stained oil layer on the surface.

Quick Review:
- Starch = Iodine (Blue-black)
- Sugar = Benedict's + Heat (Brick-red)
- Protein = Biuret (Purple)

RP 4: Effect of pH on Amylase

This practical looks at how quickly an enzyme (amylase) breaks down starch at different \( pH \) levels.

The Method

1. Place drops of iodine into every well of a spotting tile.
2. Mix amylase, starch, and a specific buffer solution (to control the \( pH \)) in a test tube.
3. Every 30 seconds, take a drop of the mixture and add it to a well in the spotting tile.
4. If the iodine stays orange, the starch has been fully broken down. If it turns blue-black, starch is still present.
5. Repeat the experiment using different \( pH \) buffer solutions.

What to Remember

We use continuous sampling (every 30 seconds) to find the exact time the reaction finishes. The rate of reaction is faster when the time taken is shorter.

RP 5: Photosynthesis and Light Intensity

This investigation shows how the distance from a light source affects how fast a plant (usually pondweed) photosynthesizes.

The Method

1. Place a piece of freshly cut pondweed in a boiling tube of water.
2. Place a lamp at a set distance (e.g., \( 10 \, \text{cm} \)) from the plant.
3. Wait for the plant to start bubbling (this is oxygen gas).
4. Count the number of bubbles produced in one minute, or use a gas syringe to measure the volume of oxygen.
5. Repeat at different distances (e.g., \( 20 \, \text{cm} \), \( 30 \, \text{cm} \)).

Did you know? Light intensity follows the inverse square law. If you double the distance, the light intensity actually drops to a quarter of what it was!

RP 6: Human Reaction Time

This is a fun practical where you investigate how a factor (like practice or tiredness) affects how quickly a person reacts.

The Ruler Drop Test

1. Person A holds a ruler vertically. Person B places their finger and thumb near the bottom at the \( 0 \, \text{cm} \) mark.
2. Person A drops the ruler without warning.
3. Person B catches the ruler as fast as they can.
4. Record the number on the ruler where it was caught. The lower the number, the faster the reaction.
5. Repeat many times to calculate a mean (average).

Important: To make this a fair test, you must use the same person catching the ruler, the same hand, and drop the ruler from the same starting height every time.

RP 7: Population Size and Distribution

In this ecology practical, you measure how many of a species live in an area and how their position changes based on a factor (like light or trampling).

Using Quadrats (Random Sampling)

1. Place two tape measures on the ground to create a grid.
2. Use a random number generator to pick coordinates.
3. Place a quadrat (a square frame) at those coordinates and count the number of the species inside.
4. Repeat at least 10 times and calculate a mean.
5. Estimate the total population: \( \text{Mean count per quadrat} \times \frac{\text{Total area}}{\text{Quadrat area}} \).

Using Transects (Distribution)

If you want to see how the species changes as you move across a field (e.g., from a shady tree to an open field), use a transect line (a long tape measure). Place quadrats at regular intervals along the line and record the changes.

Key Takeaways for Your Exam

- Independent Variable: The thing you change (e.g., the light distance or the \( pH \)).
- Dependent Variable: The thing you measure (e.g., the number of bubbles or the mass of the potato).
- Control Variables: The things you keep the same to make it a fair test (e.g., the temperature or the size of the potato).
- Repeats: Always repeat an experiment and calculate a mean to make your results more reliable and to help spot anomalies (results that don't fit the pattern).