Welcome to Your Practical Skills Guide!

In the OxfordAQA International AS Biology course, you won't sit a separate "practical exam" in a lab. Instead, your knowledge of experiments is tested directly in your written papers. This means you need to understand not just what happens, but why we use specific methods and how to handle the data.

In this chapter, we will break down the first three required practicals. Don't worry if you find the math or the technical steps a bit daunting at first—we'll take it one step at a time!

Note: For help with designing experiments or general data presentation, check out the "Planning experiments" and "Recording data" chapters in this section.

Practical 1: Investigating Enzyme-Controlled Reactions

Enzymes are biological catalysts that speed up reactions. In this practical, you investigate how a specific factor—like temperature, pH, or substrate concentration—affects the rate of reaction.

The Core Concept

We measure the rate by either seeing how fast a product is formed (e.g., oxygen gas bubbles from hydrogen peroxide) or how fast a substrate disappears (e.g., starch being broken down). We relate this to the collision frequency between the enzyme's active site and the substrate.

Step-by-Step Procedure (Example: Temperature)

1. Set up a series of water baths at different temperatures (e.g., \(10^{\circ}C\), \(20^{\circ}C\), \(30^{\circ}C\), \(40^{\circ}C\), \(50^{\circ}C\)).
2. Place your enzyme and substrate into separate test tubes and let them reach the required temperature in the water bath before mixing. This is called equilibration.
3. Mix the enzyme and substrate and start a stopwatch immediately.
4. Measure the time taken for a set change to occur (e.g., the solution turning clear) or measure the volume of gas produced every 30 seconds.

Key Calculations

To find the rate of reaction at any given point, you can draw a tangent to the curve on a graph of product against time. The slope of this tangent gives you the initial rate of reaction.

\(\text{Rate} = \frac{\text{Change in quantity}}{\text{Time}}\)

Quick Review: Control Variables

If you are changing the temperature (independent variable), you must keep the \(pH\), the enzyme concentration, and the substrate concentration the same to ensure a fair test.

Common Mistake: Forgetting to equilibrate the solutions. If you mix cold enzyme with warm substrate, the actual reaction temperature won't be what you think it is!

Practical 2: Osmosis in Plant Tissue

Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane. This practical looks at how solute concentration affects this movement.

The Core Concept

By placing plant tissue (like potato cylinders) into sugar or salt solutions of different concentrations, we can see if they gain or lose mass.

  • If the solution has a lower water potential than the plant cell, water leaves the cell (mass decreases).
  • If the solution has a higher water potential, water enters the cell (mass increases).

Step-by-Step Procedure

1. Use a cork borer to cut identical cylinders of potato. This ensures the surface area to volume ratio is similar for all samples.
2. Dry the cylinders gently with a paper towel to remove excess surface water, then record their initial mass.
3. Place the cylinders in solutions of different concentrations (e.g., \(0.0\), \(0.2\), \(0.4\), \(0.6\), \(0.8\), and \(1.0 mol \cdot dm^{-3}\) of sucrose).
4. Leave for a set time (e.g., 30 minutes).
5. Remove, dry again, and record the final mass.

Calculating Percentage Change

We use percentage change instead of just "change in mass" because the starting masses of the potato pieces might not be exactly the same. It allows for a fair comparison.

\(\text{Percentage Change in Mass} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100\)

Finding the Isotonic Point

Plot a graph of percentage change in mass (y-axis) against solute concentration (x-axis). The point where the line crosses the x-axis (where percentage change is \(0\)) is the concentration where the water potential of the solution equals the water potential of the plant tissue.

Did you know? At this "zero point," there is no net movement of water, though individual water molecules are still moving back and forth!

Practical 3: Chromatography of Leaf Pigments

Chromatography is a technique used to separate substances in a mixture based on their solubility in a solvent and their affinity for the paper (the stationary phase).

The Core Concept

Leaves contain several different pigments (like Chlorophyll a, Chlorophyll b, and Carotene). Because these pigments have different chemical structures, they travel at different speeds up the chromatography paper.

Step-by-Step Procedure

1. Crush a leaf with a small amount of solvent (like acetone) to extract the pigments.
2. Draw a line in pencil near the bottom of the chromatography paper. Why pencil? Ink from a pen would dissolve and interfere with the results!
3. Use a glass capillary tube to place a small, concentrated spot of pigment on the pencil line. This is the origin.
4. Place the paper in a boiling tube containing a small amount of solvent, ensuring the solvent level is below the pigment spot.
5. Put a lid on the tube to stop the solvent from evaporating.
6. When the solvent has nearly reached the top, remove the paper and immediately mark the solvent front with a pencil.

Calculating \(R_f\) Values

Each pigment has a specific Retention Factor (\(R_f\)) that helps us identify it. This value is always between \(0\) and \(1\).

\(R_f = \frac{\text{Distance moved by the pigment}}{\text{Distance moved by the solvent front}}\)

Quick Tip: If your pigment spot is very faint, repeat the "spotting" process several times, letting it dry between each drop, to build up a concentrated point of pigment.

Key Takeaways for Practicals 1-3
  • Enzymes: Focus on temperature control and using tangents to find initial rates.
  • Osmosis: Always calculate percentage change in mass and know how to find the isotonic point on a graph.
  • Chromatography: Remember to use pencil for the origin line and keep the pigment spot above the solvent level.

Don't worry if these seem tricky at first! The more you practice looking at specimen data and graphs, the more these procedures will make sense. You're doing great!