Welcome to Your Practical Biology Journey!

In Unit 5 of your OxfordAQA Biology course, you aren’t just learning theories—you are learning how to be a scientist. Since there is no practical exam, the written papers (especially Unit 5) will test your "hands-on" knowledge. You need to know how to design these experiments, control variables, and explain why things happen the way they do.

In this chapter, we will break down the first three required practicals: Enzymes, Osmosis, and Chromatography. Don't worry if these seem a bit technical at first; we'll take it step-by-step!

Practical 1: Investigating the Rate of Enzyme-Controlled Reactions

Enzymes are biological catalysts that speed up reactions by lowering activation energy. For this practical, you usually investigate how one factor (like temperature, pH, or substrate concentration) affects how fast an enzyme works.

How the Experiment Works

Typically, you will mix an enzyme (like amylase) with a substrate (like starch). You then measure one of two things:

1. How fast the product is formed (e.g., measuring the volume of oxygen gas produced by catalase).
2. How fast the substrate disappears (e.g., using iodine to see how quickly starch is broken down).

Key Variables to Control

If you are changing the temperature (your independent variable), you must keep everything else the same (your control variables):

  • Concentration of enzyme and substrate: More molecules mean more collisions!
  • Volume of solutions: Use a measuring cylinder or pipette for accuracy.
  • pH: Usually controlled using a buffer solution.

The Science Behind the Results

As you increase the temperature or concentration, the molecules move faster and collide more often. This increases the collision frequency between the active site and the substrate, forming more enzyme-substrate complexes.

Common Mistake to Avoid: If the temperature gets too high, the enzyme doesn't "die"—it denatures. The hydrogen bonds in the tertiary structure break, changing the shape of the active site so the substrate no longer fits.

Quick Review Tip: When calculating the rate from a graph, always try to find the initial rate. Draw a tangent to the curve at time \(t = 0\). The slope of this tangent (\(\text{change in y} / \text{change in x}\)) gives you the fastest part of the reaction.

Practical 2: Investigating 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 selectively permeable membrane. In this practical, we usually use potato cylinders or discs.

Step-by-Step Procedure

1. Cut several identical pieces of plant tissue (like potato).
2. Record the initial mass of each piece.
3. Place each piece into a different solute concentration (e.g., sucrose solutions ranging from \(0.0 \text{ mol dm}^{-3}\) to \(1.0 \text{ mol dm}^{-3}\)).
4. Leave them for a set amount of time (e.g., 30 minutes).
5. Remove, blot dry with a paper towel (to remove excess surface water), and record the final mass.

Why Use Percentage Change?

Potato cylinders won't all have the same starting mass. By calculating the percentage change in mass, you make the results comparable. Use this formula:

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

Interpreting the Graph

When you plot a graph of "Percentage Change in Mass" against "Solute Concentration":

  • Mass Increase: The water potential of the solution was higher than the potato; water moved in.
  • Mass Decrease: The water potential of the solution was lower than the potato; water moved out.
  • The X-intercept: The point where the line crosses the x-axis (0% change) is the most important! At this concentration, there is no net movement of water. This means the water potential of the solution is equal to the water potential of the plant tissue.

Key Takeaway: If the potato loses mass, the solution is "stronger" (more concentrated) than the potato cells.

Practical 3: Chromatography of Leaf Pigments

Leaves look green, but they actually contain several different pigments, such as chlorophyll a, chlorophyll b, and carotenes. Chromatography allows us to separate these based on their solubility.

How it Works

1. Extract the pigments: Crush leaves with a little solvent (like acetone).
2. Spot the paper: Use a glass capillary tube to put a concentrated small drop of pigment on a pencil line (the origin) near the bottom of the chromatography paper.
3. Run the chromatogram: Place the paper in a boiling tube with a small amount of solvent. Ensure the solvent level is below the pencil line.
4. Identify: As the solvent moves up, different pigments move at different speeds because they have different solubilities in the solvent and different affinities for the paper.

Calculating \(R_f\) Values

You can identify pigments by calculating their \(R_f\) (Relative front) value. This is a ratio and has no units!

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

Did you know? We use a pencil line because ink from a pen would dissolve in the solvent and ruin the results. Also, we mark the solvent front immediately after taking the paper out, before the solvent evaporates!

Quick Summary for Revision

  • Enzymes: Focus on collisions and active site shape. High temperatures denature the tertiary structure.
  • Osmosis: Focus on water potential. The point of zero mass change tells you the tissue's internal concentration.
  • Chromatography: Focus on solubility. \(R_f\) values allow you to compare results across different experiments.

Don't worry if these calculations feel heavy! Just remember the core principle: we are always looking for a way to measure a biological change accurately and fairly.

Note: For more on how to choose equipment or evaluate these methods, see the chapter on "Evaluating procedures and writing synoptic extended answers".