Welcome to Your IAS Core Practicals Guide!

In Unit 3 of your International A Level Biology, you aren't just expected to know facts; you need to understand how we discover them. This chapter covers the first nine "Prescribed Core Practicals" (CP1 to CP9). These are the specific experiments that the exam board wants you to know inside out. Don't worry if some of these seem complex at first—we will break them down into simple steps, focusing on what you actually need for your exam.

Note: For details on how to design an investigation, handle data, or calculate uncertainties, please see the other chapters in the "Practical Skills in Biology I" section.


1. Molecules and Diet (Practicals 1 and 2)

CP1: Estimating Concentrations of Sugars and Starch

This practical is about semi-quantitative estimation. This means you aren't getting a perfect numerical value, but you are comparing samples to "colour standards" to estimate how much of a substance is present.

The Tests:

  • Reducing Sugars: Mix the sample with Benedict’s reagent and heat in a water bath. The colour changes from blue to green, yellow, orange, or brick-red. The "redder" it is, the more sugar is present.
  • Starch: Add iodine solution (potassium iodide). A change from orange-brown to blue-black indicates starch.

How to be "Semi-Quantitative": Create a series of known concentrations (e.g., \(0.2\%\), \(0.4\%\), \(0.6\%\), etc.) to act as colour standards. Compare your unknown sample’s final colour to these standards to estimate its concentration.

CP2: Vitamin C Content in Food and Drink

We use a blue dye called DCPIP. Vitamin C is an antioxidant that "decolourises" (bleaches) DCPIP.

The Procedure:

  1. Pipette a known volume of blue DCPIP into a flask.
  2. Slowly add the food juice (e.g., orange juice) drop by drop using a burette or syringe.
  3. Gently shake the flask after each drop.
  4. Stop when the blue colour disappears (the end-point).

The Logic: If you need very few drops of juice to turn the DCPIP colourless, that juice has a high concentration of Vitamin C. If you need a lot of juice, the Vitamin C concentration is low.

Key Takeaway: For both practicals, the independent variable is usually the type of food/juice, and the dependent variable is the volume needed or the resulting colour.


2. Membranes and Enzymes (Practicals 3 and 4)

CP3: Membrane Permeability

Cell membranes are made of a phospholipid bilayer and proteins. We can test how temperature or alcohol concentration affects this "fluid mosaic" structure, usually using beetroot cells because they contain a bright red pigment called betalain.

  • The Effect of Temperature: High temperatures give molecules more kinetic energy, making the membrane more fluid. Eventually, proteins denature, creating "holes" that allow pigment to leak out.
  • The Effect of Alcohol: Alcohol dissolves lipids. Since the membrane is made of lipids, alcohol creates gaps in the structure.

Measurement: Use a colorimeter to measure the intensity of the red colour in the water surrounding the beetroot discs. More pigment leakage = higher absorbance/lower transmittance.

CP4: Initial Rate of Enzyme Reactions

Enzymes are biological catalysts. You must be able to investigate how factors like temperature, pH, enzyme concentration, and substrate concentration affect the initial rate.

Why "Initial" Rate? We measure the rate at the very start of the reaction because that is when the substrate concentration is at its highest and is not a "limiting factor." As the reaction continues, the substrate is used up, and the rate naturally slows down.

Math Tip: The rate is often calculated as \( \text{Rate} = \frac{1}{\text{time}} \). If you have a graph of "Product vs. Time," the initial rate is the gradient of the tangent at \( t = 0 \).


3. Cells and Development (Practicals 5 and 6)

CP5: Microscopy and Magnification

This practical is about using a light microscope correctly. You need to be able to draw cells and calculate their actual size.

Important Formula:

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

How to measure:

  • Eyepiece Graticule: A "ruler" inside the microscope eyepiece. It has no units until you calibrate it.
  • Stage Micrometer: A slide with a very accurate scale (e.g., \(0.1 \text{ mm}\) divisions). You use this to find out how many micrometres (\(\mu\text{m}\)) each graticule unit represents at a specific magnification.

CP6: The Root Tip Squash (Mitosis)

We use the tips of growing roots (like onions or garlic) because this is where mitosis (cell division) is most active.

Steps to remember:

  1. Acid Hydrolysis: Place the root tip in warm hydrochloric acid to break down the middle lamella (the "glue" between cells) so the tissue can be flattened.
  2. Staining: Use a stain like acetic orcein to make the chromosomes visible.
  3. Squashing: Press down firmly on the cover slip with your thumb (don't smear it!) to create a single, thin layer of cells so light can pass through.

Mitotic Index: This is a common calculation task: \( \text{Mitotic Index} = \frac{\text{number of cells containing visible chromosomes}}{\text{total number of cells observed}} \)


4. Plant Structure and Function (Practicals 7, 8, and 9)

CP7: Plant Histology (Tissue Drawings)

You need to recognize and draw the arrangement of tissues in roots, stems, and leaves. Don't worry if you aren't an artist! Scientific drawings should be clear, use single lines (no sketching), and have no shading.

Identify these structures:

  • Xylem vessels: Large, thick-walled tubes for water transport.
  • Phloem (Sieve tubes): Smaller tubes for transporting sugars.
  • Sclerenchyma fibres: Very thick-walled cells found near vascular bundles for support.

CP8: Tensile Strength of Plant Fibres

How much "pulling force" can a plant fibre (like hemp or extracted celery fibres) take before it snaps? This is tensile strength.

  • Method: Suspend a fibre between two clamps and add weights (\(10\text{g}\) or \(100\text{g}\) at a time) until the fibre breaks.
  • Safety: Place a padded box under the weights to catch them when the fibre snaps!
  • Controlled Variables: Length of fibre, diameter of fibre, and the temperature/humidity.

CP9: Antimicrobial Properties of Plants

Some plants produce chemicals to kill bacteria. We test this using aseptic technique to prevent contamination.

Aseptic Essentials:

  • Wipe surfaces with disinfectant.
  • Work near a Bunsen flame (the upward air current carries microbes away).
  • Flame the neck of glass bottles and metal loops.
  • Only open the agar plate lid slightly (the "clamshell" method).

The Test: Place paper discs soaked in plant extract onto an agar plate "seeded" with bacteria. Incubate the plate (usually at \(25^{\circ}\text{C}\) to avoid growing human pathogens). Measure the zone of inhibition (the clear area where bacteria didn't grow). A larger area means a more effective antimicrobial.


Quick Review: Common Mistakes to Avoid

1. Mixing up the variables: Always identify the Independent Variable (the thing you change) and the Dependent Variable (the thing you measure) before answering a "describe" question.

2. Forgetting Units: In calculations, always check if you need to convert millimetres (\(\text{mm}\)) to micrometres (\(\mu\text{m}\)). Remember: \(1 \text{ mm} = 1000 \mu\text{m}\).

3. Aseptic Technique: If a question asks about bacteria, you must mention safety—specifically flaming tools and not sealing the petri dish completely (to allow oxygen in and prevent anaerobic bacteria from growing).

Did you know? William Withering, an 18th-century doctor mentioned in your syllabus, used a version of "trial and error" with foxglove plants that eventually led to modern drug testing protocols. You can see the connection between his work and your CP9 experiment!