Introduction to Unit 3 Core Practicals

Welcome to your Unit 3 study guide! While Units 1 and 2 focus on the "what" of Biology, Unit 3 is all about the "how." In this chapter, we look at the 9 Core Practicals you performed during your IAS course. The exam won't just ask you to recite the method; it will ask you to apply these techniques to new situations, identify variables, and evaluate the data. Don't worry if you missed a lab session—we’ll break down each practical step-by-step!

Note: For details on how to choose variables or calculate standard deviation, see the specific chapters on "Planning an Experiment" and "Statistics."


Core Practical 1: Estimating Sugar and Starch Concentration

This practical uses two classic tests to identify carbohydrates. The syllabus focuses on a semi-quantitative method, which means we aren't just looking for a "yes" or "no," but an estimate of how much is present.

1. Reducing Sugars (Benedict's Test)

Reducing sugars (like glucose or maltose) donate electrons to Blue Copper(II) ions in Benedict’s reagent, reducing them to Red Copper(I) oxide.

  • Method: Add an equal volume of Benedict's reagent to your sample and heat it in a water bath (around \(95^{\circ}C\)).
  • Observations: The colour changes from blue \(\rightarrow\) green \(\rightarrow\) yellow \(\rightarrow\) orange \(\rightarrow\) brick red.
  • Semi-quantitative approach: To estimate concentration, you compare your result to colour standards (solutions of known concentration prepared earlier). The faster the change and the "redder" the result, the higher the sugar concentration.

2. Starch (Iodine Test)

  • Method: Add Iodine solution (iodine dissolved in potassium iodide) to the sample.
  • Observations: A change from orange-brown to blue-black indicates starch is present.

Quick Tip: If the question asks for a quantitative result, you would use a colorimeter to measure the absorbance of the remaining liquid after filtering out the precipitate.


Core Practical 2: Vitamin C Content in Food and Drink

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

  • Method: Use a titration. Pipette a known volume of DCPIP into a flask. Add the fruit juice drop by drop using a syringe or burette until the blue colour disappears.
  • Calculation: The volume of juice required is inversely proportional to the Vitamin C concentration. Less juice needed = Higher Vitamin C concentration.
  • Calibration: You must first test a standard solution of Vitamin C (e.g., \(1\ mg\ cm^{-3}\)) to know exactly how much Vitamin C is needed to decolourise the DCPIP.

Core Practical 3: Membrane Permeability

This practical often uses beetroot because its cells contain a dark red pigment called betalain inside the vacuole. If the membrane is damaged, the pigment leaks out.

  • Independent Variables: You can investigate temperature (which increases kinetic energy and denatures proteins) or alcohol concentration (which dissolves phospholipids).
  • Dependent Variable: The intensity of the red colour in the surrounding water, measured using a colorimeter.
  • Standardising: You must use a cork borer to ensure all beetroot discs have the same surface area and volume. You must also rinse the discs before starting to remove any pigment leaked during cutting.

Core Practical 4: Initial Rate of Enzyme Reactions

Enzymes (like catalase or amylase) work fastest at the very beginning because the substrate concentration is at its highest.

  • Key Concept: You must measure the initial rate. This is the gradient of the curve at \(time = 0\) on a graph of product formed against time.
  • Factors to Investigate: Enzyme concentration, substrate concentration, \(pH\), or temperature.
  • Variables to Control: If you change the enzyme concentration, you must keep the temperature and \(pH\) constant (e.g., using a water bath and a buffer solution).

Key Takeaway: Rate is often calculated as \(Rate = \frac{1}{time}\) or by measuring the volume of gas produced in the first \(30\) seconds.


Core Practical 5: Using a Light Microscope (Animal Cells)

This practical tests your ability to observe and measure microscopic structures.

  • Staining: Animal cells are often transparent. We use stains like methylene blue to make the nucleus visible.
  • Measurement: We use an eyepiece graticule (a tiny ruler inside the lens). Because the graticule doesn't have real units, we calibrate it against a stage micrometer (a slide with a known scale).
  • Formula: \(magnification = \frac{size\ of\ image}{size\ of\ real\ object}\)

Core Practical 6: Observing Mitosis (Root Tip Squash)

Why the root tip? Because that is where the meristem is—the area of active cell division.

  • The Process: 1. Place the root tip in Hydrochloric Acid (\(HCl\)) to break down the middle lamella (the "glue" between cells). 2. Stain with a DNA-binding stain (like acetic orcein). 3. Squash the tip under a coverslip to create a single thin layer of cells so light can pass through.
  • Mitotic Index: This is a common exam calculation:

    \(mitotic\ index = \frac{number\ of\ cells\ in\ mitosis}{total\ number\ of\ cells}\)


Core Practical 7: Plant Tissue Drawings

You need to be able to identify and draw three main tissues in plant stems and roots:

  • Xylem: Large, open vessels for water transport (found towards the centre).
  • Phloem: Smaller cells for transporting sugars.
  • Sclerenchyma: Thick-walled fibres for support (often found on the outer edge of vascular bundles).

Exam Tip: When drawing a plan diagram, do NOT draw individual cells. Only draw the boundaries between different tissues.


Core Practical 8: Tensile Strength of Plant Fibres

This practical tests how much "pulling force" a plant fibre (like hemp or sisal) can take before it snaps.

  • Method: Suspend a fibre between two clamps. Add masses one by one to the middle (or end) until the fibre breaks.
  • Safety: Place a box of soft padding (like sand or bubble wrap) underneath the weights to catch them when the fibre snaps.
  • Standardising: Use fibres of the same length and diameter. Ensure the age of the plant and the extraction method (retting) are the same.

Core Practical 9: Antimicrobial Properties of Plants

We test if plant extracts (like garlic or mint) can kill bacteria using aseptic technique.

Aseptic Technique (Safety First!):

  • Sterilise work surfaces with disinfectant.
  • Work near a Bunsen flame to create an upward current of sterile air.
  • Flame the necks of bottles and the metal loops used for transferring bacteria.
  • Only partially tape the lid of the Petri dish (to allow oxygen in and prevent the growth of dangerous anaerobic bacteria).
  • Incubate at \(25^{\circ}C\) (not \(37^{\circ}C\), to avoid growing human pathogens).

Measuring Success:

The "clear area" around the plant extract where no bacteria grow is called the zone of inhibition. A larger area means the plant has stronger antimicrobial properties.


Summary Checklist

Did you know? Unit 3 questions often mix these practicals. You might be asked to design an experiment for a plant extract (CP9) but use the titration method from Vitamin C (CP2)!

  • Can you name the independent, dependent, and control variables for each?
  • Do you know the safety risks (e.g., hot water, sharp scalpels, \(HCl\), bacteria)?
  • Can you calculate magnification or a mitotic index?
  • Can you explain why a specific step is taken (e.g., why rinse the beetroot)?

If you can answer "yes" to these, you are well on your way to mastering Unit 3!