Core practicals 1-8 and working scientifically

Welcome to Core Practicals & Working Scientifically!

Practical skills are at the heart of biology. In your Pearson Edexcel AS Biology B course, at least 15% of total exam marks across Paper 1 (8BI0/01) and Paper 2 (8BI0/02) directly test your understanding of core practical investigations, experimental design, and data analysis. Don't worry if experimental design feels daunting at first—once you master the key principles and the exact steps of each core practical, these exam questions become some of the easiest marks to secure!

Section 1: Working Scientifically & Experimental Design

Every successful biological experiment is built on a clear, logical structure. Examiners are very strict about scientific language, so understanding how to describe variables precisely is essential.

1. Understanding Variables

Independent Variable (IV): The factor that you deliberately change or manipulate across set intervals.
Dependent Variable (DV): The factor that you measure to assess the effect of changing the independent variable.
Control Variables (CV): Factors that could affect the dependent variable and must be kept strictly constant throughout the investigation.

Exam Golden Rule: Never write vague phrases like "to make it a fair test" or "to ensure accuracy". You will receive zero marks for these! Instead, you must always name the specific variable, describe exactly how to control it, and explain the biological reason why it must be controlled.
Example: Instead of writing "keep the temperature the same for a fair test", write: "Maintain temperature at \(25^\circ\text{C}\) using a thermostatically controlled water bath so that enzyme-substrate kinetic energy and collision rates remain constant."

2. Measurements, Calculations, and Graphing Rules

Raw Data Recording: Record all raw measurements to a consistent number of decimal places, matching the resolution of the measuring instrument used.
Calculated Means: Express calculated mean values to no more significant figures than the least precise raw measurement.
Graph Plotting Standards:
- Place the Independent Variable on the horizontal \(x\)-axis.
- Place the Dependent Variable on the vertical \(y\)-axis.
- Ensure your plotted points occupy more than 50% of the grid area in both directions.
- Label axes using the official format: \(\text{Quantity} / \text{unit}\) (for example, \(\text{Concentration} / \text{mol dm}^{-3}\) or \(\text{Time} / \text{s}\)).

Key Takeaway: Always specify the exact piece of equipment used to control a variable, and format all graph axes as \(\text{Quantity} / \text{unit}\).

Section 2: The AS Core Practicals (CP01 – CP08)

Core Practical 1: Investigating a Factor Affecting Initial Rate of Reaction

Topic Link: Topic 1 (Biological Molecules)
Purpose: To determine the initial rate of an enzyme-catalysed reaction when altering a factor such as enzyme concentration, substrate concentration, pH, or temperature.

Model Systems:
1. Trypsin and Casein: Trypsin breaks down the white, opaque milk protein (casein) into soluble peptides, turning the suspension clear.
2. Catalase and Hydrogen Peroxide: Catalase breaks down hydrogen peroxide into water and oxygen gas (\(2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2\)).

Method for Casein Breakdown:
1. Set up a colorimeter with an appropriate filter and calibrate it to zero absorbance using a reference blank (distilled water).
2. Equilibrate trypsin solution and milk suspension separately in a thermostatically controlled water bath to the chosen temperature.
3. Mix the enzyme and substrate in a cuvette, immediately place it into the colorimeter, and record absorbance (or light transmission) at regular time intervals (e.g., every \(10\text{ s}\)).
4. Repeat for different concentrations of enzyme or substrate, keeping all other variables constant.

Calculating Initial Rate:
We must measure the initial rate because the reaction is fastest at \(t = 0\), before substrate depletion or product inhibition slows it down.
- Plot a graph of \(\text{Absorbance}\) against \(\text{Time} / \text{s}\).
- Draw a tangent to the curve at time \(t = 0\text{ s}\).
- Calculate the gradient of this tangent: \(\text{Rate} = \frac{\Delta\text{Absorbance}}{\Delta\text{Time}}\).

Core Practical 2: Using a Light Microscope, Eyepiece Graticule, and Stage Micrometer

Topic Link: Topic 2 (Cells, Viruses & Reproduction)
Purpose: To make accurate observations and measurements of biological specimens using calibrated microscope scales.

The Magnification Formula:
$$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} \quad \left(M = \frac{I}{A}\right)$$
Ensure both \(\text{Image size}\) and \(\text{Actual size}\) are converted into the same units before calculating (e.g., \(1\text{ mm} = 1000\ \mu\text{m}\)).

Calibration Procedure:
An eyepiece graticule contains an arbitrary scale with divisions called eyepiece units (epu). It does not change physical size when you change magnification, so it must be calibrated against a stage micrometer (a slide with a precise, known physical scale, typically \(10\ \mu\text{m}\) per small division).
1. Line up the zero mark of the eyepiece graticule with the zero mark of the stage micrometer under a specific objective lens.
2. Find a point further along the scale where divisions on both scales align perfectly.
3. Count the number of stage micrometer divisions and calculate the true distance in micrometres (\(\mu\text{m}\)).
4. Divide the true distance by the number of eyepiece units to find the value of \(1\text{ epu}\) at that specific magnification:
$$\text{Value of } 1\text{ epu } (\mu\text{m}) = \frac{\text{Distance on stage micrometer } (\mu\text{m})}{\text{Number of eyepiece units}}$$

Core Practical 3: Root Tip Squash and Mitotic Index Calculation

Topic Link: Topic 2 (Cells, Viruses & Reproduction)
Purpose: To prepare and stain plant tissue to observe the stages of mitosis and calculate the proportion of dividing cells.

Step-by-Step Protocol:
1. Harvesting: Cut the terminal \(2\text{--}5\text{ mm}\) of an actively growing root tip (e.g., from Allium onion or garlic) because the apical meristem is the region undergoing rapid cell division.
2. Acid Hydrolysis: Warm the root tip in \(1\text{ mol dm}^{-3}\text{ HCl}\) in a water bath. This breaks down the pectins in the middle lamellae, softening the tissue so cells can separate.
3. Rinsing & Staining: Rinse the root tip in cold water and transfer it to a microscope slide. Add a chromosome stain such as acetic orcein or toluidine blue to stain the chromatin and make chromosomes visible under a light microscope.
4. Squashing: Place a coverslip over the tissue, cover it with a paper towel, and press straight down firmly with your thumb to create a single monolayer of cells.
Examiner Warning: Never twist or slide the coverslip sideways! Sideways movement shears and damages the cells, distorting chromosome arrangements.

Mitotic Index Formula:
$$\text{Mitotic Index} = \frac{\text{Number of cells containing visible chromosomes (in mitosis)}}{\text{Total number of cells observed}}$$

Core Practical 4: Effect of Sucrose Concentration on Pollen Tube Growth

Topic Link: Topic 2 (Cells, Viruses & Reproduction)
Purpose: To determine the optimum sucrose concentration for the germination and elongation of pollen tubes.

Method:
1. Prepare a range of sucrose concentrations (e.g., \(0.2\text{--}2.0\text{ mol dm}^{-3}\)) combined with a mineral salt medium.
2. Add a drop of each sucrose solution to separate cavity slides.
3. Gently transfer pollen grains from a mature anther into the solutions using a mounted needle.
4. Place the cavity slides inside a humid Petri dish (lined with moist filter paper) to prevent evaporation of the solutions.
5. Incubate for a set time at a controlled temperature.
6. Observe under a light microscope and measure pollen tube length using a calibrated eyepiece graticule.

Core Practical 5: Effect of Temperature or Chemicals on Membrane Permeability

Topic Link: Topic 2 / Topic 4
Purpose: To investigate how temperature (or alcohol/detergent concentration) alters the structure and permeability of cell membranes.

Method:
1. Use a cork borer to cut uniform cylinders of beetroot tissue, and trim them to identical lengths using a ruler and scalpel.
2. Rinse the beetroot discs thoroughly with distilled water and pat dry. This removes superficial betalain pigment released from cells damaged during cutting.
3. Place equal numbers of discs into test tubes containing equal volumes of distilled water placed in water baths across a graded temperature range.
4. After a set incubation time, remove the beetroot discs and shake the remaining liquid to disperse the pigment evenly.
5. Transfer the solution into a cuvette and measure absorbance or percentage light transmission using a colorimeter calibrated with a water blank.

Biological Explanation:
- Increasing temperature increases the kinetic energy of phospholipids, making the membrane more fluid.
- High temperatures denature membrane transport proteins, creating gaps.
- At extreme temperatures, water inside the cell expands and forms ice crystals or vaporises, rupturing the tonoplast and plasma membrane, which causes large amounts of betalain to leak out.

Core Practical 6: Determining Water Potential of Plant Tissue

Topic Link: Topic 4 (Exchange and Transport)
Purpose: To calculate the water potential (\(\psi\)) of plant epidermal cells or tissue cylinders.

Method 1: Incipient Plasmolysis (Epidermal Peels):
1. Place thin epidermal peels (e.g., red onion) into graded concentrations of sucrose solution.
2. Count the total number of cells and the number of plasmolysed cells (where the protoplast has pulled away from the cell wall).
3. Determine the point of incipient plasmolysis, defined as the concentration where exactly 50% of the cells are plasmolysed.
4. At incipient plasmolysis, the pressure potential is zero (\(\psi_{\text{p}} = 0\)), meaning the internal water potential of the cell equals the external solution's solute potential: \(\psi_{\text{cell}} = \psi_{\text{external sol}}\).

Method 2: Tissue Cylinders (Mass/Length Change):
1. Immerse pre-weighed plant cylinders into graded sucrose solutions for a set time.
2. Blot gently, reweigh, and calculate percentage change in mass: \(\frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100\).
3. Plot percentage change in mass against sucrose concentration. The point where the line crosses the \(x\)-axis (zero percentage change) represents the concentration isotonic to the tissue.

Core Practical 7: Dissection of the Insect Gas Exchange System

Topic Link: Topic 4 (Exchange and Transport)
Purpose: To observe and identify the specialised anatomical structures of the tracheal system in a locust or insect.

Key Anatomical Structures:
- Spiracles: Pores along the thorax and abdomen that open and close to regulate gas exchange and minimise water loss.
- Tracheae: Large internal tubes lined with spiral rings of chitin (visible under a light microscope), which provide structural support and keep the airways open.
- Tracheoles: Highly branched, microscopic dead-end tubes without chitin rings, in direct contact with respiring cells; filled with tracheal fluid at their tips.
- Air Sacs: Collapsible chambers that expand and compress to ventilate the tracheal system during physical activity.

Core Practical 8: Measuring Water Uptake Using a Potometer

Topic Link: Topic 4 (Exchange and Transport)
Purpose: To investigate the effect of environmental factors (light intensity, wind speed using a fan, temperature, or humidity) on the rate of water uptake in a leafy shoot.

Apparatus and Experimental Setup:
1. Cut the leafy shoot underwater at a slant. Cutting underwater prevents air bubbles entering the xylem vessels, and the slanted cut maximises the surface area for water absorption.
2. Assemble the potometer completely underwater to ensure an airtight and watertight seal.
3. Apply petroleum jelly (Vaseline) around all rubber joints and stoppers to prevent air leaks.
4. Introduce a single air bubble into the capillary tube.
5. Track the distance (\(d\)) moved by the bubble over a set time interval (\(t\)).
6. Calculate the rate of water uptake using the volume of a cylinder: \(\text{Volume} = \pi r^2 d\), giving a rate in \(\text{mm}^3\text{ s}^{-1}\) or \(\text{mm}^3\text{ min}^{-1}\).

Examiner Pitfall: A potometer does not measure transpiration directly! It measures rate of water uptake. A small fraction of the absorbed water (around \(1\text{--}2\%\)) is retained by the plant for photosynthesis and maintaining cell turgidity.

Explaining Environmental Effects: When asked to explain the effect of wind speed, do not just describe the trend. State that moving air removes the humid boundary layer of water vapour surrounding the leaf, maintaining a steep water vapour potential gradient between the air spaces inside the leaf and the external air, which increases the rate of transpiration and water uptake.

Key Takeaway: Master the specific justification for every preparation step—such as cutting stems underwater, acid hydrolysis in root squashes, and rinsing beetroot discs—as these form the core of exam mark schemes.

Section 3: Quick Summary and Revision Checklist

Before sitting your exam, make sure you can confidently:
- Identify the independent, dependent, and controlled variables for all 8 core practicals.
- Explain how to calibrate an eyepiece graticule using a stage micrometer: \(\text{Value of } 1\text{ epu } = \frac{\text{micrometer distance}}{\text{number of epu}}\).
- Calculate magnification and actual size using \(M = \frac{I}{A}\).
- Calculate the mitotic index from raw cell counts.
- State why a potometer measures water uptake rather than direct transpiration.
- Draw tangents at \(t = 0\) to determine initial rates of enzyme reactions.