Welcome to AS 3: Assessed Practical Tasks!
Hello and welcome to your practical guide for CCEA AS Biology Unit 3 (AS 3)! Practical work is where biology truly comes alive. Instead of just reading about cells, enzymes, and molecules, you get to see them and measure their actions directly in the laboratory.
Don't worry if practical exams or laboratory write-ups feel intimidating at first. By breaking down each core assessed practical into simple, logical steps, you will master the underlying science, understand why each step is performed, and easily score top marks on experimental design, data analysis, and evaluation questions.
1. Microscopy, Graticule Calibration, and Biological Drawing
Light microscopy is a fundamental skill in AS Biology. It allows us to view cells and tissues that are invisible to the naked eye.
Calibrating the Eyepiece Graticule
An eyepiece graticule is a transparent glass disc placed in the microscope eyepiece with an etched scale marked from \(0\) to \(100\) arbitrary units (graticule units). Because eyepiece units do not change when you switch objective lenses, you must calibrate the graticule against a stage micrometer (a slide with a known physical scale, usually \(1\text{ mm}\) divided into \(100\) divisions, meaning each stage division is \(0.01\text{ mm}\) or \(10\ \mu\text{m}\)).
Step-by-Step Calibration:
1. Place the stage micrometer on the microscope stage and focus on the scale using the low-power objective lens.
2. Align the zero line of the eyepiece graticule with a zero line on the stage micrometer.
3. Look along the scales to find another point where two lines perfectly overlap.
4. Count the number of eyepiece graticule units (epu) and the corresponding number of stage micrometer divisions.
5. Calculate the true distance for \(1\text{ epu}\) using the formula:
\(1\text{ epu} = \frac{\text{Distance on stage micrometer (\)\mu\text{m}\))}}{\text{Number of graticule units}}\)
6. Repeat this process every time you change the objective lens power!
Magnification Formula
To calculate magnification, image size, or actual size, use the famous I AM triangle:
\(\text{Magnification } (M) = \frac{\text{Image Size } (I)}{\text{Actual Size } (A)}\)
\(\text{Actual Size } (A) = \frac{\text{Image Size } (I)}{\text{Magnification } (M)}\)
\(\text{Image Size } (I) = \text{Actual Size } (A) \times \text{Magnification } (M)\)
Common Mistake to Avoid: Always ensure that \(I\) and \(A\) are in the exact same units before dividing! Remember: \(1\text{ mm} = 1000\ \mu\text{m}\). To convert millimeters (\(\text{mm}\)) to micrometers (\(\mu\text{m}\)), multiply by \(1000\).
Rules for Producing Clear Biological Drawings
When creating biological drawings (such as tissue plans or cellular diagrams):
- Use a sharp HB pencil and draw sharp, continuous lines (no sketchy, shaded, or feathery lines).
- Do not use any shading or coloring.
- Draw label lines with a ruler; lines must touch the structure precisely and must never cross each other.
- Always include a clear, descriptive title and a scale bar or stated magnification.
Key Takeaway: The eyepiece graticule has arbitrary units and must be calibrated using a stage micrometer for each specific objective lens before you can measure actual cell sizes.
2. Qualitative and Semi-Quantitative Biochemical Food Tests
In the laboratory, we identify unknown biological macromolecules using specific chemical reagents.
Core Biochemical Tests Summary
1. Reducing Sugars (e.g., Glucose, Maltose):
- Method: Add equal volume of Benedict's reagent to the sample and heat in a boiling water bath at \(80\text{ }^\circ\text{C} - 100\text{ }^\circ\text{C}\) for \(5\text{ minutes}\).
- Positive Result: Color changes from blue \(\rightarrow\) green \(\rightarrow\) yellow \(\rightarrow\) orange \(\rightarrow\) brick-red precipitate.
2. Non-Reducing Sugars (e.g., Sucrose):
- Method: If the initial Benedict's test remains blue, take a fresh sample, add dilute hydrochloric acid (\(\text{HCl}\)), and boil for several minutes to hydrolyze glycosidic bonds. Cool and neutralize with sodium hydrogencarbonate (\(\text{NaHCO}_3\)). Re-test with Benedict's reagent and boil.
- Positive Result: Brick-red precipitate forms (confirming that non-reducing sugars were broken down into reducing monosaccharides).
3. Starch:
- Method: Add a few drops of iodine in potassium iodide solution at room temperature.
- Positive Result: Color changes from yellow-brown to blue-black.
4. Proteins:
- Method: Add Biuret reagent (sodium hydroxide and dilute copper sulfate) at room temperature.
- Positive Result: Color changes from pale blue to purple/lilac/violet.
5. Lipids (Emulsion Test):
- Method: Add absolute ethanol to the dry sample and shake vigorously to dissolve lipids. Decant the ethanol layer into a tube containing cold water.
- Positive Result: A milky-white emulsion forms as dissolved lipids precipitate out of solution.
Semi-Quantitative Testing and Colorimetry
A standard Benedict's test gives qualitative data (presence or absence) or semi-quantitative data (judging the color change by eye). To make it fully quantitative:
1. Prepare a dilution series (e.g., serial dilutions) of known glucose concentrations (\(0.0, 0.2, 0.4, 0.6, 0.8, 1.0\text{ mol dm}^{-3}\)).
2. Carry out Benedict's test on all standards using identical volumes, heating times, and temperatures.
3. Filter or centrifuge out the brick-red precipitate.
4. Use a colorimeter (with a red filter) to measure the absorbance or percentage transmission of the remaining blue supernatant.
5. Plot a calibration curve (glucose concentration on the \(x\)-axis vs. absorbance/transmission on the \(y\)-axis).
6. Test an unknown sample, measure its absorbance, and read its concentration directly from your calibration curve.
Key Takeaway: Colorimeters remove human subjectivity, transforming visual color estimates into objective, quantitative measurements via standard calibration curves.
3. Investigating Enzyme Kinetics
Enzyme experiments assess how changing a single variable affects the initial rate of an enzyme-controlled reaction.
Common Assessed Enzyme Systems
- Catalase: Breaks down toxic hydrogen peroxide (\(\text{H}_2\text{O}_2\)) into water (\(\text{H}_2\text{O}\)) and oxygen gas (\(\text{O}_2\)). Rate is measured by collecting gas volume in a gas syringe over time or counting bubbles per minute.
- Amylase: Hydrolyzes starch into maltose. Rate is determined by sampling the reaction mixture into iodine wells at timed intervals until the iodine no longer turns blue-black (starch is fully digested).
Calculating the Initial Rate of Reaction
Why do we measure the initial rate?
As the reaction proceeds, substrate concentration rapidly decreases as it is converted into product. This substrate depletion slows the reaction rate. Therefore, measuring the rate in the first \(30 - 60\text{ seconds}\) gives the true, maximum speed under the specified experimental conditions.
To calculate rate from a graph of product volume vs. time:
\(\text{Rate} = \frac{\text{Change in Volume of Product}}{\text{Time Taken}}\)
Draw a tangent to the curve at time \(t = 0\) and calculate its gradient: \(\text{Gradient} = \frac{\Delta y}{\Delta x}\).
Controlling Experimental Variables
- Independent Variable: The factor you intentionally change (e.g., substrate concentration, enzyme concentration, temperature, or pH).
- Dependent Variable: The factor you measure (e.g., volume of oxygen produced per minute, or time for starch to disappear).
- Controlled Variables: Factors kept strictly constant to ensure a valid test (e.g., using a thermostatically controlled water bath for temperature, and buffer solutions for pH).
Key Takeaway: Always measure the initial reaction rate to prevent substrate depletion from interfering with your kinetic results.
4. Membrane Permeability and Water Potential Investigations
Investigating Membrane Permeability using Beetroot
Beetroot cells contain a vibrant red water-soluble pigment called betalain trapped within their large central vacuoles by the tonoplast and cell surface membrane.
Experimental Procedure:
1. Cut uniform cylinders of beetroot tissue using a cork borer and trim them to identical lengths with a scalpel and ruler.
2. Rinse the beetroot cylinders thoroughly with distilled water and blot dry with paper towels to remove pigment leaked from damaged cells during cutting.
3. Place cylinders into test tubes at different temperatures or in different concentrations of organic solvents (e.g., ethanol).
4. After a set incubation time, remove the cylinders and measure the absorbance of the liquid using a colorimeter (using a blue/green filter).
5. Explanation: High temperatures denature membrane transport proteins and melt membrane lipids, increasing fluidity and creating gaps. Solvents like ethanol dissolve phospholipid bilayers. Both cause severe membrane disruption, allowing betalain pigment to leak out rapidly.
Determining Water Potential in Plant Tissues (Potato Cylinders)
Water potential (\(\Psi\)) is a measure of the tendency of water molecules to move from one area to another by osmosis.
Procedure:
1. Prepare a range of sucrose concentrations (e.g., \(0.0, 0.2, 0.4, 0.6, 0.8, 1.0\text{ mol dm}^{-3}\)).
2. Cut potato cylinders to identical lengths, blot dry, and record their initial mass using an electronic balance.
3. Place cylinders into the solutions for at least \(30 - 60\text{ minutes}\).
4. Remove cylinders, blot gently to remove excess surface water, and record the final mass.
5. Calculate percentage change in mass:
\(\text{Percentage Change in Mass} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100\)
Graph Interpretation:
- Plot percentage change in mass (\(y\)-axis) against sucrose concentration (\(x\)-axis).
- The point where the curve crosses the \(x\)-axis (\(\text{Percentage Change} = 0\%\)) represents the incipient plasmolysis point / isotonic concentration.
- At this point, there is no net movement of water because the water potential of the sucrose solution equals the internal water potential of the potato cells.
Key Takeaway: Calculating percentage change in mass accounts for small starting differences between biological samples, ensuring fair and valid comparisons.
5. Chromatography of Photosynthetic Pigments
Paper or Thin-Layer Chromatography (TLC) separates complex mixtures of biological molecules based on their relative solubility in a mobile phase versus their affinity for a stationary phase.
Step-by-Step Method:
1. Grind fresh leaves with a solvent (e.g., propanone/acetone) and fine sand in a mortar and pestle to extract pigments.
2. Draw a faint pencil line (the origin) approximately \(1.5\text{ cm}\) from the bottom of the chromatography strip. (Never use pen ink, as ink contains soluble dyes that would run!).
3. Use a capillary tube to apply a small, concentrated spot of pigment extract on the pencil line, drying between successive applications to keep the spot concentrated and small.
4. Suspend the strip in a chromatography tank containing solvent. Ensure the solvent level is strictly below the pencil origin line.
5. Seal the tank with a lid to saturate the atmosphere with solvent vapor and prevent evaporation.
6. Remove the strip just before the solvent reaches the very top, and immediately mark the solvent front with a pencil.
Calculating the Retardation Factor (\(R_f\) value)
Each pigment molecule has a characteristic \(R_f\) value in a given solvent system:
\(R_f = \frac{\text{Distance moved by pigment spot from origin}}{\text{Distance moved by solvent front from origin}}\)
Note: \(R_f\) values must always be between \(0.0\) and \(1.0\). Highly soluble pigments move furthest up the plate and have higher \(R_f\) values.
Key Takeaway: Different pigments travel at different rates due to differences in molecular size, solubility in the solvent, and adsorption to the stationary phase.
6. Root Tip Squash for Observing Mitosis
To view actively dividing cells under the microscope, we examine the apical meristem found in growing root tips (e.g., garlic or onion root tips).
Key Steps and Scientific Rationale:
1. Fixation & Acid Hydrolysis: Cut the terminal \(2 - 3\text{ mm}\) of the root tip and place it in warm dilute hydrochloric acid (\(\text{HCl}\)). This dissolves the pectins in the middle lamella, breaking cell-to-cell adhesion and softening the tissue so it can be easily squashed into a single layer.
2. Staining: Stain the tissue with an acetic orcein or toluidine blue stain. These stains bind strongly to negatively charged DNA/chromatin, making chromosomes clearly visible.
3. Squashing: Place the stained root tip on a clean slide, add a coverslip, and apply firm, vertical pressure with your thumb using filter paper. (Avoid twisting or sliding the coverslip, which would shear or roll the chromosomes!).
Calculating Mitotic Index
The mitotic index represents the proportion of cells in a tissue undergoing active nuclear division:
\(\text{Mitotic Index} = \frac{\text{Number of cells with visible chromosomes (in mitosis)}}{\text{Total number of cells observed}}\)
Key Takeaway: Acid hydrolysis softens tissue by breaking down cell wall middle lamellae, enabling the creation of a single optical layer of cells during the squash.
7. Experimental Rigour: Accuracy, Precision, and Errors
Key Practical Terminology:
- Accuracy: How close a measured value is to the true biological value.
- Precision: How close repeated measurements are to each other (independent of accuracy).
- Repeatability: Obtaining consistent results when the same experimenter repeats the investigation under identical conditions.
- Reproducibility: Obtaining consistent results when different experimenters run the investigation using different equipment.
- Control Experiments: Negative controls (e.g., using boiled, denatured enzymes or distilled water instead of substrate) establish a baseline and prove that the observed change is solely due to the independent variable.
Quick Review of Common Exam Traps
- Confusing resolution with magnification: Magnification is how many times larger an image is compared to reality; resolution is the ability to distinguish between two separate points.
- Forgetting units: Always include proper biological units in tables and calculations (e.g., \(\text{s}\), \(\text{cm}^3\), \(\text{mol dm}^{-3}\), \(\mu\text{m}\)).
- Inappropriate graphs: Use line graphs for continuous data (e.g., temperature, concentration) and bar charts for discrete/discontinuous categories (e.g., different tissue types).
Final Encouragement: Success in AS 3 practical biology comes down to understanding the why behind every step: why we rinse beetroot discs, why we acid-hydrolyze root tips, and why we measure initial rates. Keep these scientific principles in mind, and you will excel in your assessed practical tasks!