AQA GCSE Biology Required Practicals Comprehensive Study Guide

General Examination Tips for Biology Practicals

• Variable Identification:     * Independent Variable: The specific factor that is changed or manipulated by the experimenter.     * Dependent Variable: The factor that is measured or observed to see how it responds to changes in the independent variable.     * Control Variables: Factors that could influence the result but are kept constant throughout the investigation to ensure the results are accurate and the test is fair.
• Equipment Specification: Always name the specific piece of equipment used for measurements. For example, instead of saying "measure the length," specify "measure the length with a ruler."
• Safety Protocols:     * Always wear goggles and gloves when working with chemicals.     * State obvious safety precautions in exam answers to ensure all potential marks are captured.
• Accuracy and Error Reduction:     * Avoid Parallax Error by ensuring your eye is level with the measurement mark on a ruler or the bottom of the meniscus in a measuring cylinder.     * Perform multiple/repeat measurements to calculate a mean value, which helps identify and reduce the impact of anomalies.
• Language and Formatting:     * Bullet points are recommended for clear organization.     * Use formal scientific English. For example, use "heat the water gently on a gauze on a tripod over a Bunsen burner flame" instead of "heat liquid with fire."

Bio 1: Microscopy

• Preparation of the Slide:     * Use a scalpel and tweezers to obtain a thin layer of onion skin.     * Place the specimen onto the microscope slide.     * Add a drop of iodine solution to stain the cells, making internal structures more visible.     * Carefully lower a cover slip onto the slide.
• Microscope Operation:     * Place the slide on the stage.     * Activate the light source or tilt the mirror to reflect light through the condenser to the slide.     * Always start with the shortest objective lens (the lowest magnification).     * Use the coarse focus knob first, then the fine focus knob to bring the specimen into clear focus.     * Switch to a higher magnification objective lens and refocus if a more detailed view is required.
• Measurement and Scale:     * A graticule (a tiny ruler on the slide) can be used to measure cell size in micrometers ($\mu m$).     * Micro denotes $10^{-6}$ in standard form.     * Example: A cell length of $2.5\,\mu m$ is equal to $2.5 \times 10^{-6}\,m$.

Bio 2: Osmosis

• Aim: To determine the concentration of sugar inside plant cells (e.g., potato cells).
• Preparation:     * Use a cork borer to cut equal-sized cylinders from a vegetable like a potato.     * Remove any non-permeable skin from the ends.     * Dab the cylinders with a paper towel to remove excess surface water.     * Weigh each cylinder using a top-pan balance.
• Procedure:     * Place cylinders into test tubes containing different concentrations of sugar solution (the Independent Variable).     * Leave the cylinders for a set duration, such as one day ($24$ hours).     * Remove the cylinders, dab off excess water, and re-weight them.
• Data Analysis:     * Calculate the percentage difference in mass for each cylinder (the Dependent Variable).     * Plot the results against the solution concentration on a graph.     * The point where the line of best fit crosses the x-axis (where the change in mass is $0$) represents the concentration where no osmosis occurs. This indicates the internal concentration of glucose within the vegetable cells.

Bio 3: Enzymes

• Aim: To determine the optimum temperature or pH for an enzyme, such as amylase acting on starch.
• Variables:     * Independent Variable: Temperature (varied using a water bath) or pH (varied using buffer solutions).     * Dependent Variable: The time taken for the starch substrate to be completely broken down.
• Method:     * Measure specific volumes of amylase and starch solutions using a syringe or measuring cylinder.     * Mix the solutions and start a stop clock.     * At $10$-second intervals, remove a sample and place a drop into a spotting tile dimple containing iodine.     * Initially, the iodine will turn black/purple, indicating starch is still present.     * Repeat every $10$ seconds until the iodine no longer changes color (the endpoint).
• Recording Data: Record the time to endpoint for various temperatures (measured with a thermometer in the test tube) or pH levels. Plot these on a graph; the optimum is located at the lowest point of the resulting curve.

Bio 4: Food Tests

• Sample Preparation: For solid food, grind the sample using a pestle and mortar, then add distilled water to create a solution.
• Starch Test: Add iodine solution. A positive result is a change to black or dark purple.
• Glucose and Simple Sugars Test:     * Add Benedict's solution.     * Heat the mixture in a water bath.     * This is a semi-quantitative test; color changes from blue (negative) to green, yellow, orange, or red depending on sugar concentration.
• Protein Test: Add Biuret reagent. A positive result turns from blue to purple.
• Fats (Lipids) Test:     * Add cold ethanol to the sample and wait one minute.     * Pour the ethanol into a test tube of water.     * A positive result appears as a cloudy emulsion.

Bio 5: Photosynthesis

• Aim: To determine the relationship between light intensity and the rate of photosynthesis.
• Variables:     * Independent Variable: The distance between the plant and the light source.     * Dependent Variable: The volume of oxygen gas produced or the number of bubbles released per minute.
• Method:     * Submerge pond weed in water inside an inverted test tube or measuring cylinder.     * Cut the stem at an angle and add sodium hydrogen carbonate ($NaHCO_3$) to the water to provide carbon dioxide ($CO_2$) and promote oxygen release.     * Place the setup in a dark room.     * Use a meter rule to measure the distance to the light source.     * Allow the plant to acclimatize for $1$ minute after turning on the light to reach a constant rate.     * Count the bubbles or measure the volume of oxygen over a set time.
• Mathematical Relationship: Light intensity follows the inverse square law relative to distance. If distance is doubled, light intensity (and the rate of photosynthesis) decreases to one-quarter ($\frac{1}{4}$) of its original value.

Bio 6: Reaction Times

• Method: The ruler drop test.     * One person holds a ruler; the subject places their thumb and finger at the zero mark without touching it.     * The ruler is dropped without warning, and the subject catches it as quickly as possible.
• Calculation: Reaction time ($t$) can be calculated using the rearranged equation of motion:     * $t = \sqrt{\frac{2s}{a}}$     * Where s = \text{distance the ruler fell in meters (m)}     * And a = \text{acceleration due to gravity (9.8\,m/s^2)}
• Factors to Investigate: Distractions (e.g., texting) or stimulants (e.g., drinking a sugary beverage) can be used as independent variables. Note that practice effects may occur, where a person improves over multiple tries.

Bio 7: Quadrats and Transects

• Quadrat Sampling: Use a random number generator to select grid coordinates to avoid bias. Aim to sample $10\%$ of the total area.     * Count organisms in the $1\,m^2$ quadrat.     * Total Population Estimate: $\text{Mean count per } m^2 \times \text{Total area in } m^2$.
• Transect Lines: Used to study how population density changes across an environmental gradient (e.g., distance from a beach).     * Move the quadrat along a line at set intervals (e.g., every $1\,m$).     * Results can be displayed using a kite graph.
• Biotic vs. Abiotic Factors:     * Biotic Factors: Biological influences such as predators.     * Abiotic Factors: Non-biological influences such as the type of surface or temperature.

Bio 8: Microbiology (Triple Science Only)

• Methods:     * Observe growth of bacterial cultures on Agar in a Petri dish.     * Create a "lawn" of bacteria to test antibiotics or antiseptics using soaked paper discs.
• Aseptic Technique (Sterility):     * Sterilize glass spreaders or rods by passing them through a Bunsen burner flame.     * Open Petri dish lids minimally and perform work near the Bunsen flame to utilize the updraft to keep microbes out.     * Secure the lid with only a few pieces of tape to allow aerobic respiration; sealing completely could encourage the growth of dangerous anaerobic bacteria.
• Analysis: Measure the diameter of the zone of inhibition (where no bacteria grow).     * Calculate area using: $Area = \pi r^2$ or $Area = \frac{\pi D^2}{4}$.

Bio 9: Germination (Triple Science Only)

• Procedure: Place seeds (e.g., cress) on damp cotton wool in a Petri dish and leave them to germinate in the dark.
• Geotropism: If the dish is rotated $90^{\circ}$, the roots will bend to continue growing downwards due to gravity.
• Phototropism: If a small hole allows light into the container, the shoots will grow toward the light source.

Bio 10: Decay (Triple Science Only)

• Aim: To model decay using milk and enzymes.
• Method:     * Measure a volume of whole milk or cream into a test tube.     * Add sodium carbonate and the indicator phenolphthalein, which turns pink at a pH above approximately $8.3$.     * Add the enzyme lipase, which breaks down lipids into fatty acids, increasing the acidity.     * The Independent Variable is temperature, controlled using a water bath.
• Endpoint: Use a stop clock to measure the time it takes for the pink solution to decolorize as the pH drops.
• Data: The rate of reaction increases with temperature until the enzyme reaches its optimum, after which the enzyme denatures.