Welcome to Your IA2 Practical Guide!
Hello! If you are preparing for your Unit 6 exam, you have come to the right place. While Unit 3 focused on the basics, Unit 6 (IA2) asks you to think like a professional researcher. You won't just need to remember what happened in these nine core practicals; you will need to know how to plan them, evaluate them, and analyse the data they produce.
Don't worry if some of these feel a bit complex—we are going to break down Core Practicals 10 through 18 into simple, easy-to-digest steps. For more detail on how to design a perfect experiment from scratch, check out our chapter on Planning Investigations and Null Hypotheses.
Core Practical 10: Factors Affecting Photosynthesis
The Goal: To see how changing light intensity, light wavelength, temperature, or \(CO_2\) availability changes the rate at which a plant photosynthesises.
How we do it: We usually use an aquatic plant (like Elodea). As the plant photosynthesises, it releases oxygen bubbles. We can measure the rate by counting bubbles per minute or, more accurately, by using a gas syringe to measure the volume of oxygen produced.
Variables to Watch:
- Independent Variable: The factor you change (e.g., distance of the lamp for light intensity).
- Dependent Variable: Volume of oxygen produced in a set time.
- Control Variables: If you change light, you must keep temperature constant using a water bath (a "heat shield" or glass of water between the lamp and plant helps block heat). Use \(NaHCO_3\) (sodium hydrogen carbonate) to provide a constant supply of \(CO_2\).
Quick Tip: Remember the Inverse Square Law! If you double the distance from the light source, the light intensity actually decreases by four times.
Core Practical 11: Ecology of a Habitat
The Goal: To study where organisms live (distribution) and how many there are (abundance) using quadrats and transects.
The Process:
- Random Sampling: Use a grid and a random number generator to place quadrats. This avoids bias.
- Systematic Sampling: Use a transect (a long tape measure) to see how species change as you move across a habitat (e.g., moving away from the shore).
Measuring Abiotic Factors: You must measure "non-living" factors like soil \(pH\), light intensity, and temperature to explain why the organisms are there.
Key Formula: You might be asked to calculate the Index of Diversity \(D\):
\(D = \frac{N(N - 1)}{\sum n(n - 1)}\)
(Where \(N\) is the total number of organisms of all species, and \(n\) is the number of individuals of one species).
Key Takeaway: Always use a large number of samples to ensure your data is representative of the whole area.
Core Practical 12: Temperature and Development
The Goal: To see how temperature affects how fast an organism grows or develops (e.g., brine shrimp hatching or seedling growth).
The Ethics: Since we are using living things, we must follow ethical guidelines: ensure they have enough food/oxygen and return them to their natural habitat (or dispose of them humanely) afterwards.
The Method: Place equal numbers of brine shrimp eggs in water at different temperatures (e.g., \(10^{\circ}C, 15^{\circ}C, 20^{\circ}C, 25^{\circ}C, 30^{\circ}C\)). Count how many have hatched after 24 hours.
The Science: This links back to Topic 5—higher temperatures increase enzyme activity (kinetic energy), which speeds up development, but too much heat will denature enzymes and stop development entirely.
Core Practical 13: Growth of Microorganisms
The Goal: To measure the growth rate of bacteria or yeast in a liquid culture (broth).
Two Main Methods:
1. Turbidity: Use a colorimeter. As the population grows, the liquid gets "cloudier" (more turbid), so it absorbs more light.
2. Cell Counts: Using a microscope and a hemocytometer (a special slide with a grid) to count cells directly.
The Math: You will likely see an exponential growth curve. You might need to use logarithmic scales on graphs because bacterial numbers grow so fast they become impossible to plot on a normal scale!
Safe Practice: Always use aseptic techniques (flaming loops, working near a Bunsen burner) to prevent contamination.
Core Practical 14: Investigating Antibiotics
The Goal: To test which antibiotic is most effective at killing a specific type of bacteria.
The Method: Spread bacteria evenly on an agar plate (creating a "lawn"). Place paper discs soaked in different antibiotics on the agar. Incubate at \(25^{\circ}C\) (never \(37^{\circ}C\) in schools to avoid growing human pathogens).
The Result: Look for the zone of inhibition (the clear area where bacteria couldn't grow).
The Calculation: Measure the diameter of the zone. Use the area of a circle formula: \(Area = \pi r^2\). The larger the area, the more effective the antibiotic.
Core Practical 15: Respiration in Yeast (Redox Indicators)
The Goal: To investigate the rate of respiration using a redox indicator like methylene blue or TTC.
How it works: During respiration, electrons are released. A redox indicator is a "hydrogen carrier" that changes colour when it is reduced (takes those electrons).
- Methylene blue: Blue \(\rightarrow\) Colourless when reduced.
Measuring the Rate: Time how long it takes for the colour change to happen.
\(Rate = \frac{1}{time}\).
Did you know? This experiment mimics the Electron Transport Chain in the mitochondria, where oxygen usually acts as the final electron acceptor!
Core Practical 16: Using a Respirometer
The Goal: To measure the rate of oxygen consumption and find the Respiratory Quotient (RQ).
The Setup: A respirometer has a chamber for the organism (like germinating seeds) and a tube containing soda lime or potassium hydroxide.
Why Soda Lime? It absorbs all the \(CO_2\) produced. Therefore, any change in gas volume is purely due to oxygen being taken in. This pulls a drop of coloured liquid along a tube.
Formula for RQ:
\(RQ = \frac{CO_2 \text{ produced}}{O_2 \text{ consumed}}\)
- \(RQ = 1.0\) (Carbohydrates)
- \(RQ = 0.7\) (Lipids)
- \(RQ = 0.8-0.9\) (Proteins)
Core Practical 17: Exercise and Spirometry
The Goal: To use spirometer traces to measure lung volumes and how they change with exercise.
Key Terms to Know:
- Tidal Volume: Volume of air in a normal breath.
- Breathing Rate: Number of breaths per minute.
- Minute Ventilation: \(Tidal \text{ } Volume \times Breathing \text{ } Rate\).
- Oxygen Consumption: Look at the "slope" of the trace. As the person breathes, they use up \(O_2\), and the soda lime in the spirometer absorbs the \(CO_2\), causing the total volume to drop over time.
Safety First: Ensure the subject is healthy, use fresh soda lime, and make sure the mouthpiece is disinfected!
Core Practical 18: Amylase in Germinating Grains
The Goal: To see how gibberellins trigger the production of the enzyme amylase in cereal grains.
The Science: When a seed starts to grow (germinate), it releases gibberellins. This hormone switches on genes that produce amylase. Amylase then breaks down stored starch into sugar for growth.
The Method:
1. Soak seeds in different concentrations of gibberellin.
2. Place them on starch agar plates.
3. After a few days, flood the plate with iodine.
4. Result: The agar turns blue-black where starch is present. A clear zone (halo) appears around the seeds where amylase has digested the starch. The larger the clear zone, the more amylase was produced.
Quick Review: Top Tips for Unit 6
1. Units Matter: Always include units in your tables (e.g., \(mm^3\), \(cm \text{ } min^{-1}\)).
2. Repeatability: Always mention doing at least three repeats to identify anomalies and calculate a mean.
3. Accuracy vs. Precision: Use the right equipment. Don't say "measure volume with a beaker"; use a measuring cylinder or a graduated pipette for better resolution.
4. The "Null Hypothesis": Remember that for most of these, you'll need to be able to write a null hypothesis (e.g., "There is no significant difference between the rate of respiration at \(20^{\circ}C\) and \(30^{\circ}C\)").