Required practicals 7-9: photosynthesis, respiration and competition

Introduction to A2 Required Practicals

Welcome to the final stretch of your practical skills journey! In Unit 5, you are assessed on your ability to plan, analyze, and evaluate experiments. This guide covers Required Practicals 7, 8, and 9, which focus on the core processes of life: photosynthesis, respiration, and competition. These practicals aren't just about following a "recipe"; they are about understanding how variables interact in living systems. Don't worry if the setups look complicated at first—we will break them down step-by-step!

Required Practical 7: Investigating Photosynthesis

The goal of this practical is to see how a limiting factor affects the rate of photosynthesis. While the syllabus suggests light intensity, you might also look at temperature or \(CO_{2}\) concentration.

The Setup: Using Pondweed

We often use aquatic plants like Elodea because they release oxygen gas as a byproduct of photosynthesis, which we can easily see as bubbles.

1. Measuring the Rate: You can count the number of bubbles released per minute, or for better accuracy, use a capillary tube and syringe to collect the oxygen gas and measure the length of the bubble (volume).
2. Controlling Variables: If you are changing light intensity (the independent variable), you must keep temperature and \(CO_{2}\) concentration constant.
3. The \(CO_{2}\) Source: We usually add a fixed volume of sodium hydrogencarbonate solution to the water. This ensures the plant has plenty of \(CO_{2}\) so that it isn't the thing slowing down the reaction.

Changing Light Intensity

To change light intensity, you move a lamp specific distances from the plant (e.g., \(10\text{ cm}\), \(20\text{ cm}\), \(30\text{ cm}\)). Key Tip: Remember the inverse square law. As the distance doubles, the light intensity actually decreases by four times!

Quick Review: Limiting Factors

If you increase light intensity and the rate of photosynthesis stops increasing, it means light is no longer the limiting factor. Something else, like temperature or \(CO_{2}\) levels, is now "holding back" the process.

Common Mistake to Avoid: Forgetting to let the plant equilibrate. Always wait a few minutes after moving the lamp for the plant to adjust to the new light level before you start counting bubbles.

Required Practical 8: Investigating Respiration

This practical looks at how variables like temperature or the type of substrate (e.g., glucose vs. sucrose) affect the rate of respiration in organisms like yeast or small invertebrates (e.g., locusts).

Using a Respirometer

A respirometer measures the change in gas volume. Here is how it works:
1. The organism uses up \(O_{2}\) during aerobic respiration.
2. The organism produces \(CO_{2}\) gas.
3. We place a chemical like potassium hydroxide (\(KOH\)) or soda lime in the chamber to absorb the \(CO_{2}\).
4. Because the \(CO_{2}\) is absorbed, the total volume of gas decreases as the \(O_{2}\) is used up. This sucks a drop of colored liquid through a capillary tube, allowing us to measure the rate.

The Respiratory Quotient (\(RQ\))

In your exam, you might be asked to calculate the \(RQ\). This tells us what substance the organism is "burning" for energy. The formula is:
\(RQ = \frac{\text{carbon dioxide produced}}{\text{oxygen consumed}}\)

Note: You must use the same units for both values! For example, if you measure both in \(cm^{3}\), the \(RQ\) is a simple ratio with no units.

Did you know?

If an organism is respiring purely anaerobically, the \(RQ\) value will be very high (or even infinite) because \(CO_{2}\) is being produced but no \(O_{2}\) is being consumed!

Required Practical 9: Investigating Competition

This is a laboratory-based study of how competition affects the growth of seedlings. Competition occurs when resources (like light, water, or minerals) are in short supply.

Intraspecific vs. Interspecific Competition

Intraspecific competition: Competition between members of the same species (e.g., two wheat seeds in one pot).
Interspecific competition: Competition between members of different species (e.g., a wheat seed and a weed seed in the same pot).

The Procedure

1. Plant seeds in pots with controlled soil type and volume.
2. Independent Variable: The density of seeds (to test intraspecific) or the mix of different species (to test interspecific).
3. Dependent Variable: Measure the "growth." Usually, this is mean biomass (the dry mass of the plants). Why dry mass? Because water content in plants can vary wildly and doesn't reflect actual organic growth.

Key Takeaway: The Effect on Growth

As competition increases (higher density of seeds), the mean mass per plant usually decreases. This is because there are fewer resources available for each individual seedling to grow to its full potential.

Common Mistake:

Students often forget that abiotic factors (like temperature and light) must be kept exactly the same for all pots. If one pot is closer to a window than another, you won't know if the growth difference is due to competition or light!

Summary of Skills for Unit 5

When answering exam questions on these practicals, keep these "Golden Rules" in mind:

• Precision: Use equipment with smaller scale divisions to reduce percentage error (e.g., a gas syringe is more precise than counting bubbles).
• Reliability: Always repeat your measurements and calculate a mean. This helps identify anomalies (results that don't fit the pattern).
• Statistical Tests: For Practical 9, you might use a t-test to see if the difference in mean biomass between two groups is statistically significant, or Spearman's Rank to see if there is a correlation between seed density and growth.
• Safety: Always mention specific hazards. For example, \(KOH\) in respirometers is corrosive, so wear goggles and gloves.

Don't worry if these experiments seem complex! Just remember the core principle: we change one thing (Independent Variable), measure the result (Dependent Variable), and keep everything else the same (Control Variables). You've got this!