Edexcel A Level (IAL) Biology -5.8 Core Practical 10: Rate of Photosynthesis- Study Notes- New Syllabus

CORE PRACTICAL 10 – Investigating Factors Affecting Rate of Photosynthesis

🌱 Introduction

This practical measures how light intensity, light wavelength, temperature, and CO₂ availability affect the rate of photosynthesis in a suitable aquatic plant (e.g., Elodea / Cabomba / pondweed).
Common measurable proxy: rate of O₂ evolution (bubble count or gas collected) or change in dissolved O₂.

🧪 Aim

Investigate one factor at a time (light intensity / wavelength / temperature / CO₂) and measure its effect on photosynthesis rate.

✅ Apparatus & Materials

• Aquatic plant (Elodea / pondweed) — healthy cut, similar size
• Beaker or conical flask (100–250 ml)
• Gas syringe or inverted graduated tube for O₂ collection / stopwatch + bubble counting
• Light source (lamp) with adjustable distance or variable power
• Coloured filters (red, blue, green) or LED light sources
• Thermometer and water bath / ice packs (temperature control)
• Sodium hydrogencarbonate (NaHCO₃) solution as CO₂ source
• Ruler or light meter for intensity measurement
• Clamp stand, boss head, stopwatch, paper towels, goggles

⚖️ Variables & Controls

🔬 Method – General (Bubble-count / Gas syringe)

1. Preparation: Cut similar lengths of pondweed and remove air bubbles by placing underwater until steady bubbling begins.
2. Place a single piece of plant in a beaker containing water with NaHCO₃ (unless CO₂ is the variable). Allow 5–10 minutes to acclimatise.
3. Setup: Invert a graduated cylinder over water trough or connect the setup to a gas syringe to collect O₂.
4. Position the light source at a measured distance or set filters/LEDs for wavelength control.
5. Run: Start the stopwatch and record O₂ volume every minute for 5–10 min, or count bubbles per minute. Repeat for each variable level three times and calculate mean values.

Example Ranges:

• Light intensity → distances: 10, 20, 30, 40, 50 cm
• Wavelength → red, green, blue filters
• Temperature → 10°C to 30°C (use water bath)
• CO₂ → increasing NaHCO₃ concentrations (0.0%–0.08%)

📐 Calibration & Measurement Notes

• If bubble size varies → use gas syringe method.
• Light intensity ∝ 1 / distance² (inverse square law).
• Keep light intensity consistent during wavelength tests.

🔢 Calculations & Data Treatment

Rate = Volume of O₂ collected (ml) ÷ Time (min)

If using bubble count: 1 bubble = x ml (calibrated).
Example: 5 ml O₂ in 5 min → rate = 1.0 ml min⁻¹
20 bubbles in 2 min (1 bubble = 0.05 ml) → 1.0 ml total → rate = 0.5 ml min⁻¹

Use means and standard deviation for repeats, plot mean ± error bars.

📈 Graphs & Expected Patterns

• Light intensity: Rises linearly then plateaus (light saturation).
• Wavelength: Peaks in blue & red; low in green.
• Temperature: Bell-shaped curve with optimum (~20–30°C).
• CO₂ concentration: Increases then plateaus (no longer limiting).

Plot: Independent variable on x-axis, rate (ml min⁻¹) on y-axis.

🔍 Sources of Error & Reductions

• Variable bubble size → gas syringe preferred.
• Plant variation → uniform samples.
• Unequal filter intensity → measure light intensity.
• Temperature drift → use thermostatic bath.
• Human error in counting → use video/automated methods.

♻️ Evaluation & Improvements

• Use more replicates and calibrate bubble volume.
• Use oxygen probe for continuous data.
• Control CO₂ precisely using buffered solutions.
• Use thermostatic water bath; avoid lamp heat with angled setup.

⚠️ Safety

• Wear goggles when handling glass and hot water.
• Keep electrical items away from water.
• Use heatproof mats for hot setups.

🧾 Example Data Table

🧠 Mnemonics

• S.P.A.C.E. → Setup, Process, All variables fixed, Collect data, Evaluate
• I-SWOT → Intensity, Spectrum, Water/CO₂, Optimum temp, Time