Thursday, 11 September 2025

Photosynthesis — Measuring Oxygen Production with Pondweed (with Time-Lapse Video)

Because watching bubbles in real time is like watching paint photosynthesise. Let’s speed it up and make the data sing.

The Big Idea

Photosynthesis makes oxygen. Pondweed (Elodea/Egeria) obligingly releases visible oxygen bubbles under light. We’ll capture the reaction as a time-lapse and measure the rate of oxygen production. Students get both the wow (bubbling plant cinema) and the why (graphs, variables, evaluation).

Learning Objectives

Link photosynthesis to measurable oxygen output.
Investigate how light intensity (or colour, or CO₂ concentration) affects the rate.
Plan a fair test (controls, repeats, anomalies, evaluation).
Present data clearly (bubbles/min or dissolved O₂ mg/L vs time/distance).

Kit List (Classroom/Lab)

Fresh pondweed (Elodea/Egeria), ~10–15 cm sprigs.
250 mL beaker or boiling tube + clamp stand.
Sodium hydrogencarbonate (bicarbonate) ~0.2% to boost CO₂.
LED lamp (stable output), metre ruler (for distance).
Black card or foil to block ambient light; thermometer.
Optional precision: dissolved oxygen sensor (e.g., PASCO Wireless Optical DO), light sensor, SPARKvue/Capstone for logging.
Smartphone or mirrorless camera on tripod for time-lapse.
White tile or sheet behind the beaker for contrast.

Setup (5–10 min)

Condition the plant: snip the stem cleanly at an angle; ensure the cut end faces up to release bubbles.
Fill beaker with bicarbonate solution; submerge pondweed fully; secure upright with a paper clip/soft wire.
Position LED lamp at a starting distance (e.g., 10 cm). Shield stray light with black card.
Check water temp (aim ~20–25 °C). If you’re running long recordings, use an LED (cool) rather than a hot halogen.

Variables (Pick One to Investigate)

Light intensity: move lamp to 10, 20, 30, 40 cm (intensity ∝ 1/d²).
Light colour: red, blue, green filters.
CO₂ availability: 0%, 0.1%, 0.2%, 0.4% NaHCO₃.
Temperature (advanced): water bath at 15, 20, 25, 30 °C.

Control other factors: same sprig length, same total time per run, same lamp, same background light, same solution volume.

Two Ways to Measure the Rate

A) Bubble Count (simple, visual)

Start the lamp → wait 2 minutes (equilibration).
Count bubbles for 2 minutes.
Repeat 3× at each distance; average to get bubbles/min.
Use time-lapse playback to verify counts or to sample longer runs quickly.

B) Dissolved O₂ Logging (quantitative, best for graphs)

Place the DO probe in the beaker (avoid blocking bubbles).
Log O₂ (mg L⁻¹) every 5–10 s for 10 minutes.
Rate = slope of O₂ vs time (mg L⁻¹ min⁻¹).
Pair with a light sensor reading at each distance for an intensity vs rate graph.

Time-Lapse Capture (the fun bit)

Goal: compress a 10–30 minute experiment into 20–40 seconds of crisp, watchable video.

Framing: side-on macro framing of the cut stem; add a ruler or grid for scale.
Background: white card; avoid reflections.
Interval: 1 frame every 1–2 s (shorter interval for faster bubbling).
Lock exposure & focus (manual) to stop flicker/breathing.
Playback speed: 20–30× looks great.
Slate each condition (e.g., “20 cm”, “30 cm”) with a quick title card so edits aren’t confusing.
For phones, use a time-lapse app; for cameras, interval timer.
Bonus: record a wide “beauty” shot of the whole rig for cutaways, and a screen capture of the live sensor graph for picture-in-picture.

Suggested Student Workflow (40–60 min lesson + homework)

Predict: “What happens to rate as light intensity increases?”
Set up, equilibrate, and record Run 1 (10 cm).
Repeat at 20, 30, 40 cm.
While the camera runs, log DO or do a manual 2-min bubble count.
Export the time-lapse and overlay labels (distance, time scale).
Plot data; fit a curve; discuss limiting factors (CO₂, temperature, chlorophyll saturation).
Evaluate: sources of error (bubble size variation, stray light, temperature drift); propose improvements.

Example Data Table (bubble count)

Distance (cm)	Bubbles/min (Trial 1)	Trial 2	Trial 3	Mean
10	96	102	98	99
20	54	57	52	54
30	31	30	28	30
40	18	20	17	18

Analysis prompts: Does mean rate scale with 1/d²? Where does it deviate (e.g., at close range from heat/CO₂ limits)?

Safety & Practical Notes

LED lamps preferred (low heat). Keep liquids away from mains.
Handle glassware carefully; dry bench and cables.
Dispose of pondweed responsibly; avoid releasing non-native species into waterways.

Extensions

Red vs Blue vs Green: link to absorption spectra of chlorophyll a/b.
Limiting factors: fix light and vary bicarbonate concentration.
Stoichiometry: estimate O₂ volume from bubble counts (assume bubble diameter) and compare to DO data.
AI/Video: students annotate their time-lapse with captions explaining what limits the rate at each stage.

Quick Teacher Script (for the voice-over)

“At 10 cm the light intensity is highest, so oxygen bubbles stream out. As we move the lamp further away, intensity drops roughly with the inverse square of distance, and so does the rate—until other limits kick in, like CO₂ or temperature. Our sensor trace confirms the trend with a steeper oxygen slope at 10 cm than at 30 or 40 cm.”

Handy Checklists

Before class

✅ Fresh pondweed snipped & upright
✅ 0.2% NaHCO₃ ready
✅ Lamp, shields, ruler positioned
✅ Camera on tripod, interval set, exposure/focus locked
✅ DO/light sensors connected (if using), logging template open

After class

✅ Export time-lapse (20–30×)
✅ Label each segment (distance/condition)
✅ Students upload graphs + 100-word evaluation to LMS

Thursday, 11 September 2025