Experiment 13: Photosynthesis — Floating Leaf Discs

Photosynthesis hides its product in plain sight: oxygen. Sink a leaf disc in bicarbonate solution and it stays sunk — until photosynthesis fills its air spaces with oxygen and it floats. The time a disc takes to rise is a speedometer for photosynthesis itself. In this investigation you compare floating times in bright light, dim light and darkness, and the Virtual Lab: Photosynthesis — Light & Oxygen Bubbles on this page shows the companion view: oxygen bubbles streaming from pondweed as you move the lamp.

The science behind the practical

Photosynthesis: carbon dioxide + water → glucose + oxygen, powered by light energy captured by chlorophyll. Leaf discs are first sunk by vacuum infiltration — a syringe pulls the air out of the leaf's spongy mesophyll and replaces it with bicarbonate solution, which also supplies CO₂. Returned to the light, each disc photosynthesises; oxygen accumulates in the air spaces, buoyancy returns, and the disc floats. The useful measure is ET50: the time for 50% of the discs to float. A *shorter* ET50 means a *faster* rate — an inverse measure, so read graphs carefully.

Rate depends on the factor in shortest supply (limiting factor): at low light, light limits; raise the light and eventually CO₂ or temperature takes over. In darkness there is no photosynthesis at all — discs respire instead, and stay sunk.

Variables, controls and fair testing

Running the investigation well

Punch discs away from thick veins, which infiltrate unevenly. Sink them properly: if a disc refuses to sink after several syringe pulls, discard and replace it — a half-infiltrated disc floats early and fakes speed. Start the stopwatch the moment the cups go under their light conditions, and record every 1–2 minutes without jiggling the cups. In the pondweed simulator, move the lamp in steps and watch the bubble rate respond — notice it rises steeply at first, then levels off as another factor begins to limit.

Worked example: a sample investigation

Discs floating (out of 10) in each condition:

Analysing the sample data

Step 1 — find ET50. 50% of 10 discs = 5 floating. Bright light reaches 5 floating at 8 minutes. Dim light reaches 5 at 16 minutes. Dark never reaches it in 20 minutes — ET50 is beyond the run (effectively infinite).

Step 2 — compare rates. Rate is proportional to 1 ÷ ET50. Bright: 1 ÷ 8 = 0.125 per minute. Dim: 1 ÷ 16 = 0.0625 per minute. The bright discs photosynthesised at twice the rate of the dim ones in this range — doubling the light (roughly) doubled the rate, the signature of light being the limiting factor.

Step 3 — describe the graph. Plot discs floating against time: bright is a steep S rising to a plateau at 10 by 14 minutes; dim is the same S stretched to the right (ET50 doubled); dark is a flat line along the axis. The plateau is not "photosynthesis stopped" — it is "all discs up": the measurement maxed out, which is why ET50, read mid-rise, is the robust measure.

Conclusion and evaluation

Conclusion: photosynthesis rate increased with light intensity — ET50 fell from 16 minutes (dim) to 8 minutes (bright), a twofold rate increase — and no photosynthesis was detected in darkness. Light is a limiting factor across this range.

Evaluation: "bright" and "dim" are descriptions, not measurements — a light meter (or fixed lamp distances) would quantify the independent variable. Temperature near the lamp could differ between cups. Improvements: lamp at measured distances (e.g. 10, 20, 30 cm — intensity follows an inverse-square pattern), a heat-shield or LED lamp, and repeats to average ET50.

Safety and ethics

Only baking soda and leaves are involved; wash hands after handling leaf material and take care with lamps and water near electrics. Spinach leaves raise no ethical issues; return pondweed used in the simulator's bench cousin to its tank.

Common mistakes

• Using discs with veins, which infiltrate unevenly and float unpredictably.
• Treating ET50 backwards — a shorter time means a *faster* rate.
• Shaking the cups while counting, knocking borderline discs up or down.
• Claiming the plateau means photosynthesis stopped, when it means the discs have all floated.

Exam Experience

Question 1 — Analysis (4 marks). A repeat gives ET50 values of 5 minutes at 10 cm lamp distance and 12 minutes at 30 cm. Calculate the relative rates (1 ÷ ET50) and state how many times faster the closer lamp's rate is. *Scoring guide:* Rates 1 ÷ 5 = 0.20 and 1 ÷ 12 = 0.083 per minute (1 mark). Ratio 0.20 ÷ 0.083 = 2.4 times faster (1 mark method, 1 mark answer). Final mark for noting light was limiting across this range, since rate still rose with intensity.

Question 2 — Evaluation and design (4 marks). Design a test of whether CO₂ concentration limits the rate at bright light, using the disc method. State your prediction and the result that would support it. *Scoring guide:* Design (up to 2 marks): same bright light, several bicarbonate concentrations, all else controlled, ET50 measured. Prediction/support (up to 2 marks): ET50 shortens as concentration rises then levels off; the levelling shows CO₂ stops limiting and another factor (light/temperature) takes over.

Multiple choice 1. Discs in darkness do not float because: A) they are respiring, using oxygen rather than producing it B) the water is too cold C) bicarbonate stops working in the dark D) leaves cannot absorb light at night. *Key: A.* Without light there is no photosynthesis; respiration consumes oxygen. The bicarbonate (C) supplies CO₂ regardless of light.

Multiple choice 2. ET50 is used because it: A) is the time for all discs to float B) gives a mid-rise time that is inversely proportional to the rate and less affected by one odd disc C) avoids using a stopwatch D) measures glucose directly. *Key: B.* The 50% point sits on the steep, most reliable part of the curve; A describes ET100, and the method plainly uses a stopwatch (C) and measures oxygen buoyancy, not glucose (D).

A second look: counting bubbles — a cruder ruler

The pondweed in the simulator invites a simpler measurement than ET50: bubbles per minute. A class running it at three lamp distances counted 42 bubbles/min at 10 cm, 18 at 20 cm and 6 at 40 cm. Compare the shape with the disc data: halving the distance roughly doubled-to-tripled the rate each time, but not by the clean factor of 4 the inverse-square law for light intensity would predict — because room light adds a background level at every distance, and because bubbles are not equal volumes: a stream of small bubbles and a stream of large ones can carry very different amounts of oxygen at the same count. This is why the leaf-disc method, for all its syringe fuss, is the better instrument: ET50 times a physical event (floating) rather than tallying events of variable size. Two methods measuring the same process, one visibly cruder — whenever a practical offers you a choice of measurements, asking "which proxy lies least?" is the mark-earning question.

Final check: the colour of light, and a limiting factor caught switching

Chlorophyll absorbs red and blue light strongly and reflects green — which is why leaves look green, and why green light should be nearly useless for photosynthesis. The discs confirm it: under red cellophane at the standard lamp distance, ET50 was 9 minutes; under blue, 10 minutes; under green, 25 minutes. Rates: red 1 ÷ 9 = 0.111, blue 0.100, green 1 ÷ 25 = 0.040 per minute. Green light sustained only 0.040 ÷ 0.111 ≈ 36% of the red rate — the absorption spectrum of a pigment, measured with floating leaves and a stopwatch.

Then the elegant sequel. At bright light, doubling the bicarbonate concentration (more CO₂) cut ET50 from 8 to 6 minutes: rate from 0.125 to 0.167 per minute, a rise of about 33%. Under dim light the same doubling changed nothing measurable. That asymmetry is the limiting-factor concept caught in the act: in bright light the discs had light to spare and were queuing for CO₂, so feeding CO₂ helped; in dim light they were queuing for photons, and extra CO₂ queued uselessly behind them. Whenever a factor "stops working", another has taken its place as the limit — the single most examinable idea in this whole investigation, and now you have watched it switch.

Final check: was it the light, or the lamp's warmth?

A sceptical examiner's question hangs over the bright-light result: the lamp that lit the discs also warmed them, and photosynthesis is enzyme-driven, so warmth alone speeds it. The worked example can be defended — but only because the defence was testable. Rerun the bright condition with an LED lamp (bright but cool) at the same distance and the ET50 lengthens slightly, from 8 to 9 minutes: rate 0.111 against the hot lamp's 0.125 per minute. The lamp's heat was contributing roughly (0.125 − 0.111) ÷ 0.125 ≈ 11% of the apparent light effect. Ninety per cent of the conclusion survives; eleven per cent is reassigned to a confound, measured rather than argued about. That is the complete pattern of good evaluation in one paragraph: suspect a variable, design the comparison that isolates it, put a number on its influence, and adjust the claim by exactly that much — no more, no less. The dim and dark conditions, far from the lamp's heat and light alike, were never under suspicion, which is why the dark result (no floating at all) remains the investigation's most bulletproof line.