DarkBright
Energy arriving at the same model leaf.
LIGHT → CARRIERS → CARBON
Photosynthesis, connected.
How do light and CO₂ supply work together to control sugar production?
ClosedOpen wide
Manually control the opening for CO₂ entry.
16 model chloroplasts
Changing the number starts a fresh trial.
Fixed leaf area · same incoming light & CO₂PREDICT · TEST · EXPLAINCan more light overcome a closed stoma? Test one control at a time.
THE MODEL LEAF
Let the carbon in.
Internal CO₂14.0relative units
New CO₂ entry0.68units / model s
CO₂ moves down its concentration gradient through the pore.
INSIDE ONE REPRESENTATIVE CHLOROPLAST
Two stages. A shared supply.
Swipe across to follow the molecules →
ATP / ADP pool35% ATP
18.9 ATP · 35.1 ADPNADPH / NADP⁺ pool35% NADPH
12.6 NADPH · 23.4 NADP⁺Light reactions recharge the carriers. The Calvin cycle uses them and returns ADP + phosphate and NADP⁺.
Pi = inorganic phosphate. One circuit represents three turns. Moving symbols represent flow packets; pool amounts and rates follow the model.
THE OUTPUT
What limits sugar production?
Current G3P rate0.00relative units / model s
Net G3P produced0.03-carbon products
6-carbon sugar equivalents0.02 net G3P supply 6 carbons
Your model leafStable referenceCondition change
Instantaneous model rate. Reference is a fixed steady rate: 65% light, 75% opening and 3 chloroplasts. This is an illustrative model, not measured plant data.
Ready. Make a prediction, then run the model.
COMPARE THE TWO STAGES
What goes in—and comes out?
Light reactions
Thylakoid membranes
INPUTS
- Light energy
- Water · H₂O
- ADP + phosphate
- NADP⁺
OUTPUTS
- Oxygen · O₂
- ATP
- NADPH
Calvin cycle
Stroma
INPUTS
- Carbon dioxide · CO₂
- ATP
- NADPH
OUTPUTS
- Net G3P · 3 carbons
- ADP + phosphate
- NADP⁺
Carbon comes from CO₂.
Three turns yield one net G3P. Two net G3P can supply the carbon for a six-carbon sugar through later reactions.
“Light-independent” means the Calvin cycle does not use photons directly. Continued production still depends on ATP and NADPH supplied by the light reactions.
THREE INVESTIGATIONS
Switch the light off.
Does production stop immediately? Use the carrier pools to explain the delay.
Close the stomata.
Keep the light bright. Follow internal CO₂ and explain the change in G3P rate.
Add chloroplasts.
Compare fresh trials with the same light and opening. Does output increase in direct proportion?
Teacher notes · CED alignment, controls and model limits
CED Topic 3.4
Aligned with the supplied 2025 AP Biology Course and Exam Description, printed pp. 73–75: light reactions capture energy in ATP and NADPH (3.4.A.3); thylakoid/stroma compartmentalization (3.4.A.2); electron transport, proton gradients and ATP synthase (3.4.B.1–5); carbohydrate production from CO₂ powered by ATP/NADPH (3.4.B.6). The student investigates changes and supports explanations with graph evidence (practice 6.B). Detailed enzyme names, intermediates and Calvin-cycle step memorization are outside the exam scope.
Controlled conditions
One schematic C3 leaf has fixed area, external CO₂, temperature and ample water. Stomatal aperture is imposed manually; natural stomatal responses to light, water status and internal CO₂ are not simulated. Changing chloroplast number starts a fresh model leaf, with carrier pools proportional to that number. These are a small representative set of chloroplasts, not a realistic count for a whole leaf. The enlarged organelle represents their shared processes; readouts are totals for the model leaf.
Carbon and energy accounting
Each net G3P consumes 3 CO₂, 9 ATP and 6 NADPH, representing three turns. ATP + ADP and NADPH + NADP⁺ pools remain conserved. G3P stays in a cumulative exported-product ledger; the “6-carbon sugar equivalents” counter is half that G3P total, not a second product or direct glucose output. Oxygen derives from water oxidation: two model NADPH regenerated correspond to one O₂ and two H₂O used. CO₂ already inside can support fixation after closure. Stored carriers can support brief fixation in darkness.
Illustrative kinetic model
Deterministic saturating rates link light, finite carrier pools and available internal CO₂. A fixed photon budget is shared across the leaf; extra chloroplasts intercept a diminishing fraction of it. Recharging slows as ADP or NADP⁺ availability falls. ATP production pools linear and ATP-balancing photophosphorylation; a measured ATP-to-photon yield is not asserted. Phosphate exchange during G3P export is pooled into the return ADP/phosphate supply; explicit proton, phosphate and water balances beyond water oxidation are not resolved. The drawn ETC/ATP synthase is contextual; the H⁺ gradient is not independently adjustable in this lab.
Graph and molecular markers
The rate is instantaneous G3P export from the coupled model. Total is its time integral. The blue reference is the same model at 65% light, 75% aperture and 3 chloroplasts after 900 model seconds, then held constant. It does not change when the experimental sliders change. The reference total is that steady rate multiplied by time. Gold starts with 35% charged pools and internal CO₂ of 14 relative units; its start-up transient is expected. Moving symbols are representative flow packets: they advance monotonically from source to destination, disappear into the indicated pool and receive new IDs when new packets form. Counters are generated by model fluxes, not by symbol counts.
Scope of the leaf model
No respiration, photorespiration, photodamage, water loss, drought signaling or carbohydrate use is included. Complete pore closure gives zero new atmospheric CO₂ entry in this model; real leaves may retain other small sources or routes. Light supply and Calvin capacity are linked without portraying unlimited accumulation of ATP or NADPH. Gas movement and molecular drawings are qualitative, not to scale. Sources: supplied AP Biology CED; Lawson et al. (2008), Journal of Experimental Botany 59:3609–3619, for stomatal gas-exchange context. Reaction parameters are teaching choices, not fitted measurements from that paper.