Cell BiologyVirtual lab18 minLesson 7 of 7

Photosynthesis

Every plant, algae, and cyanobacterium runs on the same two-stage engine. Start in the lab: change the light, CO₂, and temperature and watch leaf disks float as they fill with oxygen. Then look inside the chloroplast to see why your results turned out the way they did.

Virtual lab

The leaf disk assay

Small disks punched from a spinach leaf are soaked in bicarbonate solution (a CO₂ source) until they sink. Under a lamp, photosynthesis makes O₂, which collects inside the leaf tissue and floats the disks back up. The faster photosynthesis runs, the sooner they rise. Set up a trial, run it, and compare.

Ready0 / 10 disks floating

Light intensity70%

CO₂ (bicarbonate)60%

Temperature25 °C

Light color

Photosynthesis rate73%

Limiting factor: Light

Floating disks over time

Trial log

#	Light	CO₂	Temp	Time to 5 floating
No trials yet

Shorter time = faster photosynthesis. The graph keeps your last 4 trials.

Challenges · 0 / 4

Not yet done: Run a trial with bright light (80%+) but very little CO₂ (20% or less).
Not yet done: Run a trial under green light.
Not yet done: Run a trial at 44 °C or hotter.
Not yet done: Run a trial with the lamp switched off.

How it works

Inside the chloroplast

Drag left/right, use arrow keys, or click a tab to explore

Click a stage tab or drag the diagram to explore

Chloroplast

The chloroplast: two stages, one goal

Thylakoid membrane (left) runs the light reactions. The stroma (right) runs the Calvin cycle. Both sides always visible — because the connection between them is the whole point.

Inputs

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Outputs

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Predict before you continue

Plants release O₂. Which molecule do you think that oxygen comes from?

Three things to keep straight

Light reactions make the fuel
The thylakoid membrane splits water, ferries electrons through PS II and PS I, and uses the resulting H⁺ gradient to produce ATP and NADPH. These are the energy carriers — not glucose. Glucose comes later.
Calvin cycle spends the fuel
RuBisCO in the stroma fixes CO₂ into 3-PGA, which ATP and NADPH reduce to G3P. Three turns of the cycle fix 3 CO₂ and yield 1 net G3P. Six turns yield 1 glucose. The cycle doesn't use light directly — it uses the ATP and NADPH made by the light reactions.
Location is everything
Light reactions happen on the thylakoid membrane. The Calvin cycle runs in the stroma surrounding it. This spatial separation matters — NADPH and ATP must physically move from one compartment to the other. That handoff is the link between the two stages.

At a glance

Stage	Location	Inputs	Outputs
Light reactions	Thylakoid membrane	H₂O, light	O₂, ATP, NADPH
Calvin cycle	Stroma	CO₂, ATP, NADPH	G3P (→ glucose)
Net equation	Chloroplast	6CO₂ + 6H₂O + light	C₆H₁₂O₆ + 6O₂

Exam trap

Does the Calvin cycle need light?

Technically no — the enzymes don't use photons directly. But it stops in the dark because it depends on a constant supply of ATP and NADPH from the light reactions. This is a common AP exam trap: "light-independent" does not mean "dark-adapted." It means no photon is absorbed in that stage's chemistry.

Quick recap

1
PS II absorbs light — P680 absorbs a photon. The energy splits H₂O → 2H⁺ + ½O₂ + 2e⁻. O₂ is released. Note: PS II acts first despite its name.
2
Electron transport chain — Electrons flow PS II → plastoquinone → cytochrome b6f → plastocyanin. H⁺ pumped into lumen, building the gradient.
3
PS I + NADPH — Electrons re-energized at P700. Passed to ferredoxin → NADP⁺ reductase → NADPH formed in the stroma.
4
Chemiosmosis → ATP — H⁺ flows back through ATP synthase, spinning the rotor. Same mechanism as the mitochondrial ETC. ~3 ATP per 2e⁻.
5
Carbon fixation — RuBisCO fixes CO₂ + RuBP (5C) → 2× 3-PGA (3C). Happens once per CO₂. 3 turns = 3 CO₂ fixed.
6
Reduction — 3-PGA + ATP + NADPH → G3P. This step consumes the light-reaction outputs — the coupling moment.
7
Regeneration — G3P → RuBP (uses ATP). 5 out of 6 G3P molecules are recycled. 1 net G3P exits per 3 turns → 1 glucose per 6 turns.