AP BIOLOGY BLUEPRINT · UNIT 3

Follow the energy. Follow the electrons.

An interactive lab for the light-dependent reactions.

LIGHT REACTIONS LAB

Across the thylakoid membrane

ENLARGED SECTION
Light-dependent reactions across a thylakoid membraneStroma is above the green lipid bilayer; the thylakoid lumen is below. Electrons move from water through PSII, the electron transport chain and PSI to NADP⁺ reductase. Hydrogen ions move into the lumen and return through ATP synthase to drive ATP production in the stroma. STROMATHYLAKOID LUMEN THYLAKOID MEMBRANE PSIIETCPSINADP⁺reductase 2H₂O → O₂ + 4H⁺ + 4e⁻ETC moves H⁺stroma → lumenNADP⁺ + H⁺ + 2e⁻→ NADPHADP + Pᵢ → ATPATP synthaseH⁺ lumen → stroma H⁺ accumulation stores potential energyin the electrochemical gradient.
Photon energyElectron (e⁻)Hydrogen ion (H⁺)

Purple boxes sample equal volumes. More H⁺ markers mean a higher relative concentration; these are illustrative samples, not measured pH.

ATP production
0.0 units / model s
Waiting for a gradient
NADPH production
0.0 units / model s
Light-driven electron flow

Predict → change → explain

  1. Let NADP⁺ run out. Which production rate reaches zero first? Explain why ATP can still form.
  2. Switch off the light while a gradient is present. What happens to stored potential energy?
  3. Make the membrane leaky. Compare NADPH production with ATP production.
  4. Watch the ADP pool. Can ATP synthase keep making ATP after ADP is used up?
Show explanation

Linear electron flow requires NADP⁺ as its final electron acceptor. Once that pool is depleted, the existing H⁺ gradient can still drive ATP synthesis while ADP is available. ATP production stops when ADP runs out or the usable gradient dissipates. A leak allows H⁺ to return without driving ATP synthase.

What this model represents

Light reactions occur in the thylakoid membrane. ATP and NADPH are formed on the stromal side and can supply the Calvin cycle; oxygen is released from water splitting.

Model assumptions & scale

This qualitative model shows linear electron flow. NADP⁺ + NADPH = 40 model units, and ADP + ATP = 50 model units. Pool sizes are chosen for observation, not a biological concentration ratio. The graph reports each molecule as a percentage of its own fixed pool; read the unit values for relative amounts. Water and inorganic phosphate (Pᵢ) are assumed available. Products are not recycled; cyclic electron flow, Calvin-cycle consumption, ATP hydrolysis and ATP-synthase regulation are omitted.

Gradient energy is spent by ATP synthase or dissipated through a leak. The 0–100 scale is illustrative, not a measured voltage or pH. H⁺ sample markers, time, rates, distances and energy conversion factors are illustrative, not molecular stoichiometry. Water splitting and stromal H⁺ consumption during NADPH formation are summarized by the gradient model. The membrane diagram combines protein complexes into one teaching view; their precise distribution within thylakoids is not shown.

Biology reference: OpenStax Biology 2e · Light-dependent reactions.

LOCATE THE LIGHT REACTIONS

Inside the chloroplast

The lab shows an enlarged section of the thylakoid membrane inside a chloroplast.

Chloroplast cutaway and enlarged thylakoidA chloroplast has an outer and inner envelope surrounding stroma and stacks of thylakoids called grana. A highlighted thylakoid is enlarged on the right. Its membrane encloses the lumen, with stroma outside. The membrane hosts the light reactions. Chloroplast envelopeOuter + inner membranesGranumStack of thylakoidsStromaThylakoid membraneLight reactions happen hereThylakoid lumenH⁺ accumulates insideStroma outside Enlarged thylakoid section · not to scale

1 · ChloroplastThe organelle containing the thylakoids and surrounding stroma.

2 · Thylakoid membranePhotosystems, electron carriers and ATP synthase work across this membrane.

3 · Lumen → stromaH⁺ returns through ATP synthase; ATP is made on the stromal side.

The simulation pauses while this view is open.