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Light-Dependent Reactions (Z-Scheme)

Run the Light-Dependent Reactions MicroSim Fullscreen

About This MicroSim

This interactive step-through diagram traces the path of an electron through the light-dependent reactions of photosynthesis. The Z-scheme energy diagram (upper left) shows electron energy at each carrier molecule, while the thylakoid membrane schematic (lower left) shows the physical arrangement of PSII, cytochrome b6f, PSI, and ATP synthase. The info panel (right) explains each step with equations and running product tallies.

How to Use

  1. Click Next Step to advance through the 6 stages of the light reactions
  2. Click Previous to go back and review
  3. Click Play All to auto-advance at 2.5-second intervals
  4. Watch the orange highlighted path trace electron flow on the Z-scheme
  5. Track the product tally in the info panel (O2, H+ gradient, NADPH, ATP)

Key Concepts

  • PSII (P680) absorbs a photon, exciting an electron to high energy
  • Water splitting replaces the lost electron, releasing O2 and H+ into the lumen
  • Cytochrome b6f pumps H+ into the lumen, building the proton gradient
  • PSI (P700) absorbs a second photon, boosting the electron to very high energy
  • NADPH forms when the electron reduces NADP+ via ferredoxin
  • ATP synthase uses the H+ gradient (chemiosmosis) to produce ATP

Lesson Plan

Grade Level

9-12 (AP Biology)

Duration

10-15 minutes

Prerequisites

  • Chloroplast structure (thylakoid membranes, stroma, lumen)
  • Basic understanding of redox reactions and electron carriers
  • Concept of electrochemical gradients

Activities

  1. Step-through (5 min): Students advance through all 6 steps, reading each description and tracking where electrons, protons, and products go
  2. Prediction (5 min): Before clicking Next Step, students predict which carrier receives the electron next and whether energy goes up or down
  3. Assessment (5 min): Students answer: "Why does the Z-scheme have two upward jumps? What provides the energy for each?"

Assessment

  • Can students trace the complete electron path from H2O to NADPH?
  • Can students explain why two photosystems are needed?
  • Can students identify where the proton gradient is generated and how it drives ATP synthesis?

References

  1. Light-dependent reactions - Wikipedia
  2. Photosystem II - Wikipedia
  3. Photosystem I - Wikipedia
  4. Chemiosmosis - Wikipedia
  5. Z scheme - Wikipedia