Open with Van Helmont's willow experiment (1640s): he planted a 2.27 kg willow sapling in 90.7 kg of dried soil, watered it for 5 years, and it grew to 76.7 kg — but the soil lost only ~57 g. Ask: 'Where did the other ~74 kg of tree come from?' Take 2-3 student guesses (expect: soil, water, sunlight). Reveal: nearly all of the dry mass is carbon pulled out of the AIR as CO₂ and fixed into sugar by photosynthesis. This directly targets the misconception that plants eat soil, and frames today's question: how does a plant turn thin air and light into a solid tree? Targets SP1 and SP6 (claim–evidence reasoning).

1. 8m

   The Big Picture: Equation, Chloroplast, Two Stages

   Content

   Photosynthesis captures light energy and stores it as chemical energy in glucose. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with light energy). Critically, isotope-labeling with ¹⁸O shows the O₂ released comes from splitting H₂O, not from CO₂ — so on the reactant side we should really write 12H₂O consumed and 6H₂O regenerated, but the net equation is what students memorize. All of this happens inside the chloroplast, which has two functional compartments: the thylakoid membranes (flattened discs stacked into grana) house the light-dependent reactions, and the stroma (fluid around the thylakoids) houses the Calvin cycle. Stage 1 (light reactions, in the thylakoid) converts light energy into ATP and NADPH and releases O₂. Stage 2 (Calvin cycle, in the stroma) uses that ATP and NADPH to fix CO₂ into G3P and ultimately glucose. The two stages are physically separated but chemically coupled — the light reactions supply ATP and NADPH to the Calvin cycle right next door.

   Delivery

   Anchor the whole lesson on this equation and this compartment map — you'll return to it repeatedly. Emphasize the isotope evidence for where O₂ comes from: it heads off the misconception that O₂ comes from CO₂. Point out that 'light-independent' does NOT mean 'happens in the dark' — the Calvin cycle stops in the dark because ATP and NADPH run out. Quick check: ask 'Which stage produces O₂?' (light reactions) and 'Which stage fixes carbon?' (Calvin cycle). Targets SP1 and SP2.

2. 10m

   Light-Dependent Reactions: Electrons, Protons, ATP, NADPH

   Content

   The light reactions live in the thylakoid membrane and do four jobs: split water, move electrons, pump protons, and reduce NADP⁺. Trace one electron: (1) A photon strikes chlorophyll in photosystem II (PSII); the excited electron leaves. (2) To replace it, PSII splits water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The O₂ diffuses out — this is the source of atmospheric oxygen. (3) The excited electron travels down the electron transport chain (plastoquinone → cytochrome complex → plastocyanin). As it drops in energy, the cytochrome complex uses that energy to pump H⁺ from the stroma INTO the thylakoid lumen. (4) The electron reaches photosystem I (PSI), where another photon re-excites it. (5) The electron is passed via ferredoxin to NADP⁺ reductase, which reduces NADP⁺ + H⁺ → NADPH. Meanwhile, the H⁺ buildup in the lumen (from water splitting AND from ETC pumping) creates a steep proton gradient. In chemiosmosis, H⁺ flows back through ATP synthase into the stroma, and ATP synthase phosphorylates ADP → ATP. Bottom line: light energy in → ATP + NADPH + O₂ out.

   Delivery

   Walk the diagram step-by-step in the order an electron actually travels: water → PSII → ETC → PSI → NADP⁺. Emphasize that PHOTONS energize electrons TWICE (once at PSII, once at PSI) — this is why we need two photosystems. Stress that H⁺ ends up concentrated INSIDE the lumen, and ATP synthase sits in the thylakoid membrane like a turbine. Pre-empt the common error that ATP synthase 'makes' the gradient — it uses it. Cold-call: 'Where do the electrons that reduce NADP⁺ ultimately come from?' (water). 'What is the byproduct?' (O₂). Targets SP1 and SP2.

3. 7m

   The Calvin Cycle: Fixing CO₂ into Sugar

   Content

   The Calvin cycle runs in the stroma and turns inorganic CO₂ into organic G3P using the ATP and NADPH from the light reactions. It has three phases. (1) Carbon fixation: the enzyme RuBisCO attaches one CO₂ to a 5-carbon sugar called RuBP, producing an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (3-carbon). (2) Reduction: each 3-PGA is phosphorylated by ATP and then reduced by NADPH to form G3P (glyceraldehyde-3-phosphate). This is where the electrons/H from NADPH actually get incorporated into sugar. (3) Regeneration: for every three CO₂ fixed, six G3P are made, but only ONE G3P leaves as product — the other five are rearranged, using more ATP, to regenerate three RuBP so the cycle can keep running. To make one glucose (C₆H₁₂O₆), the cycle must turn six times, consuming 6 CO₂, 18 ATP, and 12 NADPH. Because the cycle needs ATP and NADPH constantly, it effectively halts in the dark — it is 'light-independent' but not 'dark-requiring.'

   Delivery

   Emphasize that the Calvin cycle is where the plant's MASS actually comes from — this closes the loop back to the Van Helmont hook. Nail the misconception: 'light-independent' ≠ 'dark reactions.' Ask 'Why does the Calvin cycle stop at night?' (ATP and NADPH aren't being made). Emphasize the stoichiometry: 3 CO₂ in → 1 G3P net out; 2 G3P → 1 glucose. Targets SP1.

1. 25m

   Floating Leaf-Disk Assay + Data Analysis (Investigation 5, condensed)Lab

   Set up two lab stations per group of 3-4. Pre-prepare the bicarbonate solution before class. Each group runs TWO conditions in parallel: (A) 0.2% NaHCO₃ under bright light, (B) 0.2% NaHCO₃ shaded with foil (low light). They record how many disks float at each 1-minute mark for 10 minutes, then compute ET₅₀ (time for 50% of disks to float) and graph rate (disks floated per minute) vs. condition. Targets SP3 (methods), SP4 (data/graphs), SP5 (compute rate), SP6 (argue from evidence). Student handout: Part 1 — The Assay (how it works) Leaf disks contain air in their spongy mesophyll, so they float. Pull the air out with a syringe vacuum and replace it with bicarbonate solution — now they sink. When you turn on the light, photosynthesis produces O₂ inside the disks; as O₂ accumulates, the disks become buoyant again and rise. Rate of floating = rate of photosynthesis. Part 2 — Procedure 1. Punch 20 disks from a spinach leaf (avoid big veins). Divide into two groups of 10. 2. Load 10 disks into a syringe. Draw ~5 mL of 0.2% NaHCO₃ (with soap). Hold finger over tip, pull plunger back to create vacuum, hold 10 s, release. Repeat 2-3× until ALL disks sink. 3. Pour disks + solution into a labeled cup. Repeat for the second batch in a second cup. 4. Place cup A under the lamp (bright). Cover cup B with foil (dark control). 5. Start timer. At each minute (1-10), record the number of disks floating in each cup. Part 3 — Data table (fill in as you go) - Time (min) — Bright light (# floating) — Dark (# floating) - 0 — ______ — ______ - 1 — ______ — ______ - 2 — ______ — ______ - 3 — ______ — ______ - 4 — ______ — ______ - 5 — ______ — ______ - 6 — ______ — ______ - 7 — ______ — ______ - 8 — ______ — ______ - 9 — ______ — ______ - 10 — ______ — ______ Part 4 — Analysis 1. ET₅₀ (bright) = ______ min. ET₅₀ (dark) = ______ min. (ET₅₀ = time for 5 of 10 disks to float.) 2. Rate = 1/ET₅₀ (disks/min). Compute both rates. 3. On graph paper, plot 'number of disks floating' vs. 'time' for BOTH conditions on the same axes. Label axes with units. 4. Claim–Evidence–Reasoning: Do your data support the claim that light drives photosynthesis? Cite your ET₅₀ values as evidence and explain the reasoning using PSII, ATP, NADPH, and the Calvin cycle. 5. Predict: if we bubbled N₂ gas through the bicarbonate solution to strip out CO₂, what would happen to ET₅₀ in the bright cup? Explain why. Walk around during minutes 4-8 and prompt slow groups to also start graphing while data comes in. Expect the bright cup to reach ET₅₀ around 4-8 min; the dark cup should have 0 or very few floating.

   Materials

   • Fresh spinach or ivy leaves
   • Hole punch (~1 cm diameter)
   • 10 mL plastic syringes (no needle) — one per group
   • 0.2% sodium bicarbonate solution (NaHCO₃) with 1 drop dish soap per 300 mL
   • Plain water with 1 drop dish soap (control)
   • Clear plastic cups
   • Bright LED desk lamp OR window light
   • Stopwatch or phone timer
   • Ruler and graph paper (or Google Sheets)
   • Aluminum foil (to shade one cup)

   Example outputs

   • Bright cup: disks floating at t = 0,1,2,3,4,5,6,7,8,9,10 min = 0,0,1,2,4,6,8,9,10,10,10 → ET₅₀ ≈ 4.5 min, rate ≈ 0.22 disks/min. Dark cup: 0,0,0,0,0,0,0,0,0,0,0 → no measurable rate.
   • CER answer: 'Data support the claim. In light, ET₅₀ was 4.5 min because PSII split water and drove the ETC, producing ATP and NADPH that let the Calvin cycle fix CO₂ (from bicarbonate) into sugar and release O₂, which made the disks buoyant. In the dark, no photons excited PSII, so no ATP/NADPH were made, the Calvin cycle stalled, no net O₂ accumulated, and no disks floated.'
   • Prediction: stripping CO₂ would raise ET₅₀ dramatically (approaching the dark control) because without CO₂, RuBisCO has no substrate — the Calvin cycle stops and net O₂ production collapses even under bright light.

   No-equipment fallback

   Provide pre-generated data tables from three light intensities (low/medium/high, e.g. ET₅₀ = 18 min / 8 min / 5 min) and have students graph rate vs. light intensity, identify the plateau, and argue what limits the rate at saturation (likely CO₂ availability or Calvin cycle enzyme rate).

1. A researcher grows algae in water containing H₂¹⁸O (heavy-oxygen water) and normal C¹⁶O₂. After illumination, where will the ¹⁸O label first appear, and what does this show about the mechanism of photosynthesis? (SP1, SP6)

   short answerThe ¹⁸O label appears in the O₂ gas released, not in the glucose or in water on the product side. This shows that the oxygen released during photosynthesis comes from splitting water at PSII (2H₂O → 4H⁺ + 4e⁻ + O₂), NOT from CO₂. It refutes the misconception that O₂ comes from CO₂.

2. The floating leaf-disk assay is run at five light intensities. Rates (disks/min) are: 25 µmol photons/m²/s → 0.05; 100 → 0.15; 250 → 0.28; 500 → 0.34; 1000 → 0.35. Describe the pattern and identify what is most likely limiting the rate above ~500 µmol/m²/s. (SP4, SP1)

   short answerRate rises steeply from 25 to 250 µmol/m²/s, then levels off (plateaus) between 500 and 1000 µmol/m²/s at ~0.35 disks/min. Below 250, light is the limiting factor (linear rise). Above ~500, light is saturating — additional photons don't help because another factor is limiting: most likely CO₂ (bicarbonate) availability or the maximum turnover rate of the Calvin cycle enzymes (especially RuBisCO).

3. Which statement about the Calvin cycle is correct?

   multiple choiceC. It occurs in the stroma and depends on ATP and NADPH from the light reactions, so it stops in the dark even though it does not use light directly. A. It occurs in the thylakoid lumen and directly requires photons. (WRONG — that's the light reactions.) B. It runs only at night because it is the 'dark reaction.' (WRONG — misconception; it runs whenever ATP/NADPH are available, typically in daylight.) C. It occurs in the stroma and depends on ATP and NADPH from the light reactions, so it stops in the dark even though it does not use light directly. (CORRECT) D. It produces O₂ as a byproduct of fixing CO₂. (WRONG — O₂ comes from H₂O in the light reactions.)

4. Trace an electron from H₂O to NADPH. Name each complex it passes through and explain where the proton gradient used for ATP synthesis is generated. (SP2, SP1)

   short answerH₂O → PSII (water split, e⁻ enters PSII, O₂ released) → plastoquinone → cytochrome b₆f complex (pumps H⁺ from stroma into thylakoid lumen) → plastocyanin → PSI (photon re-excites e⁻) → ferredoxin → NADP⁺ reductase → NADPH. The proton gradient is generated by (1) H⁺ released into the lumen when water is split at PSII and (2) H⁺ pumped into the lumen by the cytochrome b₆f complex. H⁺ then flows from lumen back to stroma through ATP synthase, powering ADP + Pᵢ → ATP (chemiosmosis).

• ☐Fresh spinach or ivy leaves (1-2 per group)
• ☐1 cm hole punch
• ☐10 mL plastic syringes (no needle), 1-2 per group
• ☐0.2% sodium bicarbonate solution with dish soap (prepare before class: 2 g NaHCO₃ per 1 L water + 1 drop soap per 300 mL)
• ☐Clear plastic cups (2 per group)
• ☐Bright LED desk lamps (1 per group) or window light
• ☐Aluminum foil
• ☐Stopwatches or phone timers
• ☐Graph paper or Chromebooks with Google Sheets
• ☐Student handout (data table + CER prompts, printed)
• ☐Projector for slide deck (light-reaction and Calvin-cycle visuals)