Photosynthesis.

Stroma

The fluid inside the chloroplast envelope and outside the thylakoids. This is where the Calvin cycle runs.

Thylakoid membrane

Holds chlorophyll and the photosystems, electron carriers, and ATP synthase. It separates the stroma from the lumen.

Lumen and grana

The lumen is the space inside a thylakoid. A granum is a stack of thylakoids; grana is plural.

Stomata and guard cells

Stomata are pores in the leaf epidermis; two guard cells control each pore. When guard cells take up water and become turgid, the pore generally opens. Open pores admit CO₂ and let water vapor escape. Closing them saves water but restricts CO₂ entry.

Why it matters: A separate lumen lets the chloroplast keep a high concentration of H⁺ on one side of the membrane. No compartment, no useful gradient.

Chlorophyll absorbs light energy and excites electrons in the photosystems. It absorbs strongly in the red and blue parts of the spectrum. An action spectrum shows how effectively different wavelengths drive photosynthesis, which is a different measurement from how much light a pigment absorbs.

Linear: H₂O → PSII → electron transport chain → PSI → NADP⁺ → NADPH

1. PSII goes first. Light excites electrons. Splitting water replaces those electrons and releases O₂ and H⁺ into the lumen (the O₂ can then diffuse away).
2. Electron transport builds the gradient. Energy from electron transfer helps move H⁺ from the stroma into the lumen. The lumen becomes more acidic than the stroma.
3. PSI reenergizes the electrons. After another boost from light, the electrons ultimately reduce NADP⁺ to NADPH on the stromal side.
4. ATP synthase uses the gradient. H⁺ flows from lumen to stroma through ATP synthase, which makes ATP from ADP and phosphate. This is chemiosmosis; light-driven ATP production is photophosphorylation.

Cyclic: PSI → part of the electron transport chain → back to PSI

Cyclic flow helps make extra ATP. The returning electrons do not reduce NADP⁺, and this pathway does not use PSII to split water. Linear: ATP, NADPH, O₂. Cyclic: ATP support, no net NADPH or O₂ from that route. Both routes can operate in the same chloroplast.

Predict a disruption: A leaky thylakoid membrane can reduce ATP production even if electrons still move and O₂ is still released initially. A blocked PSII cuts electron supply to linear flow, so expect less O₂ and less NADPH from that route.

The Calvin cycle runs in the stroma. It does not directly use photons, but it depends on ATP and NADPH normally made in the light. So “dark reactions” does not mean “night shift.”

1. Fixation: Rubisco attaches CO₂ to RuBP, a five-carbon acceptor. The unstable product splits into two three-carbon molecules.
2. Reduction: ATP and NADPH help convert those molecules into G3P, a three-carbon sugar phosphate. Some G3P can leave to build carbohydrates.
3. Regeneration: The rest of the G3P and more ATP help rebuild RuBP so the cycle can keep taking in CO₂.

Optional bookkeeping for your quiz: Three CO₂, nine ATP, and six NADPH yield one net G3P. Know the inputs, output, and locations before memorizing numbers. The AP framework does not require memorizing the individual steps, structures, or enzyme names of the Calvin cycle.

Rubisco can attach CO₂ or O₂ to RuBP. If it uses O₂, the plant has to salvage the resulting products. This photorespiration costs energy and releases some previously fixed carbon as CO₂. Hot, dry conditions often make it worse: stomata close to conserve water, and CO₂ becomes scarce relative to O₂ inside the leaf.

Don’t mix these up: Photorespiration is the Rubisco detour. Cellular respiration is how a cell harvests energy from organic molecules, and plants do it in light and dark.

In the water around an aquatic plant: When light is off, photosynthetic CO₂ uptake falls and cellular respiration keeps releasing CO₂. If CO₂ accumulates in weakly buffered water, the equilibrium shifts toward more H⁺: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. More H⁺ means lower pH.

• Photosynthesis exceeds respiration: net CO₂ uptake → less dissolved CO₂ → pH tends to rise.
• Respiration exceeds photosynthesis: net CO₂ release → more dissolved CO₂ → pH tends to fall. In darkness, O₂ also tends to fall if the system is closed and aerobic respiration continues.
• Rates balance: little net CO₂ change, so little CO₂-driven pH change. Buffering and gas exchange can affect what you actually measure.

Inside the chloroplast: A different pH question. When light stops, the H⁺ gradient across the thylakoid relaxes. The lumen becomes less acidic relative to its illuminated state as H⁺ redistributes. That does not contradict the surrounding water becoming more acidic from respiratory CO₂.

Talon’s grading note: “pH goes down” is not a complete thought. Tell me which compartment and where the H⁺ came from.

• Floating leaf disks: Oxygen accumulation makes disks buoyant. Compare matched disks at the same time. A light-versus-dark comparison needs the same carbon supply; a carbon comparison needs the same light. Faster floating or a lower time to 50% floating can indicate faster net O₂ accumulation under matched conditions.
• Indicators: A pH or CO₂ indicator tracks a proxy for the net CO₂ balance. A no-plant control tests whether the solution changes on its own. Use independent replicates and hold things like temperature, plant mass, and starting pH constant.
• Read a rate graph: On a graph of photosynthetic rate versus light intensity, a plateau suggests that more light no longer raises the measured rate under those conditions. Test a factor such as CO₂ before claiming it caused the plateau. A compensation point is where photosynthesis balances respiration for the measured gas.
• Make a mechanistic prediction: If a treatment collapses the H⁺ gradient, ATP production falls, which can limit carbon fixation. Give the direction, cite the comparison when data are provided, and name the link in the mechanism.
• Evolution: Photosynthesis began in prokaryotes. Oxygen released by ancient cyanobacteria helped oxygenate Earth’s atmosphere, and photosynthetic bacteria were the ancestors of chloroplasts.

Exam priority: The current AP framework calls this Topic 3.4. Prioritize chloroplast compartments, light capture, electron transfer, chemiosmosis, and how ATP and NADPH power carbon fixation. Detailed carrier names and Calvin-cycle memorization are beyond its scope. The C₃/C₄/CAM and pH sections are included for this class quiz and for applying those core ideas.