Photosynthesis Deep Dive
- Trace electrons through Photosystem II, the electron transport chain, and Photosystem I to NADPH, and link the proton gradient to photophosphorylation
- Account quantitatively for the Calvin cycle: how many turns, CO₂, ATP, and NADPH yield one G3P and one glucose
- Compare C3, C4, and CAM strategies for avoiding photorespiration
Two photosystems, one electron highway
The light reactions run noncyclic (linear) electron flow through two pigment complexes embedded in the thylakoid membrane. Confusingly, electrons pass through Photosystem II first, then Photosystem I — they are numbered by order of discovery, not order of use. At PSII the reaction-center chlorophyll is called P680 (best absorbs 680 nm light); at PSI it is P700. A photon excites P680, which ejects a high-energy electron. That electron travels down an electron transport chain (plastoquinone → cytochrome complex → plastocyanin) to PSI, where a second photon re-energizes it before it is handed off to ferredoxin and finally to NADP⁺ reductase, which builds NADPH.
Splitting water and pumping protons
P680 is left electron-deficient after firing — it is the strongest biological oxidizing agent known, strong enough to strip electrons from water. This photolysis of H₂O replaces P680’s lost electrons, dumps H⁺ into the thylakoid lumen, and releases O₂ as waste (the source of atmospheric oxygen). Meanwhile, as electrons move through the cytochrome complex, more H⁺ are pumped from the stroma into the lumen. Both effects concentrate protons inside the thylakoid, building a steep electrochemical gradient — the stored energy that will drive ATP synthesis.
Photophosphorylation: cashing in the gradient
The proton gradient across the thylakoid membrane is potential energy. H⁺ flow back down their gradient from the lumen into the stroma through ATP synthase, and this flow drives phosphorylation of ADP → ATP. Because the energy came from light, the process is photophosphorylation — mechanistically identical to chemiosmosis in respiration, only the power source differs. Note that ATP synthase does not use light or electrons directly; it is powered solely by the proton-motive force. The linear pathway thus delivers the stroma three products for the Calvin cycle: ATP, NADPH, and (as waste) O₂.
Calvin cycle carbon bookkeeping
Each turn of the Calvin cycle fixes one CO₂ onto RuBP (5C), immediately splitting into two 3-carbon 3-PGA. Because G3P is a 3-carbon sugar, you need three turns (3 CO₂) to net a single G3P to export — and it takes two exported G3P to build one 6-carbon glucose, so a full glucose costs six turns and six CO₂. Per three turns the cycle spends 9 ATP and 6 NADPH: 6 ATP and 6 NADPH reduce the six 3-PGA to six G3P, then 3 more ATP regenerate the three RuBP. Notice ATP is used more than NADPH — a detail the exam loves.
Photorespiration and the C4 / CAM workarounds
RuBisCO has a flaw: it can bind O₂ instead of CO₂. When a plant closes its stomata in heat or drought, internal CO₂ falls and O₂ (from the light reactions) rises, so RuBisCO fixes O₂ in wasteful photorespiration — consuming energy and releasing CO₂ without making sugar. C4 plants (corn, sugarcane) solve this with spatial separation: the enzyme PEP carboxylase, which ignores O₂, first fixes CO₂ into a 4-carbon acid in mesophyll cells, then ships it to bundle-sheath cells where CO₂ is concentrated for RuBisCO. CAM plants (cacti, pineapple) use temporal separation: stomata open only at night to fix CO₂ into acids, which release it to the Calvin cycle by day when stomata are shut against the heat.
How many turns of the Calvin cycle, and how many CO₂, ATP, and NADPH, are needed to synthesize one molecule of glucose (C₆H₁₂O₆)?
- 1.One net G3P (3 carbons) requires 3 turns of the cycle, fixing 3 CO₂ and spending 9 ATP + 6 NADPH.
- 2.Glucose has 6 carbons and is assembled from two exported G3P, so you need double: 6 turns fixing 6 CO₂.
- 3.Double the ATP and NADPH accordingly: 9 ATP × 2 = 18 ATP, and 6 NADPH × 2 = 12 NADPH.
- 4.Check the carbon: 6 CO₂ deliver 6 carbon atoms — exactly the carbons in one glucose.
Which sequence correctly traces electrons through the noncyclic light reactions?
ATP synthase in the thylakoid is powered only by the H⁺ gradient, not by light or electrons directly. And the O₂ released comes from splitting H₂O at Photosystem II, never from CO₂. Mixing up the electron source (water) with the carbon source (CO₂) is a classic lost point.
To produce one net molecule of G3P, the Calvin cycle turns three times. How much CO₂, ATP, and NADPH does this require?
A cactus keeps its stomata closed all day and opens them only at night, fixing CO₂ into organic acids that release it to the Calvin cycle by daylight. This strategy is best described as:
Free-response scoring hinges on the hand-off and the numbers: the light reactions (thylakoid membrane) turn light + H₂O into ATP, NADPH, and O₂; the Calvin cycle (stroma) spends 18 ATP + 12 NADPH per glucose across 6 turns / 6 CO₂. If asked why the ATP:NADPH demand is unequal, cite RuBP regeneration consuming the extra ATP.
Answer the 3 checkpoints as you read.
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