Photosynthesis
General Biology · Section 3.3 · 18 study cards
Chloroplast structure and pigments, the light reactions, Calvin cycle stoichiometry, and the C3, C4 and CAM strategies.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Hold the two stages apart by their currencies
The light reactions turn photons into two currencies, ATP and NADPH, and throw away oxygen. The Calvin cycle spends those currencies to reduce CO₂ into sugar and hands the empty carriers back. If you can state, for any question, which currency is being made or spent and in which compartment, most of the answer follows. Write the compartment next to every process: thylakoid membrane, lumen, or stroma.
Follow the electron uphill
Trace the electron from water to NADPH and notice that its energy rises twice, once at each photosystem, and falls once, through the cytochrome complex. Light is what lifts the electron; the fall between the two lifts is what pumps protons. This is the exact mirror image of respiration, where electrons only ever fall.
Do Calvin cycle arithmetic in units of three turns
- Work in blocks of three CO₂, because one G3P leaves the cycle for every three carbons fixed.
- Three CO₂ plus three RuBP gives six 3-phosphoglycerate. Spend 6 ATP and 6 NADPH to reduce them to six G3P.
- Export one G3P and keep five, then spend 3 more ATP to rearrange those five back into three RuBP.
- Totals per G3P: 3 CO₂, 9 ATP, 6 NADPH. Double everything for one glucose: 6 CO₂, 18 ATP, 12 NADPH.
- If asked about O₂ or water, remember 2 NADPH requires 4 electrons, which requires 2 H₂O split and yields 1 O₂.
Treat C4 and CAM as one problem with two answers
Both exist because rubisco also binds oxygen, and both solve it by concentrating CO₂ around rubisco using PEP carboxylase. The only thing to remember separately is the axis of separation: C4 separates in space, across two cell types, and CAM separates in time, across night and day. Any question about hot climates, closed stomata or wasteful fixation is asking about this one idea.
Definitions and theorems
Worked example
An illuminated chloroplast preparation fixes 12 molecules of CO₂ through the Calvin cycle. Determine how many G3P molecules are exported, how many ATP and NADPH are consumed, how many glucose-equivalents this represents, and how many water molecules must be split and how many O₂ released in the light reactions to supply the NADPH.
Convert CO₂ to exported G3P. Three CO₂ are fixed for every one G3P that leaves the cycle, so 12 divided by 3 gives 4 G3P exported.
Count the ATP. Each exported G3P costs 9 ATP, six in the reduction phase and three in regeneration. So 4 × 9 = 36 ATP.
Count the NADPH. Each exported G3P costs 6 NADPH, all of it in the reduction phase, since regeneration uses ATP only. So 4 × 6 = 24 NADPH.
Convert to glucose. Two G3P are combined to make one six-carbon sugar, so 4 G3P gives 2 glucose-equivalents. Check the carbon: 12 CO₂ carries 12 carbons, and 2 glucose holds 12 carbons.
Work back from NADPH to electrons. Reducing one NADP⁺ to NADPH takes 2 electrons, so 24 NADPH requires 48 electrons to be lifted through the two photosystems.
Work back from electrons to water. Splitting one H₂O yields 2 electrons, 2 protons and half an O₂. So 48 electrons require 24 H₂O split and release 12 O₂.
Cross-check against the overall equation. Two glucose should require 12 CO₂ and release 12 O₂, and the twelve-water form of the equation requires 24 H₂O split for two glucose. All three figures agree.
Common mistakes
- Calling the Calvin cycle the dark reactions and thinking it runs at night. It does not require photons directly, but it stops in the dark because ATP and NADPH run out and several of its enzymes are light-activated. Say light-independent, and explain the dependence.
- Saying the released oxygen comes from carbon dioxide. It comes from the splitting of water at photosystem II, which isotope labelling settled. The carbon and oxygen of CO₂ end up in the sugar and in water respectively.
- Mixing up the ATP and NADPH counts per G3P. They are not equal: 9 ATP but only 6 NADPH, because regeneration of RuBP consumes ATP alone. Losing the extra 3 ATP is the commonest stoichiometry error in this unit.
- Assuming photosystem I acts before photosystem II. The numbering reflects the order of discovery, not the order of operation. Electrons pass from water through photosystem II first and then through photosystem I.
- Describing C4 and CAM as different chemistries. They use the same enzyme, PEP carboxylase, and the same trick of concentrating CO₂ around rubisco. The examinable difference is the separation used: different cells in C4, different times of day in CAM.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.