Photosynthesis converts light energy into chemical energy stored in sugar. The chapter is long, but it has only four moving parts: where it happens, how light makes ATP and NADPH, how CO₂ becomes sugar, and what limits the rate. These notes follow that order.
Where it happens
- Chloroplast: double membrane, inner stroma (fluid), and stacks of thylakoids (grana) whose membranes hold the pigments.
- Light reaction → thylakoid membrane. Calvin cycle → stroma.
- Pigments: chlorophyll a (the reaction-centre pigment, blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (orange). Accessory pigments widen the absorbed range and protect chlorophyll a from photo-damage.
- Action spectrum peaks in blue and red, matching the absorption of chlorophyll a — the classic graph question.
The light reaction
Two photosystems, named for the wavelength their reaction centres absorb best: PS II (P680) and PS I (P700). Electron flow through both in the Z-scheme:
- Light hits PS II; excited electrons leave P680.
- Photolysis of water replaces them: 2H₂O → 4H⁺ + O₂ + 4e⁻. This is where oxygen is released.
- Electrons pass down an electron transport chain (plastoquinone → cytochrome b₆f → plastocyanin) to PS I, pumping H⁺ into the thylakoid lumen on the way.
- Light hits PS I; electrons are re-excited and reduce NADP⁺ to NADPH via ferredoxin.
- The H⁺ gradient drives ATP synthase — this is chemiosmosis, and the ATP made is called non-cyclic photophosphorylation.
Cyclic photophosphorylation uses PS I only: electrons return to P700, only ATP forms, no NADPH, no O₂. It happens when NADPH is plentiful or in far-red light.
Products of the light reaction: ATP, NADPH, O₂.
The Calvin cycle (C3 pathway)
In the stroma, using the ATP and NADPH just made. Three stages, and the enzyme to name is RuBisCO:
- Carboxylation: RuBisCO adds CO₂ to RuBP (5C) to give two molecules of 3-phosphoglycerate (3-PGA, 3C) — hence "C3".
- Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate using ATP and NADPH.
- Regeneration: most G3P regenerates RuBP (using ATP); the rest leaves to make glucose.
Bookkeeping worth memorising: to make one glucose (6C), 6 turns of the cycle use 6 CO₂, 18 ATP and 12 NADPH.
C4 plants and CAM plants
RuBisCO has a flaw: it also reacts with O₂ (photorespiration), wasting carbon, and this gets worse in heat when stomata close and CO₂ falls.
C4 plants (maize, sugarcane, sorghum) solve it with Kranz anatomy: bundle-sheath cells wrapped by mesophyll cells. In mesophyll, PEP carboxylase fixes CO₂ into a 4-carbon acid (oxaloacetate → malate). Malate moves to the bundle sheath, releases CO₂ right next to RuBisCO, and the Calvin cycle runs there at high CO₂. Cost: 2 extra ATP per CO₂; benefit: almost no photorespiration, higher yield in hot, bright conditions.
CAM plants (cactus, pineapple) separate the steps in time instead of space: stomata open at night to fix CO₂ into malate; by day the stomata close and the stored CO₂ feeds the Calvin cycle.
Factors affecting the rate
Blackman's law of limiting factors: when several factors affect a process, the rate is set by the one in shortest supply.
- Light: rate rises with intensity then plateaus (light saturation); at very high intensity chlorophyll can be damaged.
- CO₂: the usual limiting factor in the field (about 0.03–0.04 % in air); C3 plants respond to enrichment up to ~0.05 %.
- Temperature: the dark reaction is enzyme-driven, so it has an optimum; C4 plants have a higher optimum than C3.
- Water: shortage closes stomata, which cuts CO₂ intake.
Quick recap for the exam
- O₂ comes from water, shown by Ruben and Kamen with ¹⁸O.
- Light reaction: thylakoid; makes ATP + NADPH + O₂. Dark reaction: stroma; makes sugar; needs no light directly but depends on the light reaction's products.
- First stable product: 3-PGA in C3, oxaloacetate in C4.
- Enzymes to name: RuBisCO (C3 fixation), PEP carboxylase (C4 fixation).
- Kranz anatomy is a C4 feature; CAM plants fix CO₂ at night.