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Must-know facts
20 facts
- 1Priestley (1770): plants restore air spoilt by a candle or mouse; he discovered oxygen in 1774.
- 2Ingenhousz: sunlight is essential; only green parts release oxygen.
- 3Sachs (about 1854): green parts make glucose, usually stored as starch; chlorophyll sits in bodies later called chloroplasts.
- 4Engelmann: Cladophora plus aerobic bacteria gave the first action spectrum, with peaks in blue and red.
- 5van Niel: O₂ released in photosynthesis comes from H₂O, not CO₂.
- 6Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂.
- 7Light reactions: thylakoid membranes (grana, stroma lamellae). Carbon reactions: stroma.
- 8Chlorophyll a is the chief pigment; accessory pigments widen the spectrum and protect chlorophyll a from photo-oxidation.
- 9PS I reaction centre = P700; PS II reaction centre = P680; named in order of discovery.
- 10Water splitting (2H₂O → 4H⁺ + O₂ + 4e⁻) is linked to PS II on the lumen side.
- 11Non-cyclic: PS II + PS I, gives ATP and NADPH, releases O₂. Cyclic: PS I only, gives only ATP; likely site is the stroma lamellae.
- 12Protons accumulate in the thylakoid lumen; CF₀ is the membrane channel, CF₁ sits on the stroma side and makes ATP.
- 13Calvin: ¹⁴C tracer; first stable product in C₃ = 3-PGA; primary acceptor = RuBP (5C).
- 14Per CO₂: 3 ATP + 2 NADPH. Per glucose: 6 turns, 18 ATP, 12 NADPH.
- 15RuBisCO = RuBP carboxylase-oxygenase, the most abundant enzyme in the world.
- 16C₄: first product OAA (4C), primary acceptor PEP (3C), enzyme PEPcase in mesophyll; Calvin cycle in bundle sheath.
- 17Mesophyll of C₄ plants lacks RuBisCO; bundle sheath lacks PEPcase.
- 18Photorespiration: RuBP + O₂ → phosphoglycerate + phosphoglycolate; no sugar, no ATP; absent in C₄ plants.
- 19Light saturation at about 10% of full sunlight; C₄ saturate near 360 μL L⁻¹ CO₂, C₃ only beyond 450 μL L⁻¹.
- 20Blackman (1905): the factor nearest its minimum limits the rate.
Common traps
Where marks are lost
Assuming PS I acts first because it is numbered 1.
Numbers reflect order of discovery. In the non-cyclic Z scheme, PS II (P680) acts first and passes electrons to PS I (P700).
Saying protons pile up in the stroma during the light reaction.
They accumulate in the thylakoid lumen and flow back out through CF₀ to the stroma; CF₁ faces the stroma.
Thinking the O₂ released comes from CO₂.
It comes from water, as van Niel inferred and isotope work later confirmed. Split water, get O₂.
Confusing the first product with the primary acceptor.
C₃: acceptor RuBP (5C), first product 3-PGA (3C). C₄: acceptor PEP (3C), first product OAA (4C).
Believing C₄ plants skip the Calvin cycle.
All photosynthetic plants run the Calvin cycle. In C₄ plants it runs in the bundle sheath cells.
Crediting cyclic photophosphorylation with NADPH or O₂ production.
Cyclic flow uses only PS I and makes only ATP; no water splitting, so no O₂ and no NADPH.
Taking 'dark reaction' literally, as reactions that run in darkness.
The carbon reactions need the ATP and NADPH made in light, so they depend on light indirectly; they carry on only briefly after light is withdrawn and then stop.
Assuming light is usually the limiting factor in the field.
Light saturates at about 10% of full sunlight, so except in shade or dense forest, CO₂ is the usual limiting factor.
Putting RuBisCO in C₄ mesophyll cells.
C₄ mesophyll has PEPcase and no RuBisCO; the bundle sheath has RuBisCO and no PEPcase.
Key terms
16 terms
- Action spectrum
- A plot of photosynthetic rate against the wavelength of light used.
- Absorption spectrum
- A plot of how much light of each wavelength a pigment absorbs.
- Accessory pigments
- Chlorophyll b, xanthophylls and carotenoids, which absorb extra wavelengths and pass the energy to chlorophyll a.
- Light harvesting complex
- An antenna of hundreds of protein-bound pigment molecules feeding energy to a reaction centre.
- Reaction centre
- The single chlorophyll a molecule in a photosystem that actually loses an excited electron.
- Z scheme
- The zig-zag path of electrons from water through PS II and PS I to NADP⁺ when drawn on a redox scale.
- Photophosphorylation
- Making ATP from ADP and inorganic phosphate using light energy.
- Photolysis of water
- The PS II-linked splitting of water into protons, electrons and oxygen.
- Chemiosmosis
- ATP synthesis powered by protons flowing down a gradient through ATP synthase.
- CF₀ and CF₁
- The membrane proton channel and the stroma-facing catalytic head of chloroplast ATP synthase.
- RuBP
- Ribulose bisphosphate, the 5-carbon CO₂ acceptor of the Calvin cycle.
- RuBisCO
- The enzyme that joins CO₂ (or O₂) to RuBP; it has both carboxylase and oxygenase activity.
- PEPcase
- PEP carboxylase, the enzyme that fixes CO₂ onto PEP in C₄ mesophyll cells.
- Kranz anatomy
- The wreath of large, chloroplast-rich, thick-walled bundle sheath cells around veins in C₄ leaves.
- Photorespiration
- The oxygenase reaction of RuBisCO in C₃ plants that consumes ATP and releases CO₂ without making sugar.
- Limiting factor
- The factor, closest to its minimum, that sets the overall rate of a multi-factor process.