14.3 Photosynthesis: Light Reactions, Photophosphorylation, and the Calvin Cycle

14.3 Photosynthesis: Light Reactions, Photophosphorylation, and the Calvin Cycle

Quick Answer: The carbohydrate-metabolism area of the Biochemistry TOS (G1, 12 items) ends with photosynthesis. Know the Z scheme in order, how water is split and where the protons go, cyclic versus non-cyclic photophosphorylation, the three phases of the Calvin cycle with their ATP and NADPH costs, rubisco's oxygenase problem, and how C4 and CAM plants avoid it.

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Overview and the Chloroplast

$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2 \qquad \Delta G^{\circ\prime} \approx +2870\text{ kJ/mol}$

Isotope experiments with $H_2^{18}O$ showed that the released $O_2$ comes from water, not from $CO_2$.

Pigments and Light Harvesting

• Chlorophyll a and b are $Mg^{2+}$-chlorin pigments that absorb strongly in the blue (about 430 nm) and red (about 660 nm), so leaves look green.
• Carotenoids absorb blue-green light and protect against photo-oxidation by quenching excited chlorophyll and singlet oxygen.
• Antenna complexes pass absorbed energy to the reaction centre by resonance (Förster) energy transfer. Only the special chlorophyll pair in the reaction centre performs charge separation.

A 680 nm photon carries $\frac{119{,}627}{680} = 176$ kJ per mole of photons.

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The Light Reactions: The Z Scheme

The oxygen-evolving complex cycles through five oxidation states ($S_0$ to $S_4$, the Kok cycle). It releases one $O_2$ for every four photons absorbed by PSII; with PSI, eight photons are needed per $O_2$.

Photophosphorylation

• Non-cyclic: electrons flow from water to $NADP^+$, producing $O_2$, NADPH, and a proton gradient. CF0CF1 ATP synthase lets protons flow from lumen to stroma and makes ATP.
• Cyclic: electrons from PSI return through ferredoxin to cytochrome $b_6f$. This pumps protons and makes ATP only, with no NADPH or $O_2$. It supplies the extra ATP the Calvin cycle needs.
• Jagendorf's acid-bath experiment (1966): thylakoids soaked at pH 4 and moved to pH 8 made ATP in the dark. That supported Mitchell's chemiosmotic theory (Section 14.2).

Herbicides such as DCMU (diuron) block electron transfer from PSII to plastoquinone, and paraquat steals electrons from PSI to make damaging superoxide.

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The Calvin (Calvin-Benson-Bassham) Cycle

Stoichiometry: each $CO_2$ costs 3 ATP and 2 NADPH. One triose phosphate (3 $CO_2$) costs 9 ATP and 6 NADPH, and one hexose (6 $CO_2$) costs 18 ATP and 12 NADPH. Triose phosphate is exported to make sucrose in the cytosol or kept to make starch in the chloroplast.

Rubisco and Photorespiration

Rubisco is probably the most abundant protein on Earth. It is very slow (a few reactions per second per active site) and also reacts with $O_2$: RuBP + $O_2$ gives one 3-phosphoglycerate and one 2-phosphoglycolate. Recycling phosphoglycolate through peroxisomes and mitochondria (photorespiration) consumes ATP and releases $CO_2$, wasting fixed carbon. Oxygenation increases at high temperature and low $CO_2$, a serious problem in hot climates.

Light Regulation of the Calvin Cycle

Light activates the Calvin cycle so that it does not run in the dark:

• Proton pumping raises the stromal pH to about 8 and the $Mg^{2+}$ concentration; both activate rubisco and fructose-1,6-bisphosphatase.
• Reduced thioredoxin activates fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and phosphoribulokinase.
• Rubisco activase removes inhibitory sugar phosphates from rubisco.

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C4 and CAM Plants

PEP carboxylase does not react with $O_2$ and has a high affinity for $HCO_3^-$, so C4 plants nearly eliminate photorespiration at the cost of about 2 extra ATP per $CO_2$. CAM plants greatly reduce water loss in arid conditions.