Guenter Blobel wins the 1999 Nobel Prize in medicine for his discovery of signal sequences at the ends of proteins that target them to their intended cellular destination.

Noncyclic photophosphorylation requires both PSII and PSI (Figure 8.11):

PSII: reaction center = P680. Photoexcitation of PSII yields PSII*, which transfers electron via a redox chain involving the cytochrome b/cytochrome f complex to PSI. PSII⁺ fills its ‘electron hole’ by oxidizing H₂O to O₂.

PSI: reaction center = P700. Photoexcitation of PSI yields PSI*, which transfers its electron via a protein called ferredoxin (Fd) to NADP⁺, reducing it to NADPH.

Note that it takes two photons to send one e^- from H₂O to NADP⁺. (Therefore, it takes four photons to reduce one NADP⁺ ® NADPH.)

Electron transfer between PSII and PSI via the membrane protein complex called the cytochrome b/cytochrome f complex results in H⁺ translocation from the stroma to the lumen of the thylakoid vesicles (Figure 8.14), creating a chemi-osmotic gradient of H⁺ across the thylakoid membrane that can be trapped to drive ATP synthesis by the CF₁CF₀-ATP synthase.

Noncyclic photophosphorylation produces NADPH, ATP, and O₂.

Cyclic photophosphorylation requires only PSI (Figure 8.12:

The photo-excited electron lost from P700* returns to fill the ‘electron hole’ in P700⁺ via the redox chain of the cytochrome b/cytochrome f complex. This electron transfer creates a proton gradient that can be used to drive ATP synthesis.

Note that it takes only one photon to send one electron around cycle.

Cyclic photophosphorylation produces only ATP, no NADPH or O₂.

Figure 8.14: Chemi-osmotic ATP synthesis in chloroplasts by CF₁CF₀-ATP syntase.

The Jagendorf-Uribe Experiment was the first experimental demonstration of ATP synthesis by a chemi-osmotic mechanism (web figure).

The overall reaction:

6CO₂ + 18ATP + 12NADPH + 12H⁺ + 12H₂O ®

C₆H₁₂O₆ + 18P_i + 18ADP + 12NADP⁺

The metabolic cost of a glucose formed by CO₂ fixation is 66 ATP: Each ATP = 1ATP equivalent; each NADPH = 4ATP equivalents (an NADPH is assigned an ATP value that is 1ATP greater than an NADH)

Thermodynamics dictates that it costs more ATP equivalents to make a glucose from CO₂ than can be recovered from glucose oxidation to CO₂ via cellular respiration (max. 38 ATP). Otherwise, cells could be ‘

‘perpetual motion’ machines: cellular respiration + CO₂ fixation would form a cycle showing net synthesis of ATP.

The CO₂ Fixation Reaction: Ribulose-1,5-Bisphosphate Carboxylase (or Rubisco)

Ribulose-1,5-BisP + CO₂ ®

6-carbon intermediate ®

2 3-Phosphoglcyerates

1(5) + 1(1) ® 2(3)

Rubisco is located on the stromal surface of the thylakoid membranes. Very abundant protein: 15% of total chloroplast protein

A 15-step metabolic cycle, catalyzed by enzymes found in the stroma, that generates carbohydrate from CO₂ and replenishes the CO₂-acceptor, RuBP.

An overall summary of the Calvin-Benson cycle:

6RuBP + 6CO₂ ® 12 3-Phosphoglycerates

6(5) + 6(1) ® 12(3)

12(3) represent 36 carbon atoms.

The ATP- and NADPH-dependent reactions of the Calvin-Benson cycle:

12 of the 18 ATP are consumed in converting 12 3-phosphoglycerate molecules to 12 1,3-bisphosphoglycerate

molecules:

12 3-PG + 12 ATP ® 3-phosphoglycerate kinase®

12 1,3-BPG + 12 ADP

The 12 NADPH are used to reduce these 1,3 PBG molecules to Glyceraldehyde-3-P. The reaction is catalyzed by an NADPH-specific glyceraldehyde-3-P dehydrogenase:

12 1,3-PBG + 12 NADPH + 12 H⁺ ®

12 glyceraldehdye-3-P + 12 NADP⁺ + 12 P_i

(2 of these glyceraldehyde-3-P are converted to glucose using the reactions of glycolysis running in reverse; the remaining 10 (30C) are rearranged to form 6 Ribulose-5-P.)

Then, the remaining 6 ATP are used to phosphorylate these 6 Ru-5-P. The enzyme here is Ru-5-P kinase:

6 Ru-5-P + 6 ATP ® 6 Ru-1,5-BP + 6 ADP

Photorespiration: Light-driven O₂ uptake & CO₂ release

O₂ competes with CO₂ for binding to the rubisco active site. (Rubisco is actually an acronym for Ribulose Bisphosphate Carboxylase-Oxygenase.)

If [CO₂] levels are low and [O₂] levels are high:

Ru-1,5-BP + O₂ ® 3-phosphoglycerate + phosphoglycolate

Phosphoglycolate, a 2-carbon compound, is subsequently broken down in the mitochondria to form 2 CO₂.

Photorespiration represents a wasteful loss of the important CO₂-acceptor, RuBP. Photosynthetic yields (agricultural productivity) are diminished by photorespiration.

C₄ Plants (tropical grasses such as crabgrass and sugar cane) diminish photorespiration by spatially isolating CO₂

fixation by rubisco in the relatively low-O₂ environment of bundle-sheath cells. CO₂ is incorporated to form 4-carbon organic acids in mesophyll cells. Transport of these C₄ compounds to bundle-sheath cells, followed by decarboxylation provides CO₂ for rubisco.

CAM Plants (Crassulacean acid metabolism plants such as cacti and other succulents of semiarid and desert environments):

Diminish photorespiration by temporally isolating CO₂ fixation: CO₂ is incorporated into organic acids during the night when stomata are open. During the day, when the stomata are closed to prevent water loss, these organic acids are decarboxylated to release CO to rubisco and the Calvin-Benson cycle for CO₂ fixation into carbohydrate.