1. 3.5 x 10²¹ kcal of solar energy reaches the earth each day.
2. 1% of it is captured by photosynthetic organisms.
3. Photosynthesis: the transduction of light energy into chemical energy
4. Traditionally expressed in terms of CO₂ fixation:

   6 CO₂ + 6 H₂O + energy ® C₆H₁₂O₆ + 6 O₂
   essentially the reversal of cellular respiration.

    

5. More appropriately, since all of the O₂ evolved in photosynthesis comes from H₂O,

   6 CO₂ + 12 H₂O + energy ® C₆H₁₂O₆ + 6 O₂ + 6 H₂O

6. The energy required can be expressed as nhu, where n = some number of photons of energy hu, where h = Planck’s constant and u = the frequency of light. (The energy, E = nhu = nhc/l, where c = speed of light and l = wavelength of light.)

    

1. Review chloroplast structure
2. The light reactions involve energy capture and O₂ evolution; the light reactions are associated with the thylakoid membranes.
3. The dark reactions involve CO₂ fixation; the dark reactions take place in the stroma.

    

1. Specifically, light (radiant electromagnetic energy) is transduced by a photochemical system (thylakoid membranes) into 2 forms of chemical energy:

    

   1. ATP – phosphorylation energy
   2. NADPH – a strong reducing agent

    

2. The source of electrons to reduce NADP⁺ ® NADPH is water:

   2H₂O + 2NADP⁺ + xADP + xP_I+ nhu ® O₂ + 2NADPH + 2H⁺ + xATP

   The NADPH and ATP thus formed provide the chemical energy to drive CO₂ fixation in the dark.

    

1. Chlorophyll (Chl) resembles heme, but it has Mg²⁺ instead of Fe²⁺.
2. Possible fates of the quantum of light energy absorbed by a photosynthetic pigment molecule (web figure).
3. Resonance energy transfer (exciton transfer) is an important mechanism in harvesting light energy.

    

Photo-excitation of Chl leads to e^- transfer to a primary electron acceptor (the photochemical event), creating an ‘electron hole’ in the Chl molecule (Chl⁺). Electron transfer from water fills this ‘hole’.

1. Photosynthetic units are localized within the thylakoid membranes. Photosynthetic units consist of a specialized pair of chlorophyll a molecules (the reaction center Chl) and several hundred molecules of Chl a and accessory light- harvesting pigment molecules serving as an antenna to collect light energy and channel it to the reaction center via exciton transfer.
2. Only reaction center Chl can perform the photochemical event: conversion of light energy to chemical energy by an e^- transfer from Chl*.

    

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.

1. 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.
2. 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.)

3. 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.
4. Noncyclic photophosphorylation produces NADPH, ATP, and O₂.

    

1. 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.
2. Note that it takes only one photon to send one electron around cycle.
3. Cyclic photophosphorylation produces only ATP, no NADPH or O₂.

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