Photosynthesis: The Role of Light

• the removal of hydrogen (H) atoms from water molecules
• the reduction of carbon dioxide (CO₂) by these hydrogen atoms to form organic molecules

The second process involves a cyclic series of reactions named (after its discoverer) the Calvin Cycle. It is discussed in Photosynthesis: Pathway of Carbon Fixation. The details of the first process is our topic here.

2H₂O -> 4e^- + 4H⁺ + O₂

• They reduce NADP⁺ to NADPH for use in the Calvin Cycle.
• They set up an electrochemical charge that provides the energy for pumping protons from the stroma of the chloroplast into the interior of the thylakoid.

• They participate in the reduction of NADP⁺ to NADPH.
• As they flow back out from the interior of the thylakoid (by facilitated diffusion), passing down their concentration gradient), the energy they give up is harnessed to the conversion of ADP to ATP.
• Because it is drive by light, this process is called photophosphorylation.

  ADP + P_i -> ATP

  The ATP provides the second essential ingredient for running the Calvin Cycle.

The removal of electrons from water molecules and their transfer to NADP⁺ requires energy. The electrons are moving from a redox potential of about +0.82 volt in water to -0.32 volt in NADPH. Thus enough energy must be available to move them against a total potential of 1.14 volts. Where does the needed energy come from? The answer: Light.

The Thylakoid Membrane

1. Photosystem I

The structure of photosystem I in a cyanobacterium ("blue-green alga") has been completely worked out. It probably closely resembles that of plants as well.

• 12 different protein molecules bound to
• 96 molecules of chlorophyll a
  • 2 molecules of the reaction center chlorophyll P₇₀₀
  • 4 accessory molecules closely associated with them
  • 90 molecules that serve as antenna pigments
• 22 carotenoid molecules
• 4 lipid molecules
• 3 clusters of Fe₄S₄
• 2 phylloquinones

2. Photosystem II

• as many as 20 different protein molecules bound to
• 50 or more chlorophyll a molecules
  • 2 molecules of the reaction center chlorophyll P₆₈₀
  • 2 accessory molecules close to them
  • 2 molecules of pheophytin (chlorophyll without the Mg⁺⁺)
  • the remaining molecules of chlorophyll a serve as antenna pigments.
• some half dozen carotenoid molecules. These also serve as antenna pigments.
• 2 molecules of plastoquinone

3. & 4. Light-Harvesting Complexes (LHC)

• LHC-I associated with photosystem I
• LHC-II associated with photosystem II

5. Cytochromes b₆ and f

6. ATP synthase

How the System Works

• Light is absorbed by the antenna pigments of Photosystems II and I
• The absorbed energy is transferred to the reaction center pigment, P₆₈₀ in Photosystem II, P₇₀₀ in Photosystem I
• Activation of P₆₈₀ removes an electron from it.
• With its resulting positive charge, P₆₈₀ is sufficiently electronegative that it can remove electrons from water.
• These electrons are transferred (by way of pheophytin and plastoquinone) to the cytochrome b₆ and f complex where they provide the energy for chemiosmosis.
• Activation of P₇₀₀ enables it to pick up electrons from cytochrome f and raise them through a chain of electron transporters to a sufficiently high redox potential that
• they can reduce NADP⁺ to NADPH.

The saw-tooth shifts in redox potential as electrons pass from P₆₈₀ to NADP⁺ have caused this system to be called the Z-Scheme.

Chemiosmosis in Chloroplasts

The energy released as electrons pass down the gradient from Photosystem II to Photosystem I is harnessed by the cytochrome b₆ & f complex to pump protons (H⁺) against their concentration gradient from the stroma of the chloroplast into the interior of the thylakoid (an example of active transport). As their concentration increases inside (which is the same as saying that the pH of the interior decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in mitochondria, the energy released as these electrons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.

Antenna Pigments