It should be noted that per electron donated to the system, either NADPH or ATP is made, but not both. This puts a limit on the versatility of the system. But what would happen if both systems coexisted simultaneously? That is, if both ATP and NADPH could be formed from one electron? Additionally, these systems require compounds such as reduced sulfur to act as electron donors, not necessarily widely found compounds. What would happen if a chlorophyll
ox molecule would have a reduction potential higher than that of molecular the O
2 /H
2 O reaction? Answer, a planetary game changer.
Oxygenic photophosphorylation
Generation of nadph and atp
The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in
[link] . Protein complexes and pigment molecules work together to produce NADPH and ATP.
A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll
a molecules in the reaction center. The light excites an electron from the chlorophyll
a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain discussed below.
The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a
photosystem , two types of which are found embedded in the thylakoid membrane,
photosystem II (PSII) and
photosystem I (PSI) (
[link] ). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).
Both photosystems have the same basic structure; a number of
antenna proteins to which the chlorophyll molecules are bound surround the
reaction center where the photochemistry takes place. Each photosystem is serviced by the
light-harvesting complex , which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll
a and
b molecules as well as other pigments like carotenoids. The absorption of a single
photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.
Art connection
In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP
+ to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At the same time, splitting of water adds protons to the lumen, and reduction of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen, and a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP.
What is the initial source of electrons for the chloroplast electron transport chain?
