Non-cyclic photophosphorylation
overview
As electrons move from P680 through the ETC and P700 and then ultimately to NADPH, the chemical energy and reducing power requirements of the Calvin cycle are met. One last problem remains: the electron from P680 needs to be replaced. This replacement comes from the photolysis of water. When water is lysed using light energy, it dissociates into 2 protons, 2 electrons, and half an oxygen molecule.
H20 = 2 H+, 2e- , 1/2 O2
1/2 O2 + 1/2 O2 = O2
Oxygen leaves the chloroplast as a byproduct of the light-dependent reactions.
What are the two sources of protons in the thylakoid membrane? What two mechanisms drive the decrease in pH in the chloroplast stroma?
So the "non-cyclic" part of this process refers to the one-way transfer of electrons from water to NADPH and the concomittant phosphorylation of ADP.
To summarize the light-dependent reactions: electrons flow from water to P680 through an ETC to P700 to NADPH. A proton gradient is generated in the thylakoid membrane that is harnessed to drive the synthesis of ATP.
The ATP synthase and many of the electron carriers (quinones and cytochromes) are very similar in mitochondria and chloroplasts.
How many photons are required to move each electron from water to NADPH?
Building organic molecules from CO2
Energy in the form of ATP is not enough for plants to survive; they must also have a source of carbon to synthesize macromolecules. Autotrophs use an inorganic carbon source (carbon dioxide) and generate carbohydrates. This is an endergonic reaction. ATP is the power source. Carbon fixation is also a reduction; NADPH serves as the reducing agent. The products of the light reactions, ATP and NADPH, are consumed in the carbon fixation process.
The Calvin cycle
Overview
Similar to the Krebs cycle, the Calvin cycle must regenerate its starting material.
Carbon enters the cycle as carbon dioxide and leaves as sugar.
ATP is the energy source; NADPH is the reducing agent.
The product of the Calvin cycle is a 3-carbon sugar, glyceraldehyde 3-phosphate (G3P).
For every carbon in the manufactured sugar, a carbon dioxide molecule must enter the cycle. The cycle must be completed before another carbon dioxide molecule can be fixed.
There are three phases in the Calvin cycle.
1. Carbon fixation
2. Reduction
3. Regeneration of RuBP