Light-Dependent Reactions in Photosynthesis

Study Notes

The energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy.
It produces ATP and NADPH, crucial for the next stage of photosynthesis.
Photosystems are aggregates of pigments and proteins in the thylakoid membrane.
Light-harvesting complexes capture photons, transferring energy to the reaction center.
Reaction-center complexes consist of chlorophyll and a primary electron acceptor.
The primary electron acceptor accepts electrons from chlorophyll, raising the electron energy.
Chlorophyll absorbs photons, then transfers that energy through a series of proteins.

Basis	Photosystem II	Photosystem I
What are they?	Protein complexes absorbing light energy; dissociate water	Protein complexes absorbing light energy to convert NADP+ to NADPH
Location	Inner surface of the grana thylakoid membrane	Outer surface of the grana thylakoid membrane
Reaction center	P680	P700
Pigment	Chlorophyll b is more compared to chlorophyll a	Chlorophyll a compared to chlorophyll b
Wavelength	Approx. 680nm	Approx. 700nm
Function	Photolysis of water; ATP synthesis	Hydrolysis of water and NADPH synthesis
Photophosphorylation type	Non-cyclic	Cyclic and non-cyclic
Water Comparison	Yes	No

Chlorophyll and other light-absorbing molecules absorb energy from sunlight transferring energy to electrons, starting the ETC.
Water molecules separate into oxygen, hydrogen ions, and electrons.
Electrons move from protein to protein in the electron transport chain and the energy from the movement pumps hydrogen ions against a concentration gradient.
Hydrogen ions build up within the thylakoid, creating a concentration gradient.
The energized electrons are added to a molecule called NADP+, for NADPH creation.

In Photosystem I, chlorophyll and other light-absorbing molecules inside the thylakoid membrane absorb sunlight, exciting electrons that leave the molecules.
NADPH is produced; it functions like ATP in photosynthesis.
Energized electrons are added to NADP+, forming NADPH.

Hydrogen ions flow through a protein channel in the thylakoid membrane. This flow uses potential energy to drive ATP synthase.
ATP synthase adds phosphate groups to ADP, forming ATP.

The Calvin cycle is also known as the light-independent reactions, occurring in the stroma of the chloroplast.
Takes place in the stroma of the chloroplast.
Although it does not directly depend on light, it relies on ATP and NADPH from light-dependent reactions.
CO2 is incorporated, generating sugars from carbon dioxide.
Three stages: carbon fixation, reduction, regeneration.

CO2 is attached to a five-carbon sugar (RuBP) via the enzyme RuBisCo.
RuBisCo creates a six-carbon molecule that splits into two three-carbon molecules (3-PGA).

Phosphate groups from ATP are added to 3-PGA, forming 1,3-biphosphoglycerate.
NADPH reduces 1,3-biphosphoglycerate to G3P (glyceraldehyde-3-phosphate).
Five of the six molecules of G3P are recycled to regenerate RuBP, requiring ATP.
One G3P will create glucose.