Light Reactions in Photosynthesis

Photosynthesis consists of two main stages:
- Light Reactions: Require light.
- Calvin Cycle (Dark Cycle): Involves carbon fixation.
Light reactions convert light energy into chemical energy:
- ATP synthesis from ADP and inorganic phosphate.
- Reduction of NADP+ to NADPH.
NADPH provides energy for biosynthetic reactions, including sugar synthesis.
ATP energy is used to covalently link carbon dioxide to organic molecules during carbon fixation.
NADPH's reducing power reduces newly fixed carbon atoms to form sugars like glucose.

Photosynthetic pigments, organic molecules, and proteins are organized into photosystems within the thylakoid membrane.
Two types of photosystems:
- Photosystem I (PS1)
- Photosystem II (PS2)

Photosystems I and II have similar structures but absorb different wavelengths of light. Pigments absorb photons and transfer energy to chlorophyll a molecules in the light-harvesting complex.
Excited electrons are transferred to the reaction center complex, specifically to the primary electron acceptor.
Photosystem I absorbs light best at 700 nm (P700).
Photosystem II absorbs light best at 680 nm (P680).

When a photon hits a chlorophyll molecule:
- The molecule absorbs the photon and boosts an electron to a higher energy orbital.
- The electron is now in an excited, unstable state.
- Energy is released as heat when the electron returns to its ground state.

Photosystem II (PSII) absorbs light at 680 nm.
Pigments transfer excited electrons to a special pair of chlorophyll a molecules, which then transfer electrons to the primary electron acceptor.
PSII splits water (photolysis) into two protons, oxygen, and electrons in the oxygen-evolving complex. This process requires manganese ions as a cofactor.
$H_2O \rightarrow 2H^+ + O + 2e^-$
Released electrons are absorbed by chlorophyll a molecules and transferred to the primary electron acceptor.

Electrons from PSII are passed to plastoquinone, then to the cytochrome complex, and finally to plastocyanin.
From plastocyanin, electrons reach chlorophyll a molecules in Photosystem I.
Cytochrome complex is similar to complex II in the mitochondrial electron transport chain.

Photosystem I (PSI) absorbs light at 700 nm and transfers electrons to its primary electron acceptor.
The primary electron acceptor donates electrons to ferredoxin (an iron-sulfur protein).
Ferredoxin transfers electrons to NADP+ reductase, reducing NADP+ to NADPH.

Linear electron flow from PSII to PSI, then to ferredoxin and NADPH, is called non-cyclic electron flow.
This process synthesizes six ATP molecules and reduces six NADP+ molecules to NADPH.

Cyclic electron flow supplements ATP production to meet the demands of the Calvin cycle (which requires nine ATP).
Electrons from the primary acceptor in PSI are shunted back via ferredoxin to the cytochrome complex, then to plastocyanin, and back to PSI.
This cyclic process generates three additional ATP.
In cyclic electron flow, no NADPH is synthesized.

The electron transport chain creates a proton gradient across the thylakoid membrane.
Light energy excites electrons from chlorophyll, which are then transferred to the primary electron acceptor in PSII.
Photolysis of water generates two protons and two electrons, contributing to the high proton concentration in the thylakoid space.
Electrons move down the electron transport chain, pumping four protons into the thylakoid space, thus decreasing the pH.
Protons flow down the concentration gradient through the ATP synthase complex, generating ATP.
NADP+ is reduced to NADPH by NADPH reductase.
Both chloroplasts and mitochondria use chemiosmotic coupling to generate ATP.
Mitochondria pump protons from the matrix to the intermembrane space, while chloroplasts pump protons from the stroma into the thylakoid space.
Mitochondria transfer energy from food to ATP, while chloroplasts transform light energy into the chemical energy of ATP.