Light Dependent Reaction in Photosynthesis

Study Notes

Light Dependent Reactions

Involves absorption of light energy (photons) from the sun.
Converts light energy into chemical energy in the form of ATP and NADPH.
Two key processes: non-cyclic photophosphorylation and cyclic photophosphorylation.

Photosystems

Composed of a reaction center complex and light-harvesting complex.
Photosystem I (PSI): Contains P700 chlorophyll a, absorbs light at 700 nm.
Photosystem II (PSII): Contains P680 chlorophyll a, absorbs light at 680 nm.

Reaction Center Components

Reaction center complex consists of:
- A pair of chlorophyll a (P700 for PSI, P680 for PSII).
- Primary electron acceptor for transferring excited electrons.

Light Harvesting Complex

Includes chlorophyll a and b, along with carotenoids (carotene, xanthophyll, phaeophytin).

Steps in Light Dependent Reactions

Electron Transfer:
- Photoexcited electrons from PSII are transferred to PSI via an electron transport chain, including plastoquinone (Pq), cytochrome complex, and plastocyanin (Pc).
ATP Generation:
- Energy released during electron transfer is utilized for ATP production through chemiosmosis (ADP + Pi → ATP).
Photon Absorption:
- Light energy is absorbed by antenna pigments in PSI and transferred to P700.
Photoactivation:
- P700 undergoes photoactivation, releasing excited electrons to its primary electron acceptor, creating electron deficiency.
Electron Replacement:
- Electrons from P680 of PSII replenish those lost from P700.
Final Electron Transfer:
- Electrons from PSI's primary acceptor are passed to ferredoxin (Fd), reducing NADP+ to NADPH through the action of NADP+ reductase.

Products of Light Dependent Reactions

Non-cyclic photophosphorylation produces both ATP and NADPH, which are utilized in the Calvin cycle.
Cyclic photophosphorylation produces only ATP.

Cyclic vs Non-Cyclic Photophosphorylation

Similarities:
- Both produce ATP through photophosphorylation.
- Both occur in thylakoid membranes during light-dependent reactions.
Differences:
- Cyclic involves only PSI and produces ATP; non-cyclic involves both PSI and PSII, producing ATP and NADPH.
- Cyclic has an electron donor of P700; non-cyclic uses water as the first electron donor, releasing O2.

Calvin Cycle Overview

Also known as the C3 pathway, as it first produces a 3-carbon compound: 3-phosphoglycerate (PGA).
Occurs in the stroma of chloroplasts in C3 plants, functioning during daylight without needing light energy directly.
Comprises three phases:
- Carbon Fixation: Incorporating CO2 into organic molecules.
- Reduction Phase: Converting PGA into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
- Regeneration of RuBP: Rearranging G3P to regenerate ribulose bisphosphate (RuBP) to continue the cycle.

Description

Learn about the light dependent reaction in photosynthesis, including the conversion of light energy to chemical energy, and the processes of cyclic and non-cyclic photophosphorylation. Understand the role of photosystems I and II in this reaction.