Photosynthesis occurs in plant cells within the chloroplasts.
Two main phases: Light Reactions and Calvin Cycle.
Focus on the Light Reactions in this session.
Location
Light reactions occur in the thylakoid of the chloroplast.
Key Components and Processes
Pigments: Absorb light energy.
Photosystems: Photosystem I and II are crucial for the process.
Electron Carriers: ATP and NADPH are produced.
Photosystems
Photosystem II:
First to act in the light reactions (despite being the second discovered).
Receives photons (light energy) transferred to the chlorophyll reaction center.
Energized electrons leave Photosystem II for electron acceptor in the electron transport chain.
Water splitting occurs to replace lost electrons, releasing oxygen, hydrogen ions, and electrons.
Photosystem I:
Absorbs additional light energy.
Energized electrons move to a second electron transport chain.
Electrons from Photosystem II replace those in Photosystem I.
End process reduces NADP to NADPH using electrons and hydrogen ions.
ATP Production
Energy from electrons moving down the electron transport chain creates a hydrogen ion gradient.
ATP Synthase:
Enzyme that uses hydrogen ion gradient.
Converts ADP to ATP as hydrogen ions flow through it.
Summary of Light Reactions
Light energy and water are essential.
Produces ATP, NADPH, and oxygen (by-product).
ATP and NADPH are used in the Calvin Cycle (covered in another tutorial).
Importance: Light and water are critical for plant survival and photosynthesis continuation. Without them, photosynthesis stalls, leading to plant death.