Photosynthesis: Light-Dependent Reactions

• Photosynthesis happens in chloroplasts.
• It combines carbon dioxide, water, and energy from sunlight to synthesize glucose (C6H12O6).
• Oxygen is released as a by-product.
• Photosynthesis includes two separate sets of reactions: Light-Dependent and Light-Independent.

Light-Dependent Reactions (PHOTO)

• Occurs in the thylakoid membranes.
• Captures solar energy and transfers it to electrons by splitting water (H2O).
• Uses energy to make ATP and transfer electrons to make NADPH.
• Requires sunlight and water.
• Produces ATP, NADPH, and oxygen.
• Solar energy is trapped by chlorophyll and is used to generate high-energy compounds like ATP and NADPH.

Light-Independent Reactions (SYNTHESIS)

• Known as Carbon Fixation and happens through the Calvin Cycle.
• Occurs in the stroma.
• Uses energy in ATP and high-energy electrons in NADPH to form glucose from carbon dioxide (CO2).
• Requires ATP, NADPH, and carbon dioxide.
• Produces G3P (glyceraldehyde-3-phosphate).
• Energy from ATP and NADPH is used to reduce carbon dioxide and make glucose.

Two Photosystems

• Photosystems are clusters of chlorophyll and other pigments found in the thylakoid membrane.
• Photosystem II (PSII) comes first in the light reaction.

Stage 1: Capture Solar Energy

• Light hits chloroplasts where the photons excite chlorophyll molecules in the thylakoid membranes.
• Chlorophyll captures light energy by absorbing photons, passing the energy to electrons.
• Subsequently, energy is transferred to the reaction center.
• Light energy splits water (photolysis).
• Photolysis happens in the thylakoid lumen, where water is divided into hydrogen ions, oxygen, and electrons.
• Oxygen is released into the atmosphere, exiting through the stoma.
• Hydrogen ions (H+) remain in the lumen.
• Electrons are excited and move to chlorophyll molecules in photosystem II, which is found in the thylakoid membrane
• Excited electrons are then transferred to photosystem I through the electron transport chain.

How Electrons Move?

• Pigments in photosystem II capture light energy and transfer it to an electron.
• The electron moves down the electron transport chain (ETC) in a staircase manner.
• Electron acceptor molecules carry the electron down the ETC between photosystems.
• Electrons are passed from one molecule to the next towards photosystem I.
• Each molecule has a stronger pull on the electron and transfer it to the next molecule.

Products of Photon Activity

• Photons cause hydrolysis, splitting water into oxygen and hydrogen ions.
• Molecules donate electrons back to photosystem I.
• Oxygen is released into the environment.
• Hydrogen ions accumulate inside the thylakoid lumen and create a concentration gradient.

Stage 2: Making ATP and NADPH

• As electrons move from photosystem II to photosystem I, energy is released.
• This potential energy is then used to make ATP.
• Electrons reach photosystem I, they are hit with light again and re-excited.
• Then, these re-energized electrons move down the ETC, releasing energy.
• The energy released by photosystem I is used to rejoin high-energy electrons with hydrogen ions and NADP+ to produce NADPH.
• NADP+ gains electrons to form NADPH through reduction.
• NADPH carries electrons and their energy to the Light-Independent Reaction (Stage 3 - the Calvin Benson Cycle).

Making ATP

• The electrons are passed down the ETC from PSII to PSI, then, the energy released is used to pull hydrogen ions into the thylakoid membrane against the concentration gradient.
• As hydrogen ions build up inside the thylakoid lumen, they create a positive charge and a steep concentration gradient.
• The thylakoid membrane is impermeable to hydrogen ions.
• ATP synthase, embedded in the thylakoid membrane, provides the only pathway for hydrogen ions to move out.
• Synthesizing ATP uses the energy from the hydrogen ion gradient and the ATP synthase enzyme.
• The movement of hydrogen ions through the complexes releases energy, which is then used to combine ADP and Pi into ATP.
• Energy from light indirectly phosphorylates ADP into ATP.

Light-Dependent Reaction Summary

• Light energy and water are required at the beginning.
• Oxygen, NADPH, and ATP are produced at the end.

Explore the light-dependent reactions of photosynthesis, focusing on the roles of Photosystem II, the electron transport chain, and photolysis. Understand how these processes contribute to ATP and NADPH production within the thylakoid membrane to facilitate the synthesis of energy.