Photosynthesis 11-1

Overall Reaction:

• Reactants: Carbon dioxide + Water

• Products: Sugar + Oxygen

Two Phases: (Light and Dark Reactions)

1. Light Reactions:

   • Reactants: Light + Water (H₂O)

   • Products: Oxygen (O₂) + ATP + NADPH

   • Location: Thylakoid membranes

   • Process:

     • Photosystems are complexes of proteins and pigments that capture light energy.

     • Photosystem II (PSII):

       • First photosystem (named second in order of discovery).

       • Located in the thylakoid membrane.

       • Absorbs light energy (P680 chlorophyll a) to split water molecules into oxygen, protons, and electrons.

       • Excited electrons move through a series of proteins, creating a proton (H⁺) gradient that drives ATP synthesis via ATP synthase, similar to cellular respiration.

     • Photosystem I (PSI):

       • Also located in the thylakoid membrane.

       • Absorbs light energy to produce NADPH.

       • Peak absorption wavelength is due to chlorophyll a molecules (P700).

       • Excited electrons transfer to NADP⁺, reducing it to NADPH.

Redox Reactions Review:

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

Electron Transport Chain (ETC):

• Electrons from Photosystem II (PSII) are transferred through the electron transport chain to Photosystem I (PSI).

• The energy from these electrons is used to produce ATP and NADPH.

2. Dark Reactions (Light-Independent Reactions):

• Location: Stroma of the chloroplasts

• Process: Uses energy from ATP and NADPH to build sugars from carbon dioxide (CO₂).

• Calvin Cycle (Light-Independent Reactions):

  • Reactants: ATP + NADPH + Carbon Dioxide (CO₂)

  • Products: Sugars

  • Location: Stroma

Calvin Cycle Steps:

1. Carbon Fixation

2. Reduction

3. Regeneration of RuBP

Overall Reaction (Calvin Cycle):

Photorespiration:

• C3 plants experience photorespiration, where the enzyme RuBisCO fixes oxygen instead of CO₂.

• C4 plants and CAM plants have adapted mechanisms to minimize photorespiration.