Photosynthesis: Light and Dark Reactions [short]

Photosynthesis Overview

Stage I: Light Reactions

• The overall goal of the light reactions is to convert light energy into chemical energy in the form of ATP and NADPH.
• Using light energy harnessed by pigments (such as chlorophyll), electrons are excited and passed along the electron transport chain (ETC) to set up an electrochemical proton gradient which is used to drive ATP synthesis.
• The final electron acceptor is NADP+ which becomes NADPH.

• Pigments are organized in photosystems.
• A photosystem has two parts: the light harvesting (antenna) complex + the reaction center.
• They both:
• Absorb light energy;
• Pass it from pigment to pigment in the light harvesting complex;
• Energy ends up exciting chlorophyll a in the reaction center;
• Chlorophyll a gives off an electron to an electron acceptor.
• In photosystem II:
• Chlorophyll a regains electrons it lost by splitting water (releases O2).
• The excited electron is passed down members of the electron transport chain (ETC).
• Its energy is used to drive protons (H+) into the lumen.
• The electron ends up in chlorophyll a of photosystem I.

• In photosystem I:
• Absorbs light energy, gives off an electron and passes it to an enzyme which uses the electrons to reduce NADP+ to NADPH.
• The electrons donated by the reaction center are replaced by photosystem II.
• Proton gradient is used to generate ATP by ATP Synthase.

Stage II: Dark Reactions (Calvin Cycle)

• Happens in the stroma.
• ATP and NADPH that result from Stage I are used for carbon fixation (production of organic molecules from CO2).
• Rubisco is the main enzyme responsible for combining CO2 with an organic molecule.
• Glyceraldehyde-3-phosphate (G3P) is made. G3P is used to produce other sugars such as glucose.