Overview
This lecture covers the light-dependent reactions of photosynthesis, detailing where they occur, the key processes involved, and the main products formed.
Stages of Photosynthesis
- Photosynthesis consists of two stages: the light-dependent and light-independent reactions.
- Both stages take place in the chloroplast but in different locations.
Location of Light-Dependent Reactions
- Light-dependent reactions happen in the thylakoid membranes (also called grana; singular: granum).
Main Processes in Light-Dependent Reactions
- Light energy is required to split water and drive the production of ATP and reduced NADP (NADPH).
- Four main steps: photoionisation of chlorophyll, photolysis of water, ATP production, and chemiosmosis.
Photolysis of Water
- Photolysis means light energy splits water molecules into oxygen, electrons, and protons (hydrogen ions).
- Symbol equation: H₂O → O₂ + 2e⁻ + 2H⁺.
- Protons are picked up by NADP to form reduced NADP; oxygen is a waste product.
Photoionisation of Chlorophyll
- Photoionisation: light energy excites chlorophyll electrons, causing them to be released from the molecule.
- Excited electrons are transferred to proteins in the electron transport chain.
Electron Transport Chain & Chemiosmosis
- Electrons move down proteins in the electron transfer chain, releasing energy.
- Energy from electrons pumps protons into the thylakoid lumen, creating an electrochemical gradient.
- Protons diffuse back to the stroma via ATP synthase, driving the formation of ATP from ADP and inorganic phosphate.
- Protons and electrons at the end of the chain are picked up by NADP, forming reduced NADP.
Summary of Products
- ATP and reduced NADP (NADPH) are produced for use in the light-independent reactions.
- Oxygen is released as a by-product.
Key Terms & Definitions
- Thylakoid membrane — site of light-dependent reactions within chloroplasts.
- Photoionisation — process where chlorophyll loses electrons after absorbing light.
- Photolysis — splitting of water into oxygen, electrons, and protons using light energy.
- Electron Transport Chain (ETC) — series of proteins that transfer electrons and release energy.
- Chemiosmosis — movement of protons through ATP synthase, producing ATP.
- Reduced NADP (NADPH) — NADP after gaining electrons and protons during light-dependent reactions.
Action Items / Next Steps
- Review the steps and key terms for photosynthesis light-dependent reactions.
- Be able to identify where each process occurs within the chloroplast.
- Prepare for potential exam questions on the location and key functions of each reaction.