Overview
This lecture covers the light-dependent stage of photosynthesis, focusing on the photolysis of water, ATP production, and the formation of reduced NADP.
Structure and Location of the Light-Dependent Stage
- The light-dependent stage is the first part of photosynthesis and occurs on the thylakoid membranes inside the grana of chloroplasts.
- Thylakoid membranes are organized into stacks called grana.
Steps of the Light-Dependent Stage
- The process is divided into three main steps: photolysis of water, production of ATP (photophosphorylation), and formation of reduced NADP.
Photolysis of Water
- Photolysis splits one water molecule (H₂O) into oxygen (O₂), electrons, and hydrogen ions (protons) using sunlight energy.
- The reaction is catalyzed by an enzyme within photosystem II (PSII) on the thylakoid membrane.
- From each water molecule, two electrons and two hydrogen ions are produced.
- The oxygen formed is used by the plant for respiration or diffuses out through stomata if not needed.
- The electrons and protons generated are essential for the next step, photophosphorylation.
Production of ATP (Photophosphorylation)
- ADP (adenosine diphosphate) combines with an inorganic phosphate group in the presence of light to form ATP (adenosine triphosphate).
- This process is called photophosphorylation: "photo" for light, "phosphorylation" for adding a phosphate group.
Formation of Reduced NADP
- NADP (nicotinamide adenine dinucleotide phosphate) acts as a coenzyme and accepts hydrogen atoms, becoming reduced NADP.
- Reduced NADP is produced at the end of the light-dependent reactions.
Key Terms & Definitions
- Thylakoid membrane — site within chloroplasts where the light-dependent reactions occur.
- Grana (Granum) — stacks of thylakoid membranes.
- Photolysis — splitting of water using light energy.
- Photosystem II (PSII) — protein complex involved in capturing light and catalyzing photolysis.
- Photophosphorylation — production of ATP from ADP and phosphate using light energy.
- NADP — coenzyme that accepts hydrogen to become reduced NADP.
- Stomata — pores in leaves for gas exchange.
Action Items / Next Steps
- Review the steps and key terms of the light-dependent stage.
- Prepare for further study on the next stages of photosynthesis.