Overview
This lecture explains how plants use light energy in photosynthesis to create glucose and oxygen, focusing on the light-dependent reactions inside chloroplasts.
Introduction to Photosynthesis
- Photosynthesis converts light energy into chemical energy to produce organic molecules like sugars.
- Plants take in carbon dioxide and water to make glucose and release oxygen.
- The process has two main stages: light-dependent reactions and the Calvin Cycle.
Structure and Location of Photosynthesis
- Photosynthesis occurs in chloroplasts within plant cells.
- Chloroplasts contain thylakoids, disc-shaped structures where light is absorbed.
- Thylakoids contain pigments such as chlorophyll arranged in photosystems.
Light-Dependent Reactions
- These reactions require light and take place in the thylakoid membranes.
- Photosystem II absorbs light, energizing electrons from chlorophyll.
- Excited electrons leave chlorophyll, moving to the electron transport chain.
- Chlorophyll's lost electrons are replaced by electrons from water, splitting water into electrons, hydrogen ions, and oxygen.
- Oxygen produced is released as a byproduct.
- The electron transport chain pumps hydrogen ions into the thylakoid, creating a concentration gradient.
- Hydrogen ions flow through ATP synthase enzyme, generating ATP from ADP.
- Electrons reach Photosystem I, are re-energized by light, and move down a second electron transport chain.
- Electrons and hydrogen are transferred to NADP+, forming NADPH, an electron carrier.
- ATP and NADPH produced are used in the Calvin Cycle to synthesize sugars.
Key Terms & Definitions
- Photosynthesis — Process by which plants use light to synthesize sugars from carbon dioxide and water.
- Chloroplast — Organelle where photosynthesis occurs.
- Thylakoid — Membrane-bound compartments inside chloroplasts containing light-absorbing pigments.
- Chlorophyll — Green pigment that absorbs light energy.
- Photosystem — Cluster of pigments in thylakoid membranes that captures light energy.
- ATP synthase — Enzyme that makes ATP from ADP using a flow of hydrogen ions.
- ATP — Energy-carrying molecule produced during photosynthesis.
- NADPH — Electron and hydrogen carrier molecule produced in the light-dependent reactions.
Action Items / Next Steps
- Review the process of light-dependent reactions before learning about the Calvin Cycle.
- Optional: Explore the Photosynthesis Interactive at BioMan Biology.