Overview
This lecture explains how light energy is converted into chemical energy during photosynthesis, focusing on the light-dependent reactions in plant chloroplasts.
Photosynthesis Basics
- Photosynthesis uses light to make sugars and organic compounds in plants.
- Plants absorb carbon dioxide and water to produce glucose and oxygen gas.
Structure and Function of Chloroplasts
- Chloroplasts in plant cells contain thylakoids, which resemble solar panels and absorb light.
- Thylakoids have pigments (like chlorophyll) that capture sunlight.
Light-Dependent Reactions
- Light-dependent reactions require light and take place in the thylakoid membranes of chloroplasts.
- Pigments are arranged in clusters called photosystems, mainly Photosystem II and Photosystem I.
- When Photosystem II absorbs light, its electrons get excited and move to the electron transport chain.
- To replace lost electrons, water is split, producing electrons, hydrogen ions, and oxygen gas.
- Electrons travel the electron transport chain, powering the movement of hydrogen ions into the thylakoid.
- High hydrogen ion concentration drives ions through ATP synthase, generating ATP.
- Electrons then reach Photosystem I, are re-energized by light, and travel down a second electron transport chain.
- NADP+ accepts these excited electrons and hydrogen to become NADPH.
Products of Light-Dependent Reactions
- ATP and NADPH, produced during light-dependent reactions, are used in the next stage, the Calvin Cycle.
- Oxygen is released as a byproduct when water is split.
Key Terms & Definitions
- Photosynthesis — process by which plants use light to produce sugars and oxygen.
- Chloroplast — organelle in plant cells where photosynthesis occurs.
- Thylakoid — membrane-bound structure in chloroplasts containing photosynthetic pigments.
- Chlorophyll — main pigment that absorbs sunlight for photosynthesis.
- Photosystem — cluster of pigments in thylakoid membranes that absorb light.
- Electron Transport Chain — series of molecules that transfer electrons to produce ATP.
- ATP (Adenosine Triphosphate) — energy-carrying molecule produced in light-dependent reactions.
- NADPH — electron carrier molecule produced in light-dependent reactions.
Action Items / Next Steps
- Review the Calvin Cycle in the next lecture/video.
- Optional: Try the Photosynthesis Interactive activity at BioMan Biology.