Photosystems as Arrays of Pigment Molecules
Definition
Photosystem
A photosystem is a pigment-protein complex that absorbs light energy and transfers it to a reaction center, where it excites electrons for use in photosynthesis.
- Photosystems are pigment-protein complexes that capture light energy and use it to excite electrons.
- These electrons are then emitted and used in photosynthesis.
- In cyanobacteria (photosynthetic bacteria), photosystems are in the cell membrane or internal thylakoid-like membranes.
- In photosynthetic eukaryotes (plants, algae), photosystems are embedded in the thylakoid membranes of chloroplasts.
- Think of a photosystem as a solar panel.
- Just as a solar panel captures sunlight and converts it into electricity, a photosystem absorbs light energy and transforms it into chemical energy by exciting electrons.
Structure of a Photosystem
Photosystems are highly organized assemblies of pigments and proteins, designed to efficiently capture and convert light energy.
1. Antenna Complex (Light-Harvesting Complex)
- Contains hundreds of chlorophyll molecules and accessory pigments (carotenoids, xanthophylls).
- These pigments are arranged in a molecular array that captures light energy.
- When a pigment absorbs a photon, its electron becomes excited (higher energy state).
- The excitation energy is transferred from pigment to pigment until it reaches the reaction center.
- Energy is transferred between pigments, not electrons.
- Think of it like passing a ball, the ball (energy) moves, but the players (pigment molecules) stay in place.
How Photosystems Work
- Step 1: Light Absorption
- A pigment molecule in the antenna complex absorbs a photon of light.
- This excites an electron in that pigment to a higher energy level.
- Step 2: Energy Transfer
- The excitation energy (not the electron itself) is transferred from pigment to pigment through the antenna complex.
- Energy moves toward the reaction center.
- Step 3: Electron Excitation at Reaction Center
