Biology: Chap. 7

Photosystem Structure

• Photosystems contain proteins that organize chlorophyll.
• A cluster of proteins exists within the photosystem.
• Chlorophyll and other dyes are present, with a focus on chlorophyll.
• The arrangement of chlorophyll is highly organized, not random.
• At the base is the reaction center, composed of two special chlorophyll units.
• These units are chemically identical to other chlorophyll but are located in a special position, causing them to behave differently.

Function of Photosystems

• The role of the photosystem is to capture light energy and convert it to chemical energy.
• This conversion involves oxidation-reduction reactions.

Chlorophyll and Sunlight

• When a chlorophyll unit absorbs sunlight (a packet of sunlight):
  • An electron within the chlorophyll is promoted to a higher energy level.
• The chlorophyll doesn't prefer to remain in this state.
• The chlorophyll passes on the energy after absorption.
• The method by which chlorophyll absorbs energy depends on its location.

Three Ways Chlorophyll Transfers Energy

1. Fluorescence/Emission:

   • Occurs when chlorophyll is isolated (e.g., in a tube).
   • The electron falls back down, releasing energy as light and heat.

2. Resonance Energy Transfer:

   • Happens in the photosystem, but outside the reaction center.
   • Involves neighboring chlorophyll units.
   • When one chlorophyll's electron is promoted, its neighbor detects it.
   • This causes the neighbor's electron to be promoted, and the original electron falls back down.
   • Electrons aren't physically moving between chlorophyll molecules.
   • The energy is transferred, causing sequential electron promotions and demotions.
   • This process funnels energy towards the reaction center.

3. Oxidation in the Reaction Center:

   • The chlorophyll is oxidized.
   • The electron is promoted and then leaves to an electron acceptor.
   • This is where light energy is converted to chemical energy by creating reducing power.
   • The electron acceptor delivers electrons to other components in light-dependent reactions.

Resonance Energy Transfer Details

• Sunlight strikes a chlorophyll unit.
• Resonance energy transfer begins, creating a chain reaction.
• Energy is funneled down to the reaction center.
• The process acts as an antenna, focusing sunlight.

Photosystems Interaction

• There are two photosystems, and they interact with each other.