C1.3.9 Photosystems (HL) Notes

Photosystems as Arrays of Pigment Molecules

1. Photosystems are pigment-protein complexes that capture light energy and use it to excite electrons.
2. These electrons are then emitted and used in photosynthesis.
3. In cyanobacteria (photosynthetic bacteria), photosystems are in the cell membrane or internal thylakoid-like membranes.
4. In photosynthetic eukaryotes (plants, algae), photosystems are embedded in the thylakoid membranes of chloroplasts.

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)

1. Contains hundreds of chlorophyll molecules and accessory pigments (carotenoids, xanthophylls).
2. These pigments are arranged in a molecular array that captures light energy.
3. When a pigment absorbs a photon, its electron becomes excited (higher energy state).
4. The excitation energy is transferred from pigment to pigment until it reaches the reaction center.

How Photosystems Work

1. Step 1: Light Absorption
   1. A pigment molecule in the antenna complex absorbs a photon of light.
   2. This excites an electron in that pigment to a higher energy level.
2. Step 2: Energy Transfer
   1. The excitation energy (not the electron itself) is transferred from pigment to pigment through the antenna complex.
   2. Energy moves toward the reaction center.
3. Step 3: Electron Excitation at Reaction Center
   1. Energy reaches the special chlorophyll at the reaction center.
   2. An electron in the special chlorophyll becomes excited to a very high energy level.
4. Step 4: Electron Emission
   1. The special chlorophyll emits the excited electron to an electron acceptor molecule.
   2. The electron is now separated from the photosystem and enters the electron transport chain.
   3. The special chlorophyll is left with an "electron hole" that must be refilled (in PSII, this is done by splitting water; in PSI, by electrons from PSII).

There Are Two Types of Photosystems

Photosystem II (PSII)

1. Contains special chlorophyll P680.
2. Located in the grana (stacked regions) of thylakoids.
3. Emits electrons that enter the electron transport chain.
4. Replaces lost electrons by splitting water (photolysis—covered in next article).

Photosystem I (PSI)

1. Contains special chlorophyll P700.
2. Located in stroma lamellae (unstacked regions) of thylakoids.
3. Receives electrons from PSII (via the electron transport chain).
4. Emits electrons that are used to reduce NADP⁺ to NADPH.