Photosystems: Within the thylakoid membrane are two separate photosystems, Photosystem I (PSI) and Photosystem II (PSII). These function to harness energy from light photons.

Pigment-protein complexes also contain specific electron transfer components that are important in harnessing energy from the process of photosynthesis. The local organization within the thylakoid membrane is such that there are actually two separate photosystems (See Figure: Photosystems). Each contains an array of antenna chlorophylls and carotenoids.

In the Figure, Antenna, you can see a depiction of the arrangement of antenna pigments within a single photosystem. These pigments represent the majority of the pigment molecules in the photosystems.

They function solely to harvest light energy and transfer it to a small number of pigment-protein complexes called reaction centers. In the reaction center, the energy from the photon is used to excite an electron to a higher energy level (also a lower redox potential) so that it can be transferred to an acceptor molecule that has a higher energy level than the original reaction center. The acceptor molecule is then considered to be reduced. In this way, the energy from photons is used to 'pump' electrons to higher energy levels. Each time the reaction center loses an electron, it becomes oxidized and is able to accept electrons from an external source.

Antenna: Antenna pigments play a vital role in the harnessing of photon energy.