What is a reaction centre? Explain.

(N/A) The reaction centre is the specific site within a photosystem where the primary photochemical reaction of photosynthesis occurs.
$1$. In a photosystem,the reaction centre consists of a single molecule of chlorophyll $a$ along with primary electron acceptors.
$2$. The reaction centre is surrounded by hundreds of pigment molecules (accessory pigments) organized into Light Harvesting Complexes $(LHC)$.
$3$. These $LHC$ molecules absorb light energy of various wavelengths and transfer it to the reaction centre.
$4$. When the reaction centre receives this energy,it becomes excited and releases an electron,initiating the electron transport chain.
$5$. There are two types of photosystems: Photosystem-$I$ $(PS-I)$ and Photosystem-$II$ $(PS-II)$.
$6$. In $PS-I$,the reaction centre chlorophyll $a$ has an absorption peak at $700 \ nm$ (known as $P700$),while in $PS-II$,it has an absorption peak at $680 \ nm$ (known as $P680$).