Final answer:
The structure not found in a photosystem is the cytoplasmic matrix. Photosystems are involved in photosynthesis and are composed of a reaction center, antenna pigments, and primary electron acceptors, but not the cytoplasmic matrix.
Step-by-step explanation:
The structure that is not a component of a photosystem is the cytoplasmic matrix. Photosystems, which are integral to the process of photosynthesis, consist of various components but the cytoplasmic matrix is not one of them. Photosystems are embedded in the thylakoid membrane of chloroplasts and include a reaction center, antenna pigments, and primary electron acceptors, all crucial for capturing and converting light energy into chemical energy.