Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. These membranes are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.