The photosynthetic unit on the thylakoid of chloroplast is called photosynthetic unit. Each photosynthetic unit contains 300-400 molecules of chlorophyll-a, chlorophyll-b, carotene and xanthophyll. It contains various enzymes, phospholipids, sulfolipids, metal ions, quinones, etc. Photosynthetic units absorb light energy and cause photophosphorylation. Earlier the photosynthetic unit was called quantosome. Quantum comes from quantozome. Quantum means indivisible unit of energy.