Why organisms like seaweed and bacteria can carry out photosynthesis?

Organisms like those can carry out photosynthesis because photosynthesis is achieved in organelles called chloroplasts which contain chlorophyll; this is what turn the sunlight, water, and CO2 into glucose. And it can be found in cells, they are not made of cells. Seaweed is basically a larger form of algae or euglena which is a bacteria, and the cells of this contain chloroplasts.