The chloroplast organelle, which performs photosynthesis in plants, is more complex, structurally, than the mitochondria which use oxygen to turn food molecules into ATP (energy-carrying chemical; an abstract interface) in most eucaryotes. Eucaryotes are cells with a nucleus--cells without a nucleus are procaryotes, which come as eubacteria and archaea.
Mitochondria, themselves, appear to be bacteria captured by procaryotes very early in the evolution of cells with nuclei, and now the two live symbiotically.
Two crazy things:
Life: amazing. (Current reading is ECB2. Lots to learn.)