Chloroplasts occur in most cells of the green plant undergoing photosynthesis.
Plant chloroplasts are typically lens-shaped and bounded by a double membrane. The light reaction of photosynthesis occurs on the thylakoid membranes while the dark reaction takes place in the stroma. Mitochodrion is a structure within the cytoplasm of plant and animal cells, containing enzymes responsible for energy production during aerobic respiration. Mitochondria vary greatly in shape, size, and number but are typically oval or sausage-shaped and bounded by two membranes. They are most numerous in cells with a high level of metabolic activity.