Chloroplasts are plastid organelles found in the cells of plants and algae. They contain chlorophyll and carotenoid pigments, and an elaborate system of internal membranes called thylakoids. They are active in photosynthesis, the all-important process that converts light into chemical energy. In algae, chloroplasts exist in a wide variety of sizes and shapes. Depending on the algal species, they may be simple disks or elaborate ribbons twisted into a spiral. Plant chloroplasts are usually disk-shaped and measure between 4 and 6 micrometers in diameter. They are especially abundant in leaf mesophyll tissue where a single cell may contain as many as 50. Specialized chloroplasts lacking grana, but not chlorophyll, are found in the bundle-sheath cells of plants with the C₄ or Hatch-Slack photosynthetic pathway for carbon fixation.

The internal structure of chloroplasts is very complex, with a structure that facilitates the capture of light photons and channels the energy of the...