chloroplast

Typical plant and animal cell. Plant and animal cells share many structures, such as ribosomes, mitochondria, and chromosomes, but they also have notable differences: plant cells have chloroplasts, a large vacuole, and a cellulose cell wall. Animal cells do not have a rigid cell wall but have an outside cell membrane only.

A chloroplast is shaped like a flattened disc, with a double membrane enclosing the gel-like stroma. The stroma contains cavities, or vesicles, where photosynthesis occurs.

Structure (organelle) within a plant cell containing the green pigment chlorophyll. Chloroplasts occur in most cells of green plants that are exposed to light, often in large numbers. Typically, they are shaped like a flattened disc, with a double membrane enclosing the stroma, a gel-like matrix. Within the stroma are stacks of fluid-containing cavities, or vesicles, where photosynthesis occurs, creating glucose from carbon dioxide and water to be used in the plant's life processes. Sunlight is absorbed by chlorophyll, providing energy which is transferred to the glucose. The glucose may be converted to starch and stored. Starch can then be converted back to glucose to provide energy for the plant at a later stage.

It is thought that the chloroplasts were originally free-living cyanobacteria which invaded larger, non-photosynthetic cells and developed a symbiotic relationship with them. Like mitochondria, they contain a small amount of DNA and divide by fission. Chloroplasts are a type of plastid.