Chloroplast

**Semantic type:** Cell Component

**Definition:** Photosynthetic organelle containing chlorophyll, the photosynthetic pigment, in cells of leaves and young stems of higher plants. Lens-shaped and rather variable in size but approximately 5 m long. Surrounded by a double membrane and contains circular DNA (though not enough to code for all proteins in the chloroplast). Like the mitochondrion, it is semi-autonomous. It resembles a cyanobacterium from which, on the endosymbiont hypothesis, it might be derived. Chlorophyll is associated with the membrane of vesicles (thylakoids) that are stacked to form grana.