What is Chloroplast?

The chloroplast is the organelle of photosynthesis: a lens-shaped body some $5\,\text{µ}\mathrm{m}$ long, bounded by a double membrane, containing a fluid, the stroma, in which lie stacks of flattened membrane sacs, the thylakoids. The thylakoid membranes hold the pigments — chlorophylls a and b, green, and orange carotenoids — and the proteins that convert light energy; the stroma holds the enzymes that build sugar. A leaf cell contains tens of chloroplasts, a square millimetre of leaf some half a million.

The chloroplast is a lens-shaped organelle $3\text{ to }10\,\text{µ}\mathrm{m}$ long bounded by two envelope membranes; its interior, the stroma, holds the enzymes of carbon fixation, starch grains, ribosomes and a circular DNA; within the stroma a third membrane system, the thylakoids, forms flattened sacs stacked into grana and connected by lamellae, enclosing one continuous thylakoid lumen. The light reactions take place in the thylakoid membrane; carbon fixation in the stroma. A leaf cell holds $20\text{ to }100\,$ chloroplasts; a square millimetre of leaf, half a million.