chloroplast

noun

(Gr. chloros: light green, greenish yellow; plastos: modelled, formed) Any of the chlorophyll-containing organelles (see plastid) that are found in large numbers in those plant and algal cells undergoing photosynthesis. Plant chloroplasts are typically lens-shaped and bounded by a double membrane. They contain membranous structures called thylakoids, which are piled up into stacks (see granum), surrounded by a gel-like matrix (stroma). The light reaction of photosynthesis occurs on the thylakoid membranes while the dark reaction takes place in the stroma.