thylakoid Any of the flattened saclike membranous structures that are stacked on top of one another to form the grana (see granum) of plant chloroplasts. Chlorophyll and other photosynthetic pigments are situated in the thylakoid membranes, which are the site for the light-dependent reactions of photosynthesis (see also photophosphorylation). The photolysis of water (see photosystems I and II) occurs in the space between the thylakoid membranes.

thylakoid One of the membranaceous discs or sacs that form the principal sub-units of a granum in chloroplasts.