Any of the flattened saclike membranous structures that are stacked on top of one another to form the grana (see granum) within the stroma 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 thylakoid space between the thylakoid membranes. Thylakoid membranes also occur in photosynthetic prokaryotes such as cyanobacteria.