A
thylakoid is a membrane-bound compartment inside
chloroplasts and
cyanobacteria. They are the site of the
light-dependent reactions of
photosynthesis. Thylakoids consist of a
thylakoid membrane surrounding a
thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as
grana (singular:
granum). Grana are connected by
intergranal or
stroma thylakoids, which join granum stacks together as a single functional compartment.