Chloroplasts have a fluid-filled space called the stroma, flattened sacs, called thylakoids are stacked into structures called grana. Chlorophyll is contained within the thylacoid membranes. Mitochondria have a fluid-filled space called the matrix. The inner membrane invaginates to form cristae. Most membrane-bounded organelles are held in place by attachment to the cytoskeleton.
Organelles occupy nearly half the volume of the cell.