A stack of thylakoids.
Stacking places many thylakoid membranes in a compact region of a chloroplast. The membranes remain part of a connected internal system; the stack is a structural arrangement, whereas the thylakoid lumen is the enclosed aqueous compartment. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapters 12, 18 and 19, compartments, division and cell junctions
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