What structure is composed of stacked thylakoids within chloroplasts?

The structure composed of stacked thylakoids within chloroplasts is known as a granum. A granum is an essential component of the chloroplast's internal structure, primarily involved in the light-dependent reactions of photosynthesis. Each granum is made up of multiple thylakoids stacked together, which increases the surface area available for capturing light energy. This arrangement facilitates the efficient absorption of light and the subsequent conversion of that energy into chemical energy.

The stroma refers to the fluid matrix surrounding the thylakoids within the chloroplasts, where the light-independent reactions (Calvin cycle) occur, but it is not a stack of thylakoids. A thylakoid is an individual membrane-bound compartment within the chloroplast, containing chlorophyll and other pigments necessary for photosynthesis, but does not refer to the stacked form. Chlorophyll is the green pigment found within the thylakoid membranes, crucial for absorbing light energy, but it is not a structural arrangement. Thus, the term granum specifically denotes the stacked organization essential for maximizing light capture in the photosynthetic process.