What are the stacked discs of membranes in chloroplasts called?

The term "granum" refers specifically to the stacked discs of thylakoids found within chloroplasts. Each granum consists of multiple thylakoid membranes, which are the sites of the light-dependent reactions of photosynthesis. This stacking enhances the efficiency of light absorption by increasing the surface area available for chlorophyll and other pigments that capture light energy.

In this context, thylakoids are the individual membrane-bound structures that host the photosynthetic machinery, while the stroma is the fluid matrix surrounding the thylakoids within the chloroplast, containing enzymes and other substrates necessary for the light-independent reactions. The cytoplasm, distinct from the chloroplast structures, refers to the fluid medium within the cell that contains organelles and is responsible for cellular processes. Understanding these terms helps delineate the specific functions and organization of chloroplast components, with granum being a crucial unit in the light-harvesting process of photosynthesis.