Why do thylakoid membranes from higher plants form grana stacks?

Chloroplasts contain a system of membrane sacs, the thylakoids, some of which are stacked to form grana (singular, granum), whereas others float freely in the stroma. It is on the thylakoid membranes that the electron carriers necessary for photosynthesis reside. There has been continuous speculation and discussion about the function of the grana ever since Menke postulated their lamellar nature in 1939. On the basis of new insights into the biophysics of the two photosystems and the molecular organization of thylakoid membranes of algae that exhibit a different lateral heterogeneity from that of higher plants, we propose that the membrane stacking found in the chloroplasts of higher plants and green algae is just one way in which Nature implements a general principle, namely that of physically separating a slow (PS II) and a fast (PS I) photosystem.