For a long time, the three-dimensional structure of the thylakoid membrane system had been unknown or disputed. Many models have been proposed, the most prevalent being the helical model, in which granum stacks of thylakoids are wrapped by helical stromal thylakoids. Another model known as the 'bifurcation model', which was based on the first electron tomography study of plant thylakoid membranes, depicts the stromal membranes as wide lamellar sheets perpendicular to the grana columns which bifurcates into multiple parallel discs forming the granum-stroma assembly. The helical model was supported by several additional works, but ultimately it was determined in 2019 that features from both the helical and bifurcation models are consolidated by newly-discovered left-handed helical membrane junctions. Likely for ease, the thylakoid system is still commonly depicted by older "hub and spoke" models where the grana are connected to each other by tubes of stromal thylakoids.