Chloroplast structure: from chlorophyll granules to supra-molecular architecture of thylakoid membranes

This review provides a brief historical account of how microscopical studies of chloroplasts have contributed to our current knowledge of the structural and functional organization of thylakoid membranes. It starts by tracing the origins of the terms plastid, grana, stroma and chloroplasts to light microscopic studies of 19th century German botanists, and then describes how different types of electron microscopical techniques have added to this field. The most notable contributions of thin section electron microscopy include the elucidation of the 3-D organization of thylakoid membranes, the discovery of prolamellar bodies in etioplasts, and the structural changes in thylakoid architecture that accompany the light-dependent transformation of etioplasts into chloroplasts. Attention is then focused on the roles that freeze-fracture and freeze-etch electron microscopy and immuno electron microscopy have played in defining the extent to which the functional complexes of thylakoids are non-randomly distributed between appressed, grana and non-appressed stroma thylakoids. Studies reporting on how this lateral differentiation can be altered experimentally, and how the spatial organization of functional complexes is affected by alterations in the light environment of plants are also included in this discussion. Finally, the review points to the possible uses of electron microscope tomography techniques in future structural studies of thylakoid membranes.