Observations on the Structure of Grana-Containing Chloroplasts and a Proposed Model of Chloroplast Structure

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Abstract
Observations have been made by phase contrast and fluorescence microscopy on chloroplasts both in living cells of higher plants and in an isolated state. In the living cell the chloroplast is often surrounded by a mobile jacket of material which resembles mitochondrial substance. Many chlorplasts retain this outer jacket after isolation. The behavior of isolated chloroplasts in hypotonic sucrose solutions indicates that this jacket of mitochondria-like material is the only all-encompassing structure around the chloroplast. The jacket swells and ruptures in 0.1-0.2 M sucrose solution. At still lower s ucrose concentrations the remaining chloroplast behaves as though composed of numerous individual osmotic units. Many small blebs appear around the margin of the chloroplast and with increasing dilution these swell further, often to a larger size than the original chloroplast. The inner surfaces of these swollen blebs can fuse, but the outer surfaces do not have this property. Grana are fairly resistant to swelling in hypotonic solutions and many are well preserved even in distilled water. A model is proposed for the structure of a typical grana-containing chloroplast, and the swelling patterns which have been observed are interpreted in terms of this model. The chloroplast is depicted as a stack of flat sealed bags, each bag corresponding to a pair of stroma lamellae. The many blebs which form in sucrose concentrations below 0.1 M are envisaged as individual swollen stroma bags. It is suggested that the chloroplast jacket is commonly represented as a double-layered outer membrane in electron-micrographs.