A computer analysis of a spiral, string-of-grana model of the three-dimensional structure of chloroplasts

Using uniform grana whose diameter:thickness ratio was 5:1, a spiral, string-of-grana chloroplast model was constructed in a computer as a system of three-dimensional coordinates. None of the grana overlapped each other, and starch grains occupied the cavity beneath the spiral arrangement of grana. Visualization of internal organization was possible by examining computer-produced graphic plots of simulated sections through the model. The dimensions and form of the model were derived from light-microscopic observations of chloroplasts in living cells, except that the thickness of a thylakoid was obtained from published measurements made on electron micrographs. The computer-simulated sections, in the form of graphic plots, show a high degree of similarity to electron micrographs of in situ chloroplasts. Both have what appear to be overlapping grana; both have starch grains that appear intercalated between grana; both have grana of seemingly varying thicknesses and occasionally of irregular shape; and both exhibit long sets of continuously interconnected grana. No conflict was encountered between electron micrographs of actual chloroplasts and the simulated sections of the computer model. Therefore, the spiral model, based on conception of a string of grana of uniform thinness, appears to be a satisfactory representation of in vivo chloroplast structure.