Summary
The chloroplasts ofEuglena gracilis have been examined by freeze-cleaving and deep-etching techniques.
The two chloroplast envelope membranes exhibit distinct fracture faces which do not resemble any of the thylakoid fracture faces.
Freeze-cleaved thylakoid membranes reveal four split inner faces. Two of these faces correspond to stacked membrane regions, and two to unstacked regions. Analysis of particle sizes on the exposed faces has revealed certain differences from other chloroplast systems, which are discussed. Thylakoid membranes inEuglena are shown to reveal a constant number of particles per unit area (based on the total particle number for both complementary faces) whether they are stacked or unstacked.
Deep-etchedEuglena thylakoid membranes show two additional faces, which correspond to true inner and outer thylakoid surfaces. Both of these surfaces carry very uniform populations of particles. Those on the external surface (the A surface) are round and possess a diameter of approximately 9.5 nm. Those on the inner surface (the D surface) appear rectangular (as paired subunits) and measure approximately 10 nm in width and 18 nm in length. Distribution counts of particles show that the number of particles per unit area revealed by freeze-cleaving within the thylakoid membrane approximates closely the number of particles exposed on the external thylakoid surface (the A surface) by deep-etching. The possible significance of this correlation is discussed. The distribution of rectangular particles on the inner surface of the thylakoid sac (D surface) seems to be the same in both stacked and unstacked membrane regions. We have found no correlation between the D surface particles and any clearly defined population of particles on internal, freeze-cleaved membrane faces. These and other observations suggest that stacked and unstacked membranes are similar, if not identical in internal structure.
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Miller, K.R., Staehelin, L.A. Fine structure of the chloroplast membranes ofEuglena gracilis as revealed by freeze-cleaving and deep-etching techniques. Protoplasma 77, 55–78 (1973). https://doi.org/10.1007/BF01287292
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DOI: https://doi.org/10.1007/BF01287292