*School of Mathematical and Physical Sciences, Murdoch University, Murdoch, W.A 6150 and +Oak Ridge National Laboratory, Tennessee
High resolution images of the thylakoid and mitochondrial membranes have been obtained by scanning tunnelling and scanning force microscopy. The surface structures differ markedly, thylakoid surfaces being covered with folded structures 20 nm in diameter while the mitochondrial surfaces reveal a crystalline structure supporting filaments in a semi-ordered array, 20 to 40 nm long. In the high electric field of the STM, the mitochondrial filaments are lifted from the surface with one end anchored at the surface. There is no such field in the AFM and the underlying crystalline structure is well-defined. Repeated scanning suggests that the
filaments are of variable height, but do not translocate.
A scanning sequence of a thylakoid surface images under isotonic solution reveals structural changes in the surface suggesting that as contact sites form, surface structures, possibly proteins, unfold into the contact zone.
The structure of the thylakoid and the mitochondrial outer membranes differ, but the mitochondrial images are consistent with a model of the outer membrane supporting surface receptor proteins which participate in protein import by binding the signal peptides. The heavily folded surface suggests an abundance of import sites which is consistent with the evidence that the outer membrane is less selective in import than the inner membrane.