Isolated chloroplasts of the unicellular photosynthetic alga Chlamydomonas reinhardtii were cryofixed, freeze-fractured, and surface replicas observed by transmission electron microscopy. Examination of the plastid thylakoid membranes in this ac-5 mutant grown phototropically reveals two complementary faces termed B and C, with patterns of particle size and distribution depending on whether the thylakoids are stacked (s) or unstacked (u). See Fig 14 in Goodenough and Staehelin 1971 J Cell Biol 48:595-619.