Fundamental features of cells
Despite all the efforts of the 20th century there are many areas of basic cell biology that remain mysterious. Some of these questions, such as how secreted proteins get across the Golgi, or the source of the membranes of autophagosomes, have been debated for so long that a further century might not be enough to reach agreement. Other phenomena such as lipid rafts, exosomes, or unconventional secretion seem to have divided the protagonists into camps of believers and skeptics who consider each other's views too eccentric to even engage. Other questions, however, have started to receive attention so recently that there may not yet be sufficient entrenched views to impede progress. These include the question of how organelles and cytoskeletal structures, and indeed the cell itself, maintain constant size and shape. In addition, the extent of non-vesicular transport of lipids between organelles has only recently been appreciated, and exciting recent work has revealed the importance of membrane contact sites , and also suggested that the transport of cholesterol to the plasma membrane is mediated by oxysterol binding proteins via a counter-current of phosphoinositides , although perhaps inevitably an opposing view has formed that oxysterol binding proteins do not actually perform cholesterol transport but instead activate an as yet unidentified transport machinery .