Occurrence and loss of organelles
ABSTRACT Eukaryotic cells are distinguished from prokaryotes (Bacteria and Archaea) by the presence of organelles. Organelles are specialized parts of a cell that resemble and function as organs: that is, they are differentiated components of a cell that carry out a specific function. Most organelles are defined by the unique properties of their membranes and the specific proteins associated with these membranes. Such organelles include mitochondria, plastids, endoplasmic reticulum (ER), Golgi dictyosomes, peroxisomes, contractile vacuoles, and glycosomes. Some organelles, such as photoreceptors and ejectosomes, may be membrane-bound but are largely defined by the unique nature of their internal constituents. Other organelles such as the eukaryotic flagellum and nucleus represent a unique combination of specialized internal components (axoneme, genome) and a specialized membrane (flagellar membrane, nuclear envelope). This chapter addresses some of the current theories concerning the origins, and in some cases subsequent loss, of organelles in flagellates.
15.1 Introduction There are two major theories concerning the origin of eukaryotic organelles. The first suggests that certain organelles arose directly by sequestration of various enzymes or cellular constituents by membranes. This theory, known as the Direct Filiation or Autogenesis Theory, proposed by Cavalier-Smith and others, rests on the fact that one major distinction between prokaryotes and eukaryotes is the ability of eukaryotes to carry out endocytosis and exocytosis. Digestive enzymes, nucleic acids, and other specific cellular constituents can be isolated from the cell cytoplasm by infolding or invaginations of plasma membrane (Figure 15.1) and thus be allowed to specialize while remaining in close proximity to other cellular components. The evolution of a cytoskeleton was thought to have been a critical step in this process since it facilitates endocytosis (Mitchison, 1995).