The bottom half of my last email got cut off.
Chloroplasts are enclosed by a double-membrane
envelope composed of an outer and an inner
membrane (Fig. 10b). A common misconception seen
in most biology texts is that the double-membrane
envelope of the chloroplast is similar to the
double-membrane of the mitochondrion. This is
conceptually incorrect. It is true that the
outer mitochondrial membrane and the outer
chloroplast envelope membrane are analogous in
that both are rather minimal barriers between the
cytoplasm and the organellar interior (Salomon et
al., 1990; Sardiello et al., 2003; Weber, Chapter
14). However, the inner mitochondrial membrane
and the inner chloroplast envelope membrane are
very different in structure and function. The
inner mitochondrial membrane is the site of
oxidative phosphorylation, while the inner
chloroplast envelope membrane controls metabolite
transport (Weber, Chapter 14) and synthesizes
fatty acids, lipids, carotenoids, and prenyl
lipids (Cuttriss et al., Chapter 16; Döermann,
Chapter 17). The inner chloroplast envelope
membrane has no corresponding structure in the
mitochondrion, making the thylakoid membrane (the
site of photophosphorylation) the operative
counterpart to the inner mitochondrial membrane.
Therefore, when textbooks make statements such
as, "Like chloroplasts, (mitochondria) both have
an outer membrane and an inner membrane" (Raikhel
and Chrispeels, 2000) they run the risk of
misleading the reader into believing that the two
organelles are bounded by homologous or analogous
double-membraned structures, which they clearly
are not. (Wise, R.R. 2006. The diversity of
plastid form and function. In: Wise, R.R. and
J.K. Hoober. The Structure and Function of
Plastids, Springer, Amsterdam. pp. 3-26.)
--
Robert R. Wise
Dept. of Biology
UW Oshkosh
800 Algoma Blvd
Oshkosh, WI 54901
(920) 424-3404 (tel)
(920) 424-1101 (fax)
