untitled

part of its genome and independent existence, to

become a cellular organelle. There are likely to have

been repeated instances of endosymbiosis, leading

to the major organelles that are found in present-day

eukaryotes, and resulting in the rapid expansion of

unicellular eukaryotes which began about 2 billion

years ago. The evidence for this is still retained in

eukaryotic cells today. For example:

• Organelles such as mitochondria and chloroplasts
  are surrounded by double membranes, the outer
  membrane almost certainly being an enclosing
  membrane to “contain” the engulfed cell (as happens
  in more recent endosymbioses such as Rhizobiumcells
  in the root nodules of legumes).


• Mitochondria and chloroplasts of present-day eukary-
  otescontain a residual genome. It is not sufficient
  for an independent existence, but codes for elements
  of the electron-transport chain in mitochondria and
  some of the photosynthetic functions of chloroplasts.

150 CHAPTER 8

rumen archaea, which generate methane, according to

the equation:

4H 2 +HCO 3 −+H+→CH 4 + 3H 2 O

This methane is repeatedly belched from the animal’s
gut. Although we have accounted for all the main prod-
ucts of the mixed acid fermentation, it should be recog-
nized that the balance of these products is variable and
depends on the types of organism present and other
conditions in the rumen.

Comparison of hydrogenosomes and

mitochondria

It is now almost universally accepted that eukaryotic

(nucleate) cells originated from prokaryotic cells by the

process of endosymbiosis– when one cell engulfed

another, and the engulfed cell progressively lost all or

Fig. 8.8Diagram of the mixed-acid fermentation of the rumen chytrid Neocallimastix. The end-products of this fermenta-
tion are shown in the small shaded boxes. Part of the fermentation occurs in the cytosol, part in the hydrogenosome.
Some of the details are known (Orpin 1993; Marvin-Sikkema et al. 1994); others details are assumed, based on know-
ledge of the mixed-acid fermentation of some enteric bacteria. (After Trinci et al. 1994.)