d.w.rowlands [at] gmail.com
A bioengineer friend recently recommended that I read Power, Sex, Suicide: Mitochondria and the Meaning of Life, by Nick Lane. It was indeed an incredibly neat book. While I somewhat suspect the author of having an agenda---wanting to prove that mitochondria are responsible for essentially everything---I certainly learned a lot about biology that I hadn't known. The book is divided up into a number of topics, each of which Lane explains in terms of mitochondria.
The best-known function of mitochondria, of course, is as as location in cells where food and oxygen are converted into energy. This is usually explained as the production of ATP, the molecule that most cellular processes use as their basic fuel. However, the process in mitochondria---and in bacteria, which perform respiration without mitochondria---actually fundamentally works by performing redox reactions that produce a proton gradient, and so an electric voltage, across a membrane. In explaining this, Lane gives the first explanation for how a clay-based "metabolism-first" origin of life could have occurred that makes sense to me. Essentially, the suggestion is that the earliest biochemistry involved electrical gradients forming across the boundaries of bubbles in sediment, and that the earliest life form replicated itself in this environment before eubacteria and archaea separately invented their chemically different cell membranes.
It is generally believed that eukaryotic life---cells that have nuclei and other membrane-bound organelles---obtained their mitochondria as a symbiotic relationship between an archaea-derived host and an endosymbiont eubacteria that became the ancestor of mitochondria. Lane proposes that this step was actually the first step in the evolution of eukaryotes, before other eukaryotic traits such as nuclei and the ability to actively change the shape of the cell membrane to move or engulf food like an amoeba developed. His argument takes two forms: first, that the only rare eukaryotes that have been discovered without mitochondria appear to have had mitochondria in the past and lost or repurposed them, and second, that developing these traits required higher energy production that could only be achieved via mitochondria.
As previously noted, cellular respiration involves the production of an electric potential across a membrane. In both bacteria and archaea, the membrane in question is the outer membrane of the cell itself. This puts a limit on the size and complexity of prokaryotic cells: since surface area goes up as the square of radius and volume as the cube, cells larger than a certain size will need additional internal membranes to perform enough respiration to meet their energy budget.
Furthermore, there may be a limit to the size of respiration surface that a single bacterial chromosome or eukaryotic nucleus can control. Respiration involves a series of unusually high-energy reactions involving electron transport through a series of proteins. An imbalance in locally available proteins may make the electron transport chain back up, spewing toxic free radicals into the cell or mitochondrion. Lane suggests that the reason that, although mitochondria have lost most of their genes, all mitochondria seem to have retained the genes for a few specific proteins essential to respiration is that this allows each mitochondrion to turn on and off synthesis of these proteins independently as needed for its local conditions.
Having argued that complex cells require mitochondria, Lane then goes on to argue that mitochondria are responsible for many other properties of eukaryotic life. I don't have the time to attempt to repeat all of his arguments here, but I do strongly recommend that you all read this book.