The mitochondria were originally free-living bacteria. At some point in the distant past they became involved in a symbiotic relationship with another type of cell and this allowed the development of the eukaryotic cell. This revolutionary idea was resisted by many biologists at first but is now pretty well accepted, although there is still much uncertainty about how it happened. Lane discusses the competing theories in some detail. Here, as elsewhere in the book, he takes a historical approach, which for me is probably the best way to get an understanding of where we are today.
We start with what the mitochondria are best known for: respiration, which means oxidation of foodstuffs to generate ATP and produce energy. ATP (adenosine triphosphate) is often described as having a high-energy bond which, when broken, releases a large amount of energy. As Lane explains, this is wrong. What really happens is an extraordinary process which involves the shunting of electrons and protons in a cascade of steps culminating in the operation of an astonishing biological motor with a revolving core. The description of this process occupies three chapters; quite demanding reading in spite of Lane's clear writing, but it is essential to grasp the idea clearly since it is central to the rest of the book.
The next topic is the long-standing dispute about whether evolution inevitably tends towards increasing complexity or not. Lane thinks that if the eukaryotic cell had not arisen, largely by chance, there would have been no increase in complexity beyond that found in bacteria. Mostly this is a matter of size; eukaryotic cells can be much bigger than bacteria and so are able to engulf other cells. Predation and increasing complexity went together after this development. But if there is life on other planets, which is quite likely, it will probably have stayed at the level of bacteria. What this seems to mean is that complexity is pretty well inevitable once you have the eukaryotic cell, but there will be no real increase in complexity before this stage, and the arising of eukaryotic cells is by no means guaranteed; in fact, it is very unlikely.
The function of the mitochondria is not confined to providing energy. They also have a central role in causing cells to commit suicide (apoptosis). This is essential, for example, in preventing cancer and also in normal development of organs and structures, both in the embryo and later. For example, our fingers and toes develop as a result of apoptosis in what is initially just a webbed structure. Apoptosis results when respiration is unable to proceed normally for one reason or another. But therein lies a puzzle.
The relation between the mitochondria and the cells they inhabit is usually termed symbiosis, which implies a mutually beneficial arrangement, so it is surprising to find that it can also be lethal. How did the mitochondria acquire this role? The explanation is probably that they were originally parasitic and killed the host cell. This was before the development of multicelled organisms, so it freed the mitochondria to infect other cells. In multicellular organisms, like us, the mitochondria don't survive outside the cell; the mechanism of death was applied to the more 'altruistic' purpose of programmed cell death, for the good of the whole organism.
The widespread existence of two sexes in nature is a long-standing puzzle, because it 'ought' to be less advantageous than cloning, which produces more offspring. The usual explanation is that sex allows the elimination of faulty genes and produces genetic variety that helps in the arms race against parasites, but these ideas don't explain why there are two sexes instead of only one (or many). The explanation. at least in part, is that the discrepancy in size between egg and sperm ensures that only one set of mitochondria is transmitted to the offspring, which prevents possible conflict between different mitochondria. But, as always, the story turns out to be more complicated than appears at first. Lane doesn't shirk pointing out the existence of uncomfortable facts that don't support the argument he has just advanced.
So while the evidence suggests that mitochondria really are central to the evolution of two sexes, genomic conflict may not be all there is to it. Recent research suggests that there are other, more subtle,but probably more pervasive and fundamental reasons, too.
The final section considers the role of mitochondria in ageing and death. There has been much discussion about why we age and why different species age at such different rates. In general, smaller animals age more quickly than larger ones, although birds and bats age much less quickly than mammals of equivalent sizes. A popular idea in recent years has been that ageing is caused by accumulation of free radicals in cells, but giving antioxidants does not delay ageing (in spite of their continuing popularity in complementary medicine).
The answer to this apparent paradox, according to Lane, lies once again in the mitochondria. Free radicals do come into it but not in the simplistic way that many assume.
Mitochondria accumulate mutations through use, especially in active tissues, and these gradually undermine the metabolic activity of the tissue. Ultimately cells can only boost their failing energy supply by producing more mitochondria. As the supply of mint mitochondria dries up, cells are obliged to clone genetically damaged mitochondria. Cells that amplify seriously damaged mitochondria face an energy crisis and take the honourable exit—they commit apoptosis.Giving antioxidants cannot and should not prevent this process—it may even make it worse. The solution to the problem of ageing would be to make us more like birds. This is where research should be directed, Lane believes, rather than at attempting to combat individual diseases of old age such as osteoarthritis.
The book is written for a wide audience, including readers with little previous knowledge of science or biology (a useful glossary is included for these). That does not mean that it is elementary; its scope is so wide that even professionals are likely to find it illuminating. It has certainly done more to enhance my understanding of the basis of life than most other books I have read in recent years.
Mankind has always looked to the stars, and wondered why we are here, whether we are alone in this universe. We ask why our world is alive with plants and animals, and what were the chances against it; where we came from, who our ancestors were, what our destiny holds in store. The answer to the question of life, the universe and everything is not 42, as Douglas Adams once had it, but an almost equally cryptic shorthand. It is mitochondria. … They show us why energy-burning, warm-blooded creatures arose, thrusting off the shackles of the environment; why we have sex, two sexes, children, why we must fall in love, And they show us why our days in this firmament are numbered and why we must finally grow old and die
25-08-2016