The limits of network limitation

There are some circumstances when it is clear that supply networks do constrain function. For example, the network of microtubules within individual cells are highly efficient at distributing molecules on a small scale, but probably set an upper limit to the size of the cell, beyond which a dedicated cardiovascular system is required to meet demand. Similarly, the system of blind-ending hollow tubes, known as trachea, which deliver oxygen to the individual cells of insects, impose quite a low...

The archezoaeukaryotes without mitochondria

According to the theory put forward by Cavalier-Smith as long ago as 1983, some of the simple single-celled eukaryotes living today do still resemble the earliest eukaryotes. More than a thousand species of primitive eukaryotes do not possess mitochondria. While many of these probably lost their mitochon dria later, simply because they didn't need them (evolution is always quick to jettison unnecessary traits), Cavalier-Smith argued that at least a few of these species were probably...

Proton Power and the Origin of Life

The elementary particles of life energy-generating proteins in the mitochondrial membranes The way in which mitochondria generate energy is one of the most bizarre mechanisms in biology. Its discovery has been compared with those of Darwin and Einstein. Mitochondria pump protons across a membrane to generate an electric charge with the power, over a few nanometres, of a bolt of lightning. This proton power is harnessed by the elementary particles of life mushroom-shaped proteins in the...

The Warm Blooded Revolution

Warm-bloodedness is a misleading term. It means that the temperature of the blood, and with it the body, is maintained at a stable temperature above that of the surroundings. But many so-called 'cold-blooded' creatures, such as lizards, are really warm-blooded in this sense, for they maintain a higher temperature than their surroundings through behaviour. They bask in the sun. While this sounds inherently inefficient, at least in England, many reptiles succeed in regulating their body...

Discovery of the archaeaa missing link

Only two groups of cells have thrived in the absence of a cell wall the eukaryotes themselves, and the Archaea, a remarkable group of prokaryotes (cells that lack a nucleus, like bacteria). The Archaea were discovered by Carl Woese and George Fox at the University of Illinois in 1977, and named from the Greek for 'ancient'. Most archaea do, in fact, have a cell wall, but their walls are rather different in chemical composition from those of bacteria, and some groups (such as the boiling-acid...

List of Illustrations

1 Schematic structure of a mitochondrion, showing cristae and membranes 12 2 Schematic illustrations of a bacterial cell compared with a eukaryotic 3 Hydrogenosomes interacting with methanogens 54 Courtesy of Professor Bland Finlay, F.R.S., Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorset 4 Schematic showing the steps of the hydrogen hypothesis 58 Adapted from Martin et al. An overview of endosymbiotic models for the origins of eukaryotes, their ATP-producing organelles...

Mitochondrial recombination

If mitochondria are passed down the maternal line exclusively, then there would seem to be little possibility for recombination. Sexual recombination refers to the random swapping of DNA between two equivalent chromosomes, to make up two new chromosomes, each of which contains a mixture of genes from both sources. Clearly DNA from two distinct sources two parents is needed to make recombination possible, or at least meaningful swapping genes from two identical chromosomes makes little sense,...

Info

How life began on Earth is one of the most exhilarating fields of science today a wild west of ideas, theories, speculations, and even data. It is too large a subject to embark on in detail here, so I will limit myself to a few observations on the importance of chemiosmotics. But for perspective let me paint a quick picture of the problem. The evolution of life depends in very large measure on the power of natural selection and this in turn depends on the inheritance of characteristics that can...

Proton leak

We have seen that the muscles contribute little to the resting metabolic rate the danger of oxygen toxicity means that blood is diverted away from the muscles and into the organs, where there are relatively few mitochondria to cause damage. So what might have happened in the first mammals They had extra mitochondria in their organs to compensate for their higher aerobic capacity, but nowhere to divert the blood, which had to pass through either the organs or the...

Cell Possible Cure For Old

Ageing and age-related diseases can be ascribed to mitochondrial free-radical leakage. Unfortunately, or perhaps fortunately, the way in which the body deals with free-radical leakage from the mitochondria is far more complex than the rather naive early formulation of the mitochondrial theory would have us believe. Rather than simply causing damage and destruction, free radicals play a vital role in keeping respiration fine-tuned to needs, and in signalling respiratory deficiencies to the...

Differences between bacteria and eukaryotes

Compared with bacteria, most eukaryotic cells are enormous. Bacteria are rarely larger than a few thousandths of a millimetre a few microns or so in length. In contrast, although some eukaryotes, known as the pico-eukaryotes, are of bacterial size, the majority are ten to a hundred times those dimensions, giving them a cell volume about 10 000 to 100 000 times that of bacteria. Size is not the only thing that matters. The cardinal feature of eukaryotes, from which their Greek name derives, is...

Selfish competition

Why is uniparental inheritance so important And why are multiple sexes so uncommon, given that they expand the mating opportunities and are technically feasible The most widely accepted reason was developed as a forceful hypothesis by Leda Cosmides and John Tooby, at Harvard, in 1981. They argued that mixing the cytoplasm from two different cells creates an opportunity for conflict between different cytoplasmic genomes. These include both mitochondrial and chloroplast genomes, but also any...

Freeradical signal

We discussed the leakage of free radicals from the chain in Part 3. Paradoxically, the rate of free-radical leakage does not correspond to the rate of respiration, as one might think intuitively, but rather depends on the availability of electrons ultimately derived from food and oxygen. Because these factors vary continuously, free-radical production shifts according to circumstances. Sudden bursts of free-radical production can affect the behaviour of the...

The retrograde response

We have seen that the mitochondria operate a sensitive feedback system, in which the leaking free radicals themselves act as signals to calibrate and adjust performance. But the fact that free radicals play an integral part in mitochondrial function does not mean that they are not toxic too. Clearly they are, even if rather less so than is shouted about in health magazines. Lifespan does correlate with the rate of free-radical leakage from respiratory chains. While a good correlation doesn't...

The mitochondrial bottleneck

The fertilized egg cell the zygote contains about 100000 mitochondria, 99.99 per cent of which come from the mother. During the first two weeks of embryonic development, the zygote divides a number of times to form the embryo. Each time the mitochondria are partitioned among the daughter cells, but they don't actively divide themselves they remain quiescent. So for the first two weeks of pregnancy, the developing embryo has to make do with the 100000 mitochondria it inherited from the zygote....

Mitochondrial selection

In 2004, Douglas Wallace, the guru of mitochondrial geneticists, and his group at the University of California, Irvine, published fascinating evidence that natural selection does indeed operate on mitochondrial genes. Wallace himself, in two decades at Emory University in Atlanta, pioneered the mitochondrial typing of human populations, and his work in the early 1980s underpinned the famous 1987 Nature paper by Cann, Stoneking, and Wilson, which we considered at the beginning of this chapter....

How to lose the cell wall without dying

While many types of bacteria do lose their cell wall during parts of their life cycle only two groups of prokaryotes have succeeded in losing their cell walls permanently, yet lived to tell the tale. It's interesting to consider the extenuating circumstances that permitted them to do so. One group, the Mycoplasma, comprises mostly parasites, many of which live inside other cells. Mycoplasma cells are tiny, with very small genomes. M. geni-talium, discovered in 1981, has the smallest known...

Why insider dealing pays

The advantage of mitochondria is that they reside physically inside their host cell. Recall that mitochondria are bounded by two membranes, an outer and an inner membrane, which enclose two distinct spaces, the inner matrix and the inter-membrane space. The respiratory chains and the ATPase complexes are all embedded in the inner mitochondrial membrane, and pump protons from the inner matrix to the inter-membrane space see Figure 1, page 12 . The acid environment needed for chemiosmosis is...

Quest for a Progenitor

How did the eukaryotic cell evolve from bacteria The mainstream view assumes that it was by way of a sequence of tiny steps, through which a bacterium was gradually transformed into a primitive eukaryotic cell, possessing everything that characterises the modern eukaryotes, except for mitochondria. But what were these steps And how did they get started down a path that in the end found a way across the deep chasm separating the eukaryotes from bacteria Tom Cavalier-Smith has argued that the key...

The Power Laws of Biology

Max Kleiber Graph

They say that in London everyone lives within 6 feet of a rat. Denizens of the night, these rats are presumably dozing the day away somewhere beneath the floorboards, or in the drains. Or perhaps you're reading this in bed, in which case they may be having a riot in the kitchen in the house next door . Perhaps a few are decomposing in the drains too, as rats don't live much longer than three years. Once feared as carriers of the black plague, rats still symbolize squalor and filth, but we are...

The Hydrogen Hypothesis

Methanogen Mitochondria Tem

The quest to find the progenitor of the eukaryotic cell has run into dire straits. The idea that there might have been a primitive intermediate, a missing link with a nucleus but no mitochondria, has not been rigorously disproved, but looks more and more unlikely. Every promising example has turned out not to be a missing link at all, but rather to have adapted to a simpler lifestyle at a later date. The ancestors of all these apparently primitive groups did possess mitochondria, and their...

Parasite wars

We know that the gene for cytochrome c was brought to the eukaryotic merger by the ancestors of the mitochondria, rather than the host cell, and was later 2 Some forms of the extrinsic pathway of apoptosis, mediated by the death receptors, do bypass the mitochondria altogether, but these are likely to be refinements to the original pathway, which probably did involve mitochondria otherwise it is hard to explain why most forms of the extrinsic pathways do involve mitochondria. transferred to the...

Proton Power

Flow Protons Mitochondria

Peter Mitchell was an outsider to the field of bioenergetics. He had studied biochemistry at Cambridge during the war, and began his PhD there in 1943, as he had been injured in a sporting accident before the war and was not enlisted for service. Mitchell was a flamboyant character in the war years, well known about town for his artistic and creative flair, and impish sense of humour. He was an accomplished musician, and liked to wear his hair long in the style of the young Beethoven. Mitchell,...

A skeleton and many closets

Outside the nucleus, eukaryotic cells are also very different to bacteria. Eukaryotic cells have been described as cells with 'things inside' Figure 2 . Most of the things inside are membrane structures, made of a vanishingly thin sandwich of fatty molecules called lipids. The membranes form into vesicles, tubes, cisterns, and stacks, enclosing spaces closets that are physically separated, by the lipid barrier, from the watery cytosol. Different membrane systems are specialized for various...

Balancing gene loss and gain in bacteria

Most bacteria, of course, are not intracellular parasites, but live in the outside world. They need many more genes than Rickettsia. Nonetheless, they face a similar pressure to lose superfluous genes they just can't afford to lose as many. The tendency of free-living bacteria to lose genes can be measured in the laboratory. In 1998, the Hungarian researchers Tibor Vellai, Krisztina Takacs, and Gabor Vida, then all at the Eotvos Lorand University in Budapest, reported some simple conceptually...

Glossary

Antioxidant any compound that protects against biological oxidation, either directly by becoming sacrificially oxidised itself in place of other molecules, or indirectly by catalysing the decomposition of biological oxidants. Apoptosis programmed cell death, or cell suicide a finely orchestrated and carefully controlled mechanism for removing damaged or unnecessary cells from a multi-cellular organism. Archaea one of the three great domains of life, the other two being the eukaryotes and the...

Why There Are Two Sexes

In Chapter 13, we saw that the deepest biological difference between the two sexes relates to the inheritance of mitochondria. The female sex specializes to provide the mitochondria 100 000 of them in humans in the large, immobile egg cells, while the male sex specializes to eliminate mitochondria from tiny, motile sperm cells. We looked into the reasons for this strange behaviour, and found that it often seems to boil down to conflict between genetically different populations of mitochondria....

Part

The Ancestor's Tale A Pilgrimage to the Dawn of Life. Weidenfeld amp Nicolson, London, UK, 2004. de Duve, Christian. Life Evolving Molecules, Mind, and Meaning. Oxford University Press, New York, USA, 2002. Gould, Stephen Jay. Wonderful Life. The Burgess Shale and the Nature of History. Penguin, London, UK, 1989. Knoll, Andrew H. Life on a Young Planet The First Three Billion Years of Evolution on Earth. Princeton University Press, Princeton, USA, 2003. Lane, Nick. Oxygen The...

The fractal tree of life

Fractals from the Latin fractus, broken are geometric shapes that look similar at any scale. If a fractal is broken into its constituent parts, each part still looks more or less the same, because, as the pioneer of fractal geometry Benoit Mandelbrot put it, 'the shapes are made of parts similar to the whole in some way'. Fractals can be formed randomly by natural forces such as wind, rain, ice, erosion, and gravity, to generate natural fractals, like mountains, clouds, rivers, and coastlines....

Mitochondria

A mitochondrion one of many tiny power-houses within cells that control our lives in surprising ways Mitochondria are tiny organelles inside cells that generate almost all our energy in the form of ATP. On average there are 300-400 in every cell, giving ten million billion in the human body. Essentially all complex cells contain mitochondria. They look like bacteria, and appearances are not deceptive they were once free-living bacteria, which adapted to life inside larger cells some two billion...