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host cell was an archaeon, capable of growing
from two simple gases, hydrogen and carbon dioxide. The endosymbiont (the future mitochondrion) was a versatile bacterium (perfectly normal for bacteria), which provided its host cell with the hydrogen it needed to grow. The details of this relationship, worked out step by step on a logical basis, explain why a cell that started out living from simple gases would end up scavenging organics (food) to supply its own endosymbionts. But
complex life arose through a singular endosymbiosis between two cells only. He predicted that the host cell was an archaeon, lacking the ...
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the acquisition of mitochondria and the origin of complex life was one and the same event. And
The energy we gain from burning food in respiration is used to pump protons across a membrane, forming a reservoir on one side of the membrane. The flow of protons back from this reservoir can be used to power work in the same way as a turbine in a hydroelectric dam.
“All large, complex cells such as our own contain miniature powerhouses, which derived long ago from free-living bacteria, and which today provide essentially all our energy needs.” I could write instead: “All eukaryotes have mitochondria.”
The ancient covenant is in pieces; man knows at last that he is alone in the universe’s unfeeling immensity, out of which he emerged only by chance. His destiny is nowhere spelled out, nor is his duty. The kingdom above or the darkness below: it is for him to choose.
wrote: ‘It therefore seems likely that the precise sequence of the bases is the code which carries the genetical information.’
but in fact from the point of view of primordial biochemistry it is anything but: it is toxic and reactive. As oxygen levels rose, the textbook story goes, this dangerous gas put a heavy selection pressure on the whole microbial world. There
Nobody claims that oxygen physically drove these changes; rather, it transformed the selective landscape. Across the magnificent vistas of this unconstrained new landscape, genomes expanded freely, their information content finally unfettered.
such as amoebae make their living by physically engulfing other cells, a process called phagocytosis.
Others, such as fungi, digest their food externally – osmotrophy.
All plants, animals, algae, fungi and protists share a common ancestor – the eukaryotes are monophyletic.
a population of morphologically complex eukaryotic cells arose on a single occasion – and all plants, animals, algae and fungi evolved from this founder population.
These supergroups have names like unikonts (comprising animals and fungi), excavates, chromalveolates and plantae (including land plants and algae). Their names don’t matter, but two points are important. First there is far more genetic variation within each of these supergroups than there is between the ancestors of each group (Figure 5). That implies an explosive early radiation – specifically a monophyletic radiation that hints at a release from structural constraints. Second, the common ancestor was already a strikingly complex cell. By
All have a dynamic internal cytoskeleton, capable of remodelling itself to all shapes and requirements. All have motor proteins that shuttle objects back and forth on cytoskeletal tracks across the cell. All have mitochondria, lysosomes, peroxisomes, the machinery of import and export, and common signalling systems.
We even see miniature eyes, replete with a ‘lens’ and a ‘retina’, in some single-celled protists.
In short, evolutionary theory predicts that there should be multiple – polyphyletic – origins of traits in which each small step offers a small advantage over the last step.
bacteria and eukaryotes is competition. Once the first true eukaryotes had evolved, the argument goes, they were so competitive that they dominated the niche of morphological complexity. Nothing else could compete. Any bacteria that ‘tried’ to invade this eukaryotic niche were given short shrift by the sophisticated cells that already lived
To use the parlance, they were outcompeted to extinction.
Why is all complex life monophyletic, arising just once in 4 billion years? Why do prokaryotes not continuously, or even occasionally, give rise to cells and organisms with greater complexity? Why do individual eukaryotic traits such as sex, the nucleus and phagocytosis not arise in bacteria or archaea? Why did eukaryotes accumulate all these traits?
Energy has to be absorbed from the surroundings, lowering their entropy, cooling them down. Writers of horror stories grasp the central point in their chilling narratives – almost literally. Spectres, poltergeists and Dementors chill, or even freeze, their immediate surroundings, sucking out energy to pay for their unnatural existence.
To unfold the protein returns it to a state more similar to a soup of amino acids, increasing its entropy. But doing so also exposes the hydrophobic amino acids to water, and this physically uncomfortable state sucks in energy from outside, decreasing the entropy of the surroundings, cooling them down – what we might call the ‘poltergeist effect’.
Thus, seeds, spores and viruses are not perfectly stable in today’s oxygen-rich environment. Their components will react with oxygen – oxidise – slowly over time, and that ultimately erodes
a reaction will take place on its own accord only if G is negative. For that to be the case, either the entropy of the system must rise (the system becomes more disordered) or energy must be lost from the system as heat, or both. This means that local entropy can decrease – the system can become more ordered – so long as H is even more negative, meaning that a lot of heat is released to the surroundings.
Just think of the stars. They pay for their ordered existence by releasing vast amounts of energy into the universe. In our own case, we pay for our continued existence by releasing heat from the unceasing reaction that is respiration.
And those suitably reactive precursors will also form spontaneously, given a highly reactive environment. Thus, ultimately, the power for growth comes from the reactivity of the environment, which fluxes continuously through living cells (in the form of food and oxygen in our case, photons of light in the case of plants).
Living cells couple this continuous energy flux to growth, overcoming their tendency to break
ATP works like a coin in a slot machine. It powers one turn on a machine that promptly shuts down again afterwards. In the case of ATP, the ‘machine’ is typically a protein. ATP powers a change from one stable state to another, like flipping a switch from up to down. In the case of the protein, the switch is from one stable conformation to another.
A single cell consumes around 10 million molecules of ATP every second! The number is breathtaking. There are about 40 trillion cells in the human body, giving a total turnover of ATP of around 60–100 kilograms per day – roughly our own body weight.
60 grams of ATP,
ADP + Pi + energy ATP
130 watts for an average person weighing 65 kg, a bit more than a standard 100 watt light bulb.
but per gram it is a factor of 10,000 more than the sun
(only a tiny fraction of which, at any one moment, is undergoing nuclear fusion). Life is not much like a candle; more of a rocket launcher.
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This is what happens when substances such as iron react with oxygen – they pass electrons on to oxygen, themselves becoming oxidised to rust. The substance that receives the electrons, in this case oxygen, is said to be reduced.
In the end, respiration and burning are equivalent; the slight delay in the
middle is what we know as life.
The fact that many electron donors and acceptors are both soluble and stable, entering and exiting cells without much ado, means that the reactive environment required by thermodynamics can be brought safely inside, right into those critical membranes.
That makes redox chemistry much easier to deal with than heat or mechanical energy,
‘Anoxygenic’ forms of photosynthesis use hydrogen sulphide or iron as electron donors, leaving behind brimstone or rusty iron deposits as waste.8 Oxygenic photosynthesis uses a much tougher donor, water, releasing oxygen gas as waste.
Without a high flux of carbon and energy that is physically channelled over inorganic catalysts, there is no possibility of evolving cells. I would
coli and harmless common strains is not reflected in the ribosomal RNA, but in the acquisition of other genes that confer aggressive growth – as much as 30% of the genome can vary in different strains of E. coli – that’s 10 times the variation between us and chimpanzees, yet we still call them the same species!
carbon flux, energy flux, catalysis, DNA replication, compartmentalisation and excretion
