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host cell was an archaeon, capable of growing from two simple gases, hydrogen and carbon dioxide.
We know that complex cells arose on just one occasion in 4 billion years of evolution, through a singular endosymbiosis between an archaeon and a bacterium
So it seems to me there are two big unknowns at the very heart of biology today: why life evolved in the perplexing way it did, and why cells are powered in such a peculiar fashion.
hypothesis as an imaginative leap into the unknown. Once the leap is taken, a hypothesis becomes an attempt to tell a story that is understandable in human terms. To be science, the hypothesis must make predictions that are testable. There’s no greater insult in science than to say that an argument is ‘not even wrong’, that it is invulnerable to disproof.
Erwin Schrödinger’s book What is Life?
Genomes do not predict the future but recall the past: they reflect the exigencies of history.
In all forms of photosynthesis, the energy of light is used to strip electrons from an unwilling donor. The electrons are then forced on to carbon dioxide to form organic molecules. The various forms of photosynthesis differ in their source of electrons, which can come from all kinds of different places, most commonly dissolved (ferrous) iron, hydrogen sulphide, or water. In each case, electrons are transferred to carbon dioxide, leaving behind the waste: rusty iron deposits, elemental sulphur (brimstone) and oxygen, respectively.
Life, as biochemist Albert Szent-Györgyi observed, is nothing but an electron looking for a place to rest.
By oxidising methane, oxygen removed a potent greenhouse gas from the air, triggering the global freeze.3
Oxygen levels rose twice, in the Great Oxidation Event 2.4 billion years ago and again towards the end of the eternal Precambrian period, 600 million years ago
least two components of eukaryotic cells were derived from endosymbiotic bacteria – the mitochondria (the energy transducers in complex cells), which derive from α-proteobacteria; and the chloroplasts (the photosynthetic machinery of plants), deriving from cyanobacteria.
none of the archezoa is a real missing link, which is to say that they are not true evolutionary intermediates.
All plants, animals, algae, fungi and protists share a common ancestor – the eukaryotes are monophyletic.
causing devastation to life over hundreds of square miles. Oil and water are said to be immiscible – the physical forces of attraction and repulsion mean they prefer to interact with themselves rather than each other.
the release of heat when oil and water separate actually increases entropy. In terms of overall entropy, then, and taking all these physical interactions into consideration, an ordered oily membrane around a cell is a higher entropy state than a random mixture of immiscible molecules, even though it looks more ordered.2
This physically ‘uncomfortable’ state costs energy. If a physically comfortable state releases energy into the surroundings as heat, a physically uncomfortable state does the opposite.
That’s why we can survive starvation for a while, by breaking down the protein in our muscles and using it as a fuel. This energy does not come from the protein itself, but from burning up its constituent amino acids.
ATP powers a change from one stable state to another, like flipping a switch from up to down.
A single cell consumes around 10 million molecules of ATP every second!
The energy of respiration – the energy released from the reaction of food with oxygen – is used to make ATP from ADP and Pi
Life is not much like a candle; more of a rocket launcher.
cells derive their energy from just one particular type of chemical reaction known as a redox reaction,
As the donor passes on electrons, it is said to be oxidised.
In the end, respiration and burning are equivalent; the slight delay in the middle is what we know as life.
reductions are sometimes defined as the transfer of a hydrogen atom.
all life is driven by redox chemistry, via remarkably similar respiratory chains.
Life doesn’t use plain chemistry, but drives the formation of ATP by the intermediary of proton gradients across thin membranes.
For each pair of electrons that passes through the first complex of the respiratory chain, four protons cross the membrane.
A single cell contains hundreds or thousands of mitochondria. Your 40 trillion cells contain at least a quadrillion mitochondria, with a combined convoluted surface area of about 14,000 square metres; about four football fields.
Their job is to pump protons, and together they pump more than 1021 of them – nearly as many as there are stars in the known universe – every second.
Pumping protons across a sealed membrane achieves two things: first, it generates a difference in the proton concentration between the two sides; and second, it produces a difference in electrical charge, the outside being positive relative to the inside.
‘Chemiosmotic’ is the term Mitchell used to refer to the transfer of protons across a membrane.
Essentially all life uses redox chemistry to generate a gradient of protons across a membrane. Why on earth do we do that?
In most modern organisms, carbon metabolism is quite separate from energy metabolism.
The main point is that sustained and predictable physical structures can be produced by energy flux.
In the absence of genes or information, certain cell structures, such as membranes and polypeptides, should form spontaneously, so long as there is a continuous supply of reactive precursors – activated amino acids, nucleotides, fatty acids; so long as there is a continuous flux of energy providing the requisite building blocks.
the point is that replication in water needs a continuous and liberal supply of both organic carbon and something much like ATP, in the same environment.
At the origin of life, natural rates of carbon delivery and waste removal must have dictated a small cell volume.
Without a high flux of carbon and energy that is physically channelled over inorganic catalysts, there is no possibility of evolving cells.
that olivine reacts with water to form serpentinite. The waste products of this reaction are key to the origin of life. Olivine is rich in ferrous iron and magnesium.
Mantle rocks are rarely exposed directly – water percolates down beneath the sea floor, sometimes to depths of several kilometres, where it reacts with olivine. The warm, alkaline, hydrogen-rich fluids produced are more buoyant than the descending cold ocean water, and bubble back up to the sea floor. There they cool, and react with salts dissolved in the ocean, precipitating into large vents on the sea floor.
Thermodynamics determines which states of matter are more stable – which molecules will form, given unlimited time. Kinetics relates to speed – which products will form in a limited time.
The point is that a molecule with a negative reduction potential will tend to get rid of its electrons, passing them on to any molecule with a more positive reduction potential,
The more acidic the solution, the easier it is to transfer electrons on to CO2
Can there possibly be any mighty import about the fact that the reduction potential of hydrogen falls with pH? Yes! Yes! Yes! Under alkaline hydrothermal conditions, H2 should react with CO2 to form organic molecules.
The serious problem is that these vents are rich in hydrogen gas, but hydrogen will not react with CO2 to form organics. The beautiful answer is that the physical structure of alkaline vents – natural proton gradients across thin semiconducting walls – will (theoretically) drive the formation of organics. And then concentrate them. To my mind, at least, all this makes a great deal of sense.
Rock, water and CO2: the shopping list for life.
:)
the last universal common ancestor, fondly known as LUCA, was the common ancestor of bacteria and archaea.
is almost as hard to define death as life, but the irrevocable collapse of membrane potential comes pretty close.
