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Ribosomes are orders of magnitude smaller still. You have 13 million of them in a single cell from your liver.
In the case of banded-iron formations – stunningly beautiful in their stripes of red and black – bacteria strip electrons from iron dissolved in the oceans
From the late 1960s, Lynn Margulis argued that this view is in any case misguided: that eukaryotic cells did not arise via standard natural selection, but through a series of endosymbioses, in which a number of bacteria cooperated together so closely that some cells physically got inside others. Such ideas trace their roots back to the early twentieth century to Richard Altmann, Konstantin Mereschkowski, George Portier, Ivan Wallin and others, who argued that all complex cells arose through symbioses between simpler cells.
Margulis did indeed hold firm to her belief that eukaryotes are a rich and varied tapestry of endosymbioses.
The story hinges on a large group of species (a thousand or more in number) of simple single-celled eukaryotes that lack mitochondria.
All of them retain structures that are now known to derive from mitochondria by reductive evolution – either hydrogenosomes or mitosomes.
The common ancestor of all eukaryotes quickly gave rise to five ‘supergroups’ with diverse cellular morphologies, most of which are obscure even to classically trained biologists. These supergroups have names like unikonts (comprising animals and fungi), excavates, chromalveolates and plantae (including land plants and algae).
Barring the simpler archezoa (which turn out to be scattered widely across the five supergroups,
All are sexual, with a life cycle involving meiosis (reductive division) to form gametes like the sperm and egg, followed by the fusion
The ‘oxygen holocaust’, which supposedly wiped out most anaerobic cells, can’t be traced at all: there is no evidence from either phylogenetics or geochemistry that such an extinction ever took place.
The idea of endothermy in plants might seem surprising, but it is known in many different flowers,
Some plants like the sacred lotus (Nelumbo nucifera) are even capable of thermoregulation, sensing changes in temperature and regulating cellular heat production to maintain tissue temperature within a narrow range.
Beyond a general requirement for eating, we need specific vitamins in our diet, without which we succumb to nasty diseases like scurvy. Vitamins are compounds that we can’t make for ourselves from simple precursors, because we have lost our ancestors’ biochemical machinery for synthesising them from scratch. Without the external props provided by vitamins, we are as doomed as a virus without a host.
A single cell consumes around 10 million molecules of ATP every second!
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. In fact, we contain only about 60 grams of ATP, so we know that every molecule of ATP is recharged once or twice a minute.
To fuel its growth E. coli consumes around 50 billion ATPs per cell division, some 50–100 times each cell’s mass.
We use about 2 milliwatts of energy per gram – or some 130 watts for an average person weighing 65 kg, a bit more than a standard 100 watt light bulb. That may not sound like a lot, but per gram it is a factor of 10,000 more than the sun
‘the most counterintuitive idea in biology since Darwin’, according to the molecular biologist Leslie Orgel.
One mitochondrion contains tens of thousands of copies of each respiratory complex. 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.
Hydrogen gas, hydrogen sulphide and ferrous iron are all electron donors, as we’ve already noted. They can pass their electrons into a respiratory chain, so long as the acceptor at the other end is a powerful enough oxidant to pull them through. That means bacteria can ‘eat’ rocks or minerals or gases, using basically the same protein equipment that we use in respiration.
But the point is that all of these different types of photosynthesis obviously derive from respiration. They use exactly the same respiratory proteins, the same types of redox centre, the same proton gradients over membranes, the same ATP synthase – all the same kit.
‘tails’ (fatty acids in bacteria and eukaryotes, and isoprenes in archaea).
Dissipative structures are produced by the flux of energy and matter. Hurricanes, typhoons and whirlpools are all striking natural examples of dissipative structures.
Rocks derived from the mantle, rich in minerals such as olivine, react with water to become the hydrated mineral serpentinite.
Olivine is rich in ferrous iron and magnesium. The ferrous iron is oxidised by water to the rusty ferric oxide form. The reaction is exothermic (releasing heat), and generates a large amount of hydrogen gas, dissolved in warm alkaline fluids containing magnesium hydroxides.
The combination of high CO2, mildly acidic oceans, alkaline fluids, and thin, FeS-bearing vent walls is crucial, because it promotes chemistry that would otherwise not happen easily.
olivine and water are two of the most abundant substances in the universe.
These apparently ancient cells generate a proton gradient across a membrane (we’ll come to how they do that), reproducing exactly what alkaline hydrothermal vents provide for free. The proton gradient drives the acetyl CoA pathway, by way of an iron–sulphur protein embedded within the membrane – the energy-converting hydrogenase, or Ech for short. This protein funnels protons through the membrane on to another iron–sulphur protein, called ferredoxin, which in turn reduces CO2
A rare successful loss of the cell wall might therefore have permitted the evolution of phagocytosis – an innovation that Oxford biologist Tom Cavalier-Smith has long argued was key to the evolution of eukaryotes.
some bacteria and archaea do now turn out to have straight chromosomes and ‘parallel processing’,
venerable eukaryote hypothesis.
Some scientists like to view the eukaryotes as descending from the very base of the tree of life, for what I see as basically emotional reasons.
at least 25 different groups of modern bacteria appear to have contributed genes to eukaryotes.
It looks like the first eukaryotes picked up thousands of genes from prokaryotes, but then ceased to ply any trade in prokaryotic genes.
Despite this basic limitation of chemistry, cells still grow from these redox couples perfectly happily. They do so because proton gradients across membranes are by definition gradations. The beauty of chemiosmotic coupling is that it transcends chemistry. It allows cells to save up ‘loose change’.
Many cells do exactly this – when infected, they form pores in the membrane. These collapse the proton-motive force, which in turn trips the latent death machinery. Proton gradients have become the ultimate sensors of cellular health, the arbiters of life and death, a role that will loom large later in this chapter.
By our calculations, eukaryotes have up to 200,000 times more energy per gene than prokaryotes.
Each peptide bond requires at least 5 ATPs to seal, five times as much as is needed to polymerise nucleotides into DNA.
there is a straight correlation between ribosome number and the burden of protein synthesis.
There are about 13,000 ribosomes in an average bacterium such as E. coli; and at least 13 million in a single liver cell, about 1,000 to 10,000 times as many.
There are some giant bacteria out there; they’re not common, but they do exist. Two species have been studied in detail. Epulopiscium is known only from the anaerobic hind gut of surgeonfish. It is a battleship of a cell – long and streamlined, about half a millimetre in length, just visible to the naked eye. That’s substantially larger than most eukaryotes, including paramecium (Figure 23). Why Epulopiscium is so big is unknown. Thiomargarita is even larger. These cells are spheres, nearly a millimetre in diameter and composed mostly of a huge vacuole.
One relatively simple factor that sets eukaryotes apart from bacteria is a dynamic internal cytoskeleton, capable of remodelling itself and changing shape
How much actin could we make for 580,000 ATPs per second? Actin is a filament composed of monomers joined together in a chain; and two such chains are wound around each other to form the filament. Each monomer has 374 amino acids, and there are 2 × 29 monomers per micrometre of actin filament. With the same ATP cost per peptide bond, the total ATP requirement per micrometre of actin is 131,000. So in principle we could make about 4.5 micrometres of actin per second.
Some large amoebae have as many as 300,000 mitochondria.
some bacteria, such as cyanobacteria, are perfectly capable of internalising their bioenergetic membranes, twisting their inner membrane into baroque convolutions, expanding their surface area considerably.
The mitochondrial inner membrane has an electrical potential of about 150–200 millivolts. As the membrane is just 5 nanometres thick, we noted that this translates into a field strength of 30 million volts per metre, equal to a bolt of lightning.
On the few occasions that cells lost genes from the mitochondria altogether, they also lost the ability to respire.
The more complex cyanobacteria often have several hundred copies of their complete genome.
A large cell needs to ship cargo to all quarters, and eukaryotes do exactly that.
‘Big fleas have little fleas upon their backs to bite ’em; little fleas have smaller fleas, and so ad infinitum.’
