Summary
In which we see the cell membrane as the defining boundary of life but also as a confining barrier that must be overcome to allow substances in and out of the cell. This is the ‘containment paradox’. Single-celled animals ultimately developed the ability to bring components from outside the membrane into their substance through mechanisms such as ‘endocytosis’ and ‘phagocytosis’. This was the ‘invention’ of eating and significantly enhanced the organism’s capacity to assimilate nutrients. The ability to ingest particles had enormous significance for the evolution of life. Whole bacteria that came to live as ‘endosymbionts’ within the cell took on the energy conversion processes from the surrounding surface membrane (where this previously occurred) and enabled it to take on additional roles. Importantly, phagocytosis led to the ingestion of whole living organisms. This was to alter the relationship between lifeforms with consequences that would lead ultimately to the requirement of both a gut and an immune system.
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- 1.
Cell (from Latin—cella—meaning ‘small room’). The concept of the cell literally becomes the living room of life. Different types of cell are identified by the suffix ‘-cyte’ from the Greek word ‘Kytos’ meaning ‘container’.
- 2.
The thickness of the membrane can be estimated on the basis of an experiment performed by the American diplomat and polymath Benjamin Franklin on a pond in Clapham Common in the early 1770s! He poured a teaspoon of oil onto the pond and estimated that it covered about half an acre when it spread out—from this you can estimate the thickness of a single layer of molecules. The experiment was not designed for this purpose though but to see if there was any truth in the efficacy of pouring oil on water to calm choppy seas.
- 3.
The bubbles that are made by blowing through a film of washing up liquid between our fingers or in children’s bubble toys are also a lipid bilayer, but the exact opposite of the cell membrane. Here the double layer is made up of the fat-loving ends pointing outwards from the membrane with a thin layer of water sandwiched in between the water-loving ends in the middle.
- 4.
All proteins can be described by a given name, or by individually naming the amino acids that make them up—given that there are over 30,000 in titin this would therefore make an extremely long name indeed (over 189,000 letters) and some have suggested that this is the longest word in the English language, or indeed any. I think it is stretching it a little though to use a chemical formula as a word.
- 5.
At its simplest, life could be seen as the emergence of self-replicating molecules. Such a concept was promulgated by Richard Dawkins in ‘The Selfish Gene’ in 1976 in which he envisaged the genetic material as the being of life on which evolutionary selection worked, and the organism around it as the mere container or vehicle.
- 6.
A broader perception is that of David Hull who added the necessity for the ‘interactor’ to the ‘replicator’: ‘A process is a selection process because of the interplay between replication and interaction. The structure of replicators is differentially perpetuated because of the relative success of the interactors of which the replicators are part. In order to perform the functions they do, both replicators and interactors must be discreet individuals which come into existence and cease to exist’.
- 7.
Michelangelo painted the ceiling of the Sistine chapel between 1508 and 1512. It depicts the Creation as told in the book of Genesis and the Creation of Adam is the centrepiece—famously as God and Adam nearly touch fingers in the giving of life. The image of God is surrounded by a red mantle—whilst many have pointed out the anatomical similarity of the shape of this ensemble to the human brain it also closely resembles the form of the uterus after giving birth. Which of these was intended is not known—is it possible that Michelangelo’s genius was to combine both forms?
- 8.
‘Frankenstein; or the Modern Prometheus’ was written by the young Mary Shelley who published it in 1818 at the age of 20. She referred to Victor Frankenstein’s monster in different tellings of the tale as ‘Adam’ and the monster refers to himself in talking to Frankenstein as ‘The Adam of your labours’. Interestingly, the actual animation of the monster is not described in her book, although she hints strongly that it was connected to Victor Frankenstein’s interest in electricity.
- 9.
The coupling of the proton gradient to the synthesis of ATP is known as the ‘Chemiosmotic hypothesis’ and was first espoused by Peter D Mitchell in 1961—he was awarded the Nobel Prize in Chemistry for his work in 1978.
- 10.
In 1908 the Daily Mail newspaper offered £500 for anyone achieving a powered flight across the English Channel—Le Matin paper in Paris said that it could never be won. The prize was doubled to £1000 in 1909 when on 25th July, Louis Bleriot claimed it by flying his mark XI monoplane from near Calais to Dover in a time of 36 min. He crash-landed as he had not previously reconnoitred a landing site on the English side. Perhaps he never expected to make it across…. The radial engine, designed by Alessandro Anzani, became the prototype for all aircraft engines until long after the Second World War.
The rotating three step mechanism of the ATP synthase was first described by Paul Boyer at UCLA and confirmed by John Walker from Cambridge following elucidation of the structure of the molecule—they shared the 1997 Nobel prize in Chemistry for their achievement.
- 11.
These difficulties are faced by exo-biologists looking for traces of life on nearby planets and moons of the solar system, but compounded by the necessity of transporting a miniature laboratory across space, and constrained by knowledge of the molecules associated with life on earth. An example of such potential confusion is the presence—or not—of ‘nanobacteria’. These are tiny structures which, by their complexity and ability to perpetuate themselves, have been considered by some to represent small organisms, about 100 times smaller than most bacteria. Nanobacteria have been implicated as the cause of certain diseases in humans—including calcifying conditions such as atherosclerosis and kidney stones. They have also been found on material emanating from other worlds (the ‘Alan Hills 84001’ meteorite from Mars that was found in Antarctica) and considered as possible evidence for the existence of extra-terrestrial life. Whilst the debate continues, it is most likely that these structures on Earth are simply mineral-protein complexes that are capable of self-propagation whilst the appearances on the Mars rock are now thought to be artefacts from the preparation for electron microscopy.
- 12.
Charles Darwin put this much more eloquently—“As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.” Darwin, The Origin of Species (1872), 104f. Darwin was a student of theology at Christ’s College in Cambridge between 1828 and 1831 (having failed to show any interest or aptitude in medical studies at Edinburgh) and embarked on the legendary voyage of the Beagle shortly after leaving Cambridge. His student rooms have been restored and are well worth a visit, as is the statue of him sitting on a bench made by Anthony Smith in gardens in the college recreated with species identified from his voyage.
- 13.
Robert Brown described the first eukaryotic organelle—the nucleus—in 1833. Like Darwin, he also dropped out of medical studies in Edinburgh, as he was more interested in botany. He also described ‘Brownian motion’—the random motion of particles suspended in fluid due to collision with fast moving molecules. Although mitochondria were first spotted inside the cell in the 1850s, their definitive description fell to the German pathologist, Richard Altmann in 1890. He called them ‘bioblasts’ and thought that they were ‘elementary organisms’ living independently within the cell. He would have been gratified to know that this is concurrent with present thinking, even though his theory was initially ridiculed (as all the best are!). Mitochondria are now thought to have evolved from formerly free-living bacteria. The term ‘mitochondrion’ was coined in 1898 by Benda, from the Greek for ‘thread’ and ‘granule’ after the appearance of the mitochondria in the tail of the sperm.
- 14.
Carl Woese (1928–2012), along with George Fox at the University of Illinois pioneered the use of 16s ribosomal RNA to understand the relationship of different bacterial and archaeal species over time. 16s ribosomal RNA is a structural nucleotide that makes up part of the ‘ribosome’ a subcellular structure responsible for the translation of the nucleic acid genetic code into proteins—effectively the ‘protein factory’ within cells. It mutates very slowly due to its critically important role and therefore changes in the 16s rRNA can be used to define different species. In common with many other original thinkers in science who challenge the established paradigm, Woese was considered to be something of a crank—but later recognised as a ‘scarred revolutionary’ as his theory became accepted by the end of the 1980s.
- 15.
Camillo Golgi (1843–1926) was an Italian pathologist who studied the nervous system, having invented a stain that demonstrated nerve fibres and for which he received the Nobel prize for his work in 1905. The stain also demonstrated the intracellular apparatus that bears his name and is his most lasting legacy.
- 16.
The originator of the term ‘phagocytosis’ was the Ukrainian biologist Elie Metchnikoff who noticed accumulations of cells around thorns that he pinned into starfish larvae in 1882. He postulated that these cells were attempting to engulf and destroy the foreign bodies. For this work he was awarded the Nobel prize in 1908 and is known as ‘The father of natural immunity’. He also thought that aging could be prevented by drinking sour milk every day and attributed the longevity of Bulgarian peasants to this. This theory has not been substantiated to date! However, the sour milk theory may just be one of the first steps towards our current enthusiasm for ‘probiotic’ remedies.
- 17.
The delightfully named ‘Vampirococcus’ adheres to other bacteria then digests them from outside. Bdellovibrio burrows through the outer surface membrane of bacteria that are double membraned and secretes digestive chemicals from its protected niche to destroy the cell. Only Daptobacter, described in 1986 has been reliably shown to invade other bacteria.
- 18.
Whilst we have words for most forms of eating, based on the suffixes ‘–vory’ (such as carnivore, omnivore etc) or ‘–phagy’ being the respective Latin or Greek roots, interestingly there is no word that expresses eating something alive. Perhaps ‘Vitavory’ would be the appropriate expression.
- 19.
Philip Siekevitz (1918–2009) first called the mitochondrion the ‘powerhouse of the cell’ in 1957.
- 20.
The ‘words’ of the DNA code are each made up of three letters. Each of the three letter combinations codes for a different amino acid—these are then added together in a chain to make a protein. The sequence of letters that codes for that polypeptide is called a ‘gene’. There are differences in the code between different types of organisms. Eukaryote codes have a set starting point and therefore always see the same three letter words and the sentence always translates into the same sequence of amino acids to make up the protein. Bacteria also always start the protein with a variant of the amino acid methionine called N-formyl methionine. Mitochondria also initiate proteins with this molecule, but eukaryotes and archaeans do not. This supports current evolutionary theories of the relationship of bacteria with mitochondria, and of eukaryotes with archaeans.
- 21.
Rickettsia is a genus of bacteria that can only live within other cells. That this genus includes R. prowazeckii which is the causative organism of louse-borne epidemic Typhus fever is perhaps pertinent to our story. Both of its discoverers—HT Rickett and SJM Prowazek—succumbed to the disease which is undoubtedly one of the greatest epidemic scourges of mankind. It was responsible for about 3 million deaths in Eastern Europe after the First World War but has been responsible for changing the course of history on many occasions—for instance it led to the surrender of Prague to the French in 1741 when the Austrian army was decimated by the infection. The Nobel prize in 1928 went to Charles Nicolle for the discovery of its mode of transmission.
- 22.
The first person to describe the theory of ‘endosymbiosis’ was the Russian botanist Constantin Mereschkowsky (1855–1921) who postulated this mechanism for the origin of chloroplasts in plants. Lynn Margulis (1938–2011) reinvigorated the endosymbiosis theory and added the possibility of endosymbiotic origin of mitochondria. Her seminal paper on the subject was published in 1967 (under the name Lynn Sagan, as she was at the time married to the astronomer, Carl Sagan) after being turned down by 15 journals. Perhaps as a result of this experience she became drawn to other ideas and theories rejected by mainstream thinking such as the ‘Gaia hypothesis’ that have attracted less support over the following years. She was refreshingly outspoken and unafraid to be proven wrong—but then it took many years for the scientific community to accept the evidence for endosymbiosis.
- 23.
Edward Drinker Cope (1840–1897) was a critic of Darwinian natural selection and a proponent of Lamarckism—namely the inheritance of acquired characteristics. A prolific author (he published over 1400 papers over 40 years) and theorist on paleontology, his professional life was characterised by his rivalry with Charles Othniel Marsh and on his deathbed he left his skull to medical science in a challenge to his great rival to compare the size of their brains!
- 24.
To a certain extent this is mirrored in the bacterial world. The small size of Bdellovibrio is necessary in order to gain entry to the space between the two bacterial membranes. However, there are plentiful examples of bacterial sticking together in filaments after replication to increase their size and introducing predatory bacteria into culture systems appears to select for these larger forms—even in the absence of phagocytosis.
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Woodward, J. (2021). The Invention of Eating. In: The Gastro-Archeologist. Springer, Cham. https://doi.org/10.1007/978-3-030-62621-1_1
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