These primeval cells would have been prokaryotic, coming from the Greek meaning ‘before a nucleus’(class notes). These make up one of two cell groups, the other being eukaryotic, taken from the Greek for ‘possessing a true nucleus’(classnotes).
It is thought that eukaryotic cells emerged from an endosymbiotic relationship between the ancestors of modern day eukaryotic cells and cyanobacteria, in order to be able to provide their own energy, due to the increased size of the eukaryotic cells. This basically means that the cells would have ‘engulfed’ the cyanobacteria, and put them to work as part of it’s own functions, producing energy for the cell(Dawkins).
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Figue One: Tree of Evolution of cells, showing the ‘capture’ of cyanobacteria for use in eukaryotic cells as mitochondria and chloroplasts
In terms of function, prokaryotic and eukaryotic cells do similar things, in similar ways, due to basic structural features that they share. They both contain cytoplasm (a site for chemical reactions to occur within the cell), enclosed within a plasma membrane which is made up of a phospholipid bilayer. This is an arrangement of phospholipids, in a double layer. Phospholipids are made up of a phosphate head which is hydrophilic, and as such can face towards the water on both the outside and the inside of the cell, with the hydrophobic fatty acid tails facing in towards each other. In both cases, this layer is responsible for isolating the cells contents from the outside world, and allowing transport into and out of the cell, as well as providing a shape for the cell. It is arranged in what is known as the ’Fluid Mosaic Model’. This is due to the arrangement of proteins set within the two phospholipid layers, which can move freely around within the membrane. Prokaryotic cells are also contained within a cell wall, as well as a cell membrane, a feature found in some eukaryotic cells. Another feature sometimes shared by prokaryotic and eukaryotic cells are flagella, which are small projections from the cell which enable locomotion.
Both prokaryotic and eukaryotic cells make use of ribosomes for the purpose of protein synthesise. Ribosomes are composed of two subunits, which are made in the nucleolus and are used to form proteins when the ribosomes attach to messenger RNA(biology.about). Both kinds of cells store energy in the form of ATP. ATP is the product of aerobic respiration, facilitated by mesosomes in prokaryotes, and cristae within mitochondria, thus supporting the idea of endosymbiosis in early eukaryotic cells.
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Figure two: a simple diagram demonstrating the principle of endosymbiosis
The most obvious differences between prokaryotic and eukaryotic cells are size and complexity. Possibly the most important difference (suggested by their names) are the way in which the two cells store their DNA. DNA is the same no matter what kind of cell it is in, made up of the ‘letters’ A, T, C and G, arranged in the famous double helix, as discovered by Watson and Crick in the 1950’s(The Double Helix). However, as the name suggests, eukaryotic cells house their DNA within it’s own membrane, where prokaryotic cells have their DNA floating freely within the cytoplasm(Michael Kent).
The nucleus of a eukaryotic cell is just one of many membrane bound organelles contained within the cell, all working together to produce proteins, which the prokaryotic cells do with ribosomes found in the cytoplasm, smaller than those found in eukaryotic cells.
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Figure three: a basic diagram illustrating the differences between prokaryotic and eukaryotic cells.
Many prokaryotic cells are bacteria. Eukaryotic cells can be divided into two main groups: plant and animal cells. Both plant and animal cells contain a true nucleus housing it‘s DNA, along with mitochondria, which are responsible for the synthesis of ATP (a form in which energy can safely be stored and transported). They both contain an organelle known as the endoplasmic reticulum, takes part in synthesising proteins and lipids, in the rough and smooth endoplasmic reticulum respectively, and send them off to the Golgi apparatus. In both plant and animal cells, this is the organelle responsible for processing proteins, and enzymes, and is also involved in producing lysosomes - a very small organelle responsible for ridding the cell of waste organelles and involved in the destruction of the cell itself (Class notes).
Silvertown, J. (2008) 99% Ape: How Evolution Adds Up. 2nd ed. London: Natural History Museum. 41-46
Bryson, B. (2003) A Short History of Nearly Everything ….ed. Great Britain: Black Swan. 350.
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Lane, N. (2009) Life Ascending: The Ten Great Inventions of Evolution ….ed. London: Profile Books Ltd
Watson, J.D. (1968) The Double Helix: The Discovery of the Structure of DNA Great Britain : Phoenix
Kent, M. (2000) Advanced Biology 20th ed. Oxford: Oxford University Press
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