Natural selection has generally favoured those prokaryotic cells that can reproduce the fastest by taking up raw materials from their environment and replicating themselves most efficiently, at the maximal rate permitted by the available food supplies. Small size implies a large ratio of surface area to volume, thereby helping to maximise the uptake of nutrients across the plasma membrane and boosting a cell’s reproductive rate. Presumably for these reasons, most prokaryotic cells carry very little superfluous baggage; their genomes are small and compact, with genes packed closely together and minimal quantities of regulatory DNA between them. From DNA sequencing of the three classes of cell, we can see that, in truth all three classes differ from each other by the same degree genetically, whilst all retaining unmistakable genetic similarities.
There are a number of common features that both eukaryotes and prokaryotes share. These include ribosomes, which are the sites of protein synthesis, and an essential activity of all cells. Since both cells contain DNA, ribosomes are essential as these are the sites at which strings of amino acids are combined using peptide bonds to form polypeptides, based on genetic information on mRNA. In this way, the DNA in each of the cells controls the cells by regulating which proteins are made. Both cells need the DNA if the cells are to survive and reproduce. DNA is a self-replicating molecule, which enables new cells to be formed, which are identical genetically to the parent cell.
Both cells also contain a cytoplasm, which makes up the bulk of the cell, and provides a site for metabolic activity and contains vital biochemicals in solution. The cytoplasm is surrounded by a plasma membrane in both cases, and is essential in that it holds the cell together (compartmentalisation), and acts as a permeability barrier between the cell and its external environment, regulating movement of materials into and out of the cell. This is necessary to maintain an environment inside the cell which is different to that outside, thus creating electrochemical gradients for diffusion of essential molecules.
Cell walls are always present in prokaryotes and also in some, but not all eukaryotic cells e.g. animal cells do not have a cell wall. In prokaryotae, the cellw all consists of peptidoglycan (murein), a polymer of sugars and amino acids in a continuous two-dimensional array. More specifically, it is an alternating co-polymer of N-acetyl glucosamine and N-acetyl muramic acid, cross-linked with short peptide chains, often containing unusual amino acids. In eukaryotes, the cell wall, often found in plant and fungal cells, is made of polysaccharide, cellulose (polymerised glucose) in plants and chitin (polymerised N-acetyl-glucosamine) in some fungi. This cell wall prevents the cell from bursting if it is exposed to a solution of higher water potential.
Eukaryotes and prokaryotes are also similar in that they can combine together to form whole organisms. Although most prokaryotic organisms are unicellular, prokaryotic cells can join together in string of cells, to form filaments as in cyanobacteria. All eukaryotic organisms are formed from many cells clustered together, each with their own specific role, and often gathered into groups of similar cells, called organs. In this way, it can be seen, that both types of cell, can act as building blocks for larger organisms.
These common features may be thought to be related to the idea of a common ancestor. This theory suggests that all living organism originated from a common cell, which then divided into different groups via speciation methods.
Common features are also related to the endosymbiotic theory, which proposes that eukaryotes evolved from prokaryotes. Eukaryotes contain ribosomes that are semi-autonomous, and have their own DNA, ribosomes, mRNA and tRNA. It was gradually realised that DNA, RNA and protein synthesis in mitochondria and chloroplast display more similarities to the cognate processes in prokaryotic cells than to those in eukaryotic cells. From this realisation, came the endosymbiotic theory which adds to the argument that prokaryotes are the same as eukaryotes, because from this theory, one can see that eukaryotes are heavily dependent on the processes that prokaryotes have given them in this symbiosis. It is the transformation of chloroplasts and mitochondria from prokaryotes that has allowed eukaryotes to respire especially in atmospheres lacking in oxygen; and since these features are two of the main features which distinguish eukaryotes from prokaryotes, it is possible to see that, eukaryotes and prokaryotes are in fact very similar, and were once dependent on each other. It can be seen that if this theory was to be viable, it has to be this way round, since eukaryotes are bigger in size and have internal organelles that prokaryotes lack. For instance, the nucleus may have arisen by internalisation of infoldings of the plasma membrane enveloping the genetic material. The nuclear membrane may then have become distinct from the plasma membrane.
The endoplasmic reticulum and endomembrane system is also thought to have evolved from infoldings of the plasma membrane. The mitochondria and chloroplasts are likely to represent the early uptake of two sorts of prokaryotes by eukaryotes.
Since both eukaryotes and prokaryotes make up living functioning organisms, it is inevitable that both will contain similar factors, because of the common requirements and features that are true for all living organisms. For example, all living organisms have the potential to reproduce, asexually or sexually, to produce further offspring and therefore maximise the frequencies of their alleles in the population. Hence the reason why both cells contain self-replicating DNA, and ribosomes.
All living organisms also must be able to respire and hence both organisms contain essential respiration biochemicals free within their cytoplasm. Both also contain a plasma membrane which emphasises the idea of a form of homeostasis necessary in all living organisms, if they are to be able to survive in changeable conditions.
Both eukaryotic and prokaryotic cells may have accessory structures, which distinguish their specific roles. Often eukaryotic structures are larger however. Cilia are present in large numbers on a cell, often of the order of 100 per cell. They beat at right angles to the length, each cilium slightly out of phase with its neighbour producing a wave-like motion that either propels the cell or the medium near it. Prokaryotic cells sometimes have cilia attached which are approximately 12-18nm whereas eukaryotic cilia are larger, being about 200nm. This makes sense since the eukaryotic cilia have to propel a larger mass and volume around, and so need to displace more fluid to do so.
As mentioned earlier, both cells reproduce, however in completely different ways. Prokaryotic cells divide by binary fission, the best-known example of which is that of Escherichia coli. Eukaryotic cells however divide by mitosis and meiosis. This results in a difference in the segregation of genetic material: prokaryotic cells merely replicate their DNA snd divide by fission, with one molecule of DNA going to each daughter cell. Eukaryotic cells also replicate their DNA, but their daughter cells obtain DNA equitably. Even the expression of genes in the two cells are different. Eukaryotic cells tend to transcribe genetic material in the nucleus into large RNA molecules and depend on laer processing and transport processes to deliver RNA molecules of the proper sizes to the cytoplasm for protein synthesis. By contrast, prokaryotic cells transcribe very specific segments of genetic information into RNA messages, and little or no processing or selection appears to be either nesseccary or possible. In fact, because there is a lack of nuclear membrane makes it possible for new messenger RNA molecules to become involved in the process of protein synthesis even before they themselves are completely synthesised. Prokaryotic cells are in fact, so unique that they provide their own group as one of the five classifications of living organisms and are made up primarily by bacteria.
Although prokaryotes are similar to eukaryotes in many ways, they are distinct groups morphologically, and have more that is not in common than that isn’t. For example, not only do eukaryotes have a double membrane bound nucleus, but the cells make extensive use of internal membrane systems such as golgi apparatuses, endoplasmic reticulum, and membranes also delimit organelles such as mitochondria, chloroplast, lysosomes, and perioxisomes, as well as other kinds of vacuoles and vesicles. Each of these organelles has its own characteristic membrane (or pair of membranes in the case of mitochondria and chloroplasts), similar to other membranes in basic structure but often with its own specific chemical composition and proteins.
Also found within the cytoplasm of eukaryotic cells are several non-membranous structures that are involved cellular contraction and motility. These include the microtubules found in the cilia and flagella of many cell types, the microfilaments of actin found in muscle fibrils, and also the intermediate filaments which are found in areas subject to stress.
A feature of eukaryotes is also their ability to transport materials into and out of the cell in bulk, i.e. not just by diffusion by carrier proteins in the cell membrane. They can do this via endocytosis and exoxytosis, processes which are unique to eukaryotes. In endocytosis, portions of the membrane invaginate and are pinched off to form membrane-bounded cytoplasmic vesicles, containing substances that were previously on the outside of the cell. Exocytosis is the opposite of this process and involves membrane-bounded vesicles inside the cell fusing with the plasma membrane and releasing their contents to the outside of the cell.
As a summary, it has been shown that eukaryotes and prokaryotes have many cellular similarities, as do many different organisms, but in general, they differ greatly in their morphological structure, and hence the reason they are classed into different taxonomist classifications.
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