Comparing Prokaryotes And Eukaryote Cells

The cell is the basic functioning unit of organisms in which chemical reactions take place. These reactions involve an energy release needed to support life and build structures. The cell consists of membrane bound organelles, which are responsible for the division of labour. There are two main classes of cells- Prokaryotes which are cells without a nucleus, where the DNA is spread around the cytoplasm of the cell, an example of a prokaryote is a bacterium (See Figure 1). The other class is the Eukaryotes which are the cells of plants and animals, and example is a palisade cell (See Figure 2). Both have similarities and differences in their functions and structures.

Both Prokaryote and Eukaryote cells consist of Cell wall, (however this is not present in animal cells) - this is made of Peptidoglycan (though the plant cell is made of cellulose microfibrils embedded in a layer of calcium pectate and hemicelluloses). The cell wall provides support for a plant cell, and
controls what goes in and out, allowing the cell to build a hydrostatic skeleton, giving the cell shape and rigidity. The Cells also contain a cell membrane made of phospholipids and proteins, designed to control what goes in and out of the cell. DNA is also found in both cells, which carries the genetic information for the cell, allowing for replication. Like DNA, ribosomes are also found in both cells. The ribosome is made of rRNA and they synthesise protein, as they are the site where mRNA meets tRNA so that amino acids are bonded together. However, the size of the ribosome differs- in a prokaryote the ribosome is around 70 Svedberg's, while in a Eukaryote it's larger at around 80 Svedberg's. Both cells have similar metabolisms and are amazingly diverse in their forms- for example there are dangerous prokaryotes such as Streptococcus gonorrhoea, which causes gonorrhoea, and useful prokaryotes such as Nitrosomonas used in the nitrogen cycle. Diverse Eukaryotes range from unicellular yeast for making bread, to multicellular humans.

However, both eukaryotes and prokaryotes have great differences.
Most notably the lack of a nucleus in prokaryotes and membrane bound organelles. The DNA of prokaryotes floats freely around the cell; the DNA of eukaryotes is held within its nucleus. The organelles of eukaryotes allow them to exhibit much higher levels of intracellular division of labour than is possible in prokaryotic cells. Another physical difference is the size of both cells. A prokaryote is only 0.5-5 μm, whilst Eukaryotes are on average 10 times bigger at up to 40μm. Mitochondria are absent from Prokaryotes, yet found in Eukaryotes, as are chloroplasts (though chloroplasts are not found in animal cells). One difference as far as existence is concerned, is that prokaryotes are believed to be the first form of life around 3.5 billion years ago. Eukaryotes are much younger, and believed to have existed around 1.5 billion years ago when the cells developed a nucleus- multicellular organisms are younger still at 0.5 billion years
old.

One theory as to why cells developed organelles such as mitochondria and chloroplasts are suggested in the theory of endosymbiosis. The theory suggests that mitochondria and chloroplasts were free living bacteria (prokaryotes) and ended up becoming part of an early cell. This is theory is supported by the fossil record showing that oxygen began to accumulate between the fossil records of prokaryotes and the later record of eukaryotes. This also is proven by the early world of around 3-4 billion years ago having no oxygen. When the oxygen began to build up, many bacteria needed to evolve- some of which became aerobic bacteria that needed oxygen to respire. The theory of endosymbiosis (See figure 3) suggests an aerobic bacterium was engulfed by an anaerobic bacterium (doesn't use oxygen to respire), such as an amoeba- these two different types benefited from each other, and after the original bacteria replicated, the aerobic bacteria inside it replicated too. Eventually these two lost their ability to function without the other- this relationship where both partners benefit is what is called symbiosis.

In conclusion, cells consist of two main groups- Prokaryotes (without a nucleus) and Eukaryotes (with a nucleus). While they have many similarities, there are also many differences to their structures. It is likely that eukaryotes evolved from two prokaryotes at around the stage in the earth's history when oxygen became available. However, one thing for sure is that cells are found in all living organisms- be they single celled amoeba, to multi cellular humans. The Cell is therefore the basic unit of life.