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Biology Essay Ap

By Njkeys4 Oct 14, 2013 775 Words
1. There are two main types of cells, prokaryotic and eukaryotic. The connection between the two can be seen right from their prefixes, pro meaning before and eu meaning true or good. This is because prokaryotic cells are before a nucleus and eukaryotic cells possess a true nucleus. This has led to many theories that eukaryotic cells have evolved from prokaryotes. The evidence for this is most noticeable in the structure of the cells, which will be compared below.

Prokaryotic cells have no nuclei and contain few organelles as shown below.

Eukaryoitc cells have a nuclei and contain many organelles.

While the two cells might appear very different they do have many things in common and perform most functions in the same ways. Both prokaryotic and eukaryotic cells are enclosed by plasma membranes, filled with cytoplasm, and are filled with small structures called ribosomes. Both also contain DNA, which carries the archived instructions for operating the cell. The similarities go beyond visible things, as physiologically they are also very similar. An example of this is the DNA in the two cell types is precisely the same kind of DNA, and the genetic code for a prokaryotic cell is exactly the same genetic code used in eukaryotic cells. Other things in the two cells that may appear a difference aren’t, an example being prokaryotic cells have a cell wall, and while not all eukaryotic cells have a cell wall, many do including plant cells.

There are also many differences between the two cells, the most obvious being the size of the two and with the difference in complexity. The most distinct difference is that Eukaryotic cells have a true nucleus bound by a double membrane. Prokaryotic cells do not have one. Another difference is despite the fact they both have DNA, eukaryotic DNA is complexed with proteins called histones and is organized into chromosomes. Prokarytoic DNA is naked, meaning it has not histones associated with it, and it is not formed into chromosomes. Eukarytoic cell contains a number of chromosomes; whereas prokaryotic cell contain only one circular DNA molecule and a varied assortment of much smaller ciclets of DNA called plasmids. The simpler prolaryotic cell does not need nearly as many genes to operature which is why the DNA is much simpler. While both cells contain many ribosomes the one in eukaryotic cells are much larger and more complex than those of prokaryotic cells. A eukaryotic ribosome is composed of 5 kinds of rRNA and about eighty kinds of proteins. Prokaryotic ribosomes are composed of one 3 kinds of rRNA and about fifty kinds of proteins. Another difference is that the cytoplasm of eukaryotic cells is filled with a contains a collection of organelles, many of which are enclosed in their own membranes. Prokaryoitic cells contain no membrane-bound organelles not including the plasma membrane. A final major difference in structure is memesomes are not present in most prokaryotic cells. A esosome is an elaboration of the plasma membrane that intrudes into the cell.

In essence eukaryotic cells are a complex version of prokaryotic cells. It is quite easy to see where the theory of eukaryotic cells evolving from prokaryotic cells came from and the evidence that could support it. For one, the size of prokaryotic cell is around 100 nanometers to 10 micrometers, whereas a eukaryotic cell ranges from 10 micrometers to 100 micrometers. However, the size of individual organelles like the mitochondria and nucleus that are unique to eukaryotic cells are about the same size as a prokaryotic cell. This supports the idea that Eukaryotic cells evolved from prokaryotes living symbiotically inside a larger cell. The individual organelles could have been formed by the small prokaryotic cells. Another characteristic is that organelles like mitochondrion act in similar ways to prokaryotic cells. A mitochondria can move around, alter shape, and even divide, just like a living prokaryotic cell. This theory is called endosymbiosis, where one organism lives inside the other and they act as a single organism. This theory is most prevalent in mitochondrion and chloroplasts. The endosymbiosis theory postulates that the mitochondria evolved from aerobic bacteria living within their host cell. It also postulates that chloroplasts of red algae green algae, and plants evolved from endosymbiotic cyanobacteria. These theories are supported by the fact they only way to two can arise is through themselves. It is also supported because they have their own genomes that closer resemble bacteria. Lastly it is supported because both mitochondrion and chloroplast have their own protein-synthesizing machinery. It is most likely that they were living in a symbiotic relationship that was continually beneficial so it became permanent.

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