Describe the difference between prokaryotic and eukaryotic cells. Describe the theory of endosymbiosis and the evidence for and against this theory.
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Describe the difference between prokaryotic and eukaryotic cells. Describe the theory of endosymbiosis and the evidence for and against this theory. All living organisms are made up of cells. The cells can be divided into two groups: prokaryotic and eukaryotic. Prokaryotes date back to 3,500 million years ago, whilst eukaryotes are only as old as 1,500 million years. As prokaryotes come before eukaryotes, they are much simpler. Eukaryotes have become more developed and complex since prokaryotes. The two types of cells have similarities as well as differences. The similarities are that they both have a cell surface membrane, cytoplasm, DNA and ribosomes. However, even these similarities are in their own way, slightly different. For example, both cells contain DNA, but the structure of the DNA differs (explained later in more detail). As you will read below, eukaryotes are much more complex and contain many more organelles than the prokaryotes. Size Prokaryotes are usually much smaller than eukaryotes. Its average size ranges from 1 to 10 µm, whilst the eukaryotic cell is typically 10 to 150 µm. This actually gives prokaryotes a large surface-to-volume ratio, making it a simple, but efficient organism (absorbed nutrients have to travel only a small distance to diffuse completely throughout the inside of the cell).
Animal cells are spherical in shape as they lack the cell wall. Bacteria cells come in three common shapes: coccus (round), bacillus (rod) and spirilum (helical); as well as two different types of cell walls: gram-positive and gram-negative. Cytoskeleton Eukaryotic cells have a number of protein fibres that help give the cell its shape and support. These include microtubules and filaments for inner cell movement. Prokaryotes are lacking in cytoskeletons. Flagella / Cilia / Pili The flagella (and cilia in eukaryotes) aids in cell movement. In eukaryotic cells, they have a distinct arrangement of 9+2 microtubules. In a prokaryote, the flagella (if present) consist of a singe fibril (a cylinder of protein subunits, about 20 nm thick and several µm long). Flagella can be found in prokaryotes all over the cell, or in a group at one or both ends, or as a single tail. Flagella rotate around a 'bearing' anchored in the cell wall, producing a corkscrew motion that can propel it through a fluid medium. Prokaryotic cells also have pili (filamentous structures projecting out of the cell wall). These are involved in transferring plasmids between two prokaryotic cells during mating and attaching the cell to potential hosts. Ribosomes The ribosomes of prokaryotes are smaller and float around freely in the cytoplasm.
It seems likely that mitochondria and chloroplasts were once prokaryotes as they possess many features similar to that of prokaryotic cells: * Outer membrane is similar to plasma membrane (could have been pinched off to protect prokaryote during early stages of ingestion). * Inner membrane is similar to bacterial membrane (could have been the original prokaryotic cell membrane, retained even as it becomes the organelle of the eukaryote). * Own DNA strands are circular like that in prokaryotes, instead of linear in eukaryotes. * Own ribosomes are smaller than eukaryote's ribosomes, but similar sized to prokaryotes'. They are also able to self-replicate independently from the eukaryotic cell because they have their own sets of genes (more similar to prokaryotic genes than eukaryotic genes). It is also highly possible that this theory occurred as eukaryotes are able extremely able with the process of endocytosis. Evidence against the theory The mitochondria and chloroplasts could be made from invaginations of the plasma membrane. If the plasma membrane is able to develop into the Golgi apparatus and ER, then it could be possible that mitochondria and chloroplasts are even more highly developed invaginations of the plasma membrane. By relieving the cell of some of its functions, more energy can be directed to evolving highly specialized organelles. 7 November 2002 Nicole Lai 12.4 Unit 1: Molecules and Cells - - Topic 1.3 Cellular Organization - 1 -
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