Endosymbiosis Essay, Research Paper

Scientists have formulated many theories about the origin of life and how it evolved into the various forms known today. These ideas come from the evidence of the fossil record, from laboratory simulations of conditions on the earth, and from consideration of the structure and function of cells.

The earth was created more than 4 billion years ago, although more than 2 billion years probably passed before life was developed. Scientists believe that the atmosphere of the young earth was mostly water vapor, methane, and ammonia, with very little gaseous oxygen. Laboratory simulations have shown that all major classes of organic molecules could have been generated from this atmosphere by the energy of the sun or by lightning and that the lack of oxygen would prevent newly formed organic molecules from being broken down by oxidation.

Randomly formed aggregations that could harness energy to grow and reproduce themselves would eventually far outnumber other combinations. DNA may have been an important component of the self-reproducing aggregates and RNA is the only organic molecule able to duplicate themselves. The absence of oxygen from the atmosphere of the young earth meant that no ozone layer existed to screen out ultraviolet radiation and no oxygen was available for aerobic respiration. Therefore, the first cells were probably photosynthetic and used ultraviolet light. Because photosynthesis generates oxygen, the oxygen content of the atmosphere gradually increased. As a result, cells that could use this oxygen to generate energy, and photosynthetic cells that could use light other than ultraviolet, eventually became predominant.

Eukaryotes may have evolved from prokaryotes. This idea comes from speculation about the origin of mitochondria and chloroplasts. These organelles may be the descendants of aerobic and photosynthetic prokaryotes that were engulfed by larger prokaryotes but remained alive within them (endosymbiosis). Over the years the host cell became dependent on the endosymbionts for energy (ATP), while they in turn became dependent on the host for most other cell functions. The fact that mitochondria and chloroplasts are surrounded by two membranes, as if they had originally entered the cell by phagocytosis, supports this theory. In addition, these organelles contain their own DNA and ribosomes, which resemble the DNA and ribosomes of bacteria more than those of eukaryotes do. It is possible that other eukaryotic organelles originated similarly.

The theory of endosymbiosis is supported in part by the observation that the genetic material of plastids, the general term for the photosynthetic organelles of plants, is often different from the genetic material of the “host cell”, even after millions of years of evolving together. Research is still being conducted to determine if the diversity of plastids is the result of one eukaryote engulfing a prokaryote, and then a bigger eukaryote engulfing that cell, or the result of many different engulfments of diverse forms of bacteria and host cells.

Dr. Lynn Margulis, a Distinguished Professor of Biology at The University of Massachusetts at Amherst and a member of the National Academy of Sciences, played a crucial role in introducing the theory that eukaryotic cells (cells with nuclei: protists, fungi, plants and animals) evolved through a symbiotic relationship between different kinds of prokaryotic cells (cells without nuclei: bacteria and blue-green algae). The so-called “endosymbiosis theory” is now becoming widely accepted by biologists. The previously accepted theory, which we will call the “infolding theory,” suggested that the complex internal structures of eukaryotic cells–such as the nuclear envelope, endoplasmic reticulum, chloroplasts, and mitochondria, evolved as infoldings of the cell membrane that allowed for the separate packaging of cellular processes.

The endosymbiosis theory suggests an explanation for the existence of certain cellular structures in eukaryotic cells. According to the theory, complex eukaryotic cells evolved from the symbiotic relationship (living together) between larger cells and smaller cells that started as either undigested meals or parasites of the larger cells. Endosymbiosis (living together inside) is not uncommon in nature; however, this theory suggests that mitochondria and chloroplasts originated as free-living organisms, and that modern eukaryotic cells are essentially communities of different cell types living cooperatively.

The evidence for mitochondria and chloroplasts are as follows. Both mitochondria and chloroplasts can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes encode only some of the proteins of which they are made. Both mitochondria and chloroplasts have their own genome and it resembles that of prokaryotes not that of the nuclear genome. Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it resembles that of prokaryotes not that found in the cytoplasm of eukaryotes. Their ribosomal RNA and the structure of their ribosomes resemble those of prokaryotes, not eukaryotes. A number of antibiotics that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes. Outer membrane is similar to the plasma membrane.

Mitochondria looks a lot like bacteria and chloroplasts look a lot like blue-green algae. These organelles are similar to prokaryotes in that: Both have circular, naked DNA. RNA is similar. Both have prokaryotic type and size ribosomes. Inner membrane lipids of mitochondria and chloroplasts are similar to their prokaryotic counterparts. Membrane proteins are highly similar. Mitochondria and chloroplasts seem to divide independently of the rest of the eukaryotic cell. Eukaryotes are very good at endocytosis.

Professor Kwang Jeon’s Supporting Discovery at the University of Tennessee. In 1987, Professor Jeon noticed that his collections of amoebas were developing a large number of dots. This large number of dots turned out to be bacteria, which were quickly killing off Jeon’s collection. Jeon noted the least sick ones and began keeping records of their progress. The least sick ones apparently were more resistant to the bacteria since they survived and returned to their normal modes. However, some 40,000 of the invading bacteria were still present within each of the surviving amoebas! Through transplanting experimentation, Jeon found that the nucleus of the amoebas could not live without the once pathogenic bacteria. Jeon’s accidental discovery proves that it is possible for an organism to become dependent on and a functional part of invading organisms. Instead of eliminating competitors, evolution eliminated competition itself, forming the basis of symbiotic relationships.