Prokaryotic and Eukaryotic Protists, Essay Example

Describe the sequence of appearance and the diversity of prokaryotic (Archaea and Eubacteria) and eukaryotic protists. Include in your answer: a. the reasons we believe that the first cellular life forms were anaerobic heterotrophs, b. the general nature of prokaryotic metabolic evolution and the diversity early bacterial metabolic pathways, c. the role of endosymbiosis and the roles of the symbiotic partners, d. and the evidence for the sequence of events from the fossil record.

The bacteria are from the Kingdom Prokaryote due to the major feature, the prokaryotic cell, which has no membrane-bound nucleus. Fossil bacteria date back to 3.5 million years ago and are considered the earliest signs of life. On the other hand, the Eukaryotes, from the Kingdom Animalia, did not appear unlit 1.5 billion years after the prokaryotes. The bacteria are more numerous than other types of organisms and are found in every type of habitat. The first discovery of bacteria was from the scientists, Anton van Leeuwenhoek in 1676. Leeuwenhoek constructed his own microscope and examined different types of samples and found different variations of microorganisms. In the 1860s, Louis Pasteur and Robert Koch illustrated the role of bacteria in food spoilage and diseases. Prokaryotes differ than eukaryotes in several ways. First there is the absence of a nuclear envelope separating the genetic material from the cytoplasm. Prokaryotes are also very small compared to eukaryotes and they have one circular DNA structure. In addition, they have smaller ribosomes and different external features in their cell wall.

 Prokaryotic evolution has occurred due to natural selection. The ability of a prokaryotic cell to transfer genes from cell to cell played a major role in its evolution. Prokaryotes have a rapid reproduction. Some bacteria are capable of dividing every 20-30 minutes and this can result in millions of cells in only two days. Due to the large amount of cells in a population, even rare mutations can produce genetic variation in individual prokaryotes. In addition, bacteria have more different variations of energy metabolism. Many of the bacteria species obtain their energy through fermentation, (which is the breakdown of carbon containing subtances using carbon containing molecules) and others use cellular respiration using compounds that do not contain carbon as their electron acceptors. There are aerobes that require oxygen for cellular respiration, obligate anaerobes that are killed by oxygen, facultative anaerobes that can take or leave oxygen. In fact, the first bacteria that have been identified on Earth are believed to be the anaerobic bacteria. This is due to the fact that oxygen was not required on Earth and actually toxic to organisms. The gas was even rare in the atmosphere during that time period. It is also believed that cyanobacteria from the Precambrian period produced oxygen as a waste material and helped form the aerobic ecosystem. Cyanobacteria are easily found in the fossil records because they g grew in shallow sea water where they formed mats and used the sunlight for photosynthesis. If a mat was covered by mud or sand, light did not penetrate and the bacteria did. A new mat then formed on the old mat and the fossilized structure of millions of layers of bacteria formed what is called Stromatolites.

There are some major lineages to bacteria based on their features. The archeobacteria resemble other prokaryotes with the absence of a nucleus and the small size of the cells. However, their ribosomes are different. In some respects, archeobaceteria are like nothing else and it is said that they may have diverged from another organisms in evolution. There are three different types of archeobacteria, known as acidophiles (acid-likeing), methanogens (produce methane), and extreme halophiles (like salt). Eubacteria is another type of prokaryote that falls into a second main evolutionary group. These bacteria are the more familiar bacteria, such as the photosynthetic and chemosynthetic bacteria. In the Eubacteria, there are two different methods for distinguishing them through the gram stain. The gram negative cells have an outer membrane covering their cells and the gram positive do not. The cyanobacteria are the blue-green bacteria. These bacteria are photosynthetic and compared with other bacteria have larger cells.

Due to the bacteria’s large cell walls, they cannot engulf large food particles. Instead, they absorb small molecules and ions through their walls. Bacteria are divided into two groups, autotrophs and heterotrophs. Autotrophs make their own food from inorganic compounds and heterotrophs absorb organic food from other organisms. There are different types of autotrophic bacteria to include, photoautotrophs, that use light as the energy source and chemoautotrophs that use chemicals are their energy source. Most of the bacteria are heterotrophic bacteria and use food made by other organisms as their main energy source.

Prokaryotes are hard to classify because classifying an organisms is usually based on a shared gene pool; however, the bacteria may have produced dozens of generations each week for 3.5 billion years. In addition, bacteria can receive DNA from other individuals. Therefore, the bacteria have been placed into groups based on their shapes, bacillus (rod), coccus (spherical) and spirillum (spiral).

There are many different kinds of bacteria that live in other organisms and form a symbiotic relationship that is parasitic, commensal or mutualistic. Many of the symbiotic bacteria are part of an animal, living on or within it. For instance, humans have normal bacteria, Staphylococcus epidermidis on the skin. These are not lethal to the host because they form populations that prevent other pathogens from entering or settling on the host. There is also evidence of evolution of eukaryotic organelles that have evolved from prokaryotic symbionts living inside ancient eukaryotic cells. This is referred to as the endosymbiont theory. This indicates that some prokaryotes live inside eukaryotic cells today, that plastids and mitochondria are the size of prokaryotic cells and contain circular DNA without histone proteins, just like prokaryotic DNA. In addition, the theory explains that plastids and mitochondria make their own proteins, organelles and prokaryotic ribosomes are the same size and chloroplast and prokaryotic ribosomes can be hybridized. Furthermore, antibiotics were shown to block protein synthesis in ribosomes of prokaryotes and organelles, but not by the ribosomes of eukaryotes and nucleotide sequencing shows a similarity between ribosomal and transfer RNAs of many cyanobacteria, red algae and chloroplasts of many plants.

It has been theorized that the ancestors of mitochondria may have been accepted into the host cells because they could detoxify Oxygen. Then it is said that the evolved a way in which they could exchange ADP and ATP with cytoplasm and become the hosts main energy supply. This evidence further indicates that the mitochondrial ancestors moved into the eukaryotic host cells before the ancestral plastids. This is because all organisms with plastids also have mitochondria; however, not all organisms with mitochondria have plastids. In addition, mitochondria are less like free living bacteria, resulting in the notion that they have a longer history as symbionts. Furthermore, chloroplasts still make their own membrane lipids; whereas mitochondria import most of theirs from the cytoplasm.