The Endosymbiotic Theory

Scientists discovered that chloroplasts and mitochondria contain DNA similar to the DNA of certain bacteria, just as Margulis’s theory had predicted. Upon this data, Margulis extended her book and entitled it Symbiosis in Cell Evolution. Margulis then traveled, presenting and defending her theory, and her theory was becoming well accepted by her peers. “Margulis added that flagella and cilia (appendages and protuberances that many microbes use for motility) originated from spiral bacteria called spirochetes (Markov 10).” The second edition of her book, published in 1993, Margulis suggested that “symbiogenesis is a major factor in evolution, leading to the creation of new species via natural selection (Markov

When life arose on Earth about 4 billion years ago, the first types of cells to evolve were prokaryotic cells. For approximately 2 billion years, prokaryotic-type cells were the only form of life on Earth. The oldest known sedimentary rocks found in Greenland are about 3.8 billion years old. The oldest known fossils are prokaryotic cells, 3.5 billion years in age, found in Western Australia and South Africa. The nature of these fossils, and the chemical composition of the rocks in which they are found, indicates that these first cells made use of simple chemical reactions to produce energy for their metabolism and growth. Eukaryotic cells evolved into being between 1.5 and 2 billion years ago. Eukaryotic cells appear to have arisen from prokaryotic cells, specifically out of the archaea. Indeed, there are many similarities in molecular biology of contemporary archaea and eukaryotes. However, the origin of the eukaryotic organelles, specifically chloroplasts and mitochondria, is explained by evolutionary associations between primitive nucleated cells and certain respiratory and photosynthetic bacteria, which led to the development of these organelles and the associated explosion of eukaryotic diversity. Today Prokaryotes

According to Margulis, the pre-eukaryotic cell engulfed an aerobic bacterium, but rather than digest and kill the bacterium, a symbiotic relationship was born. This relationship, the aerobic bacterium provided energy through ATP and the eukaryotic cell provided an environment to live while protecting the new symbiont from harm in environmental factors such as oxygen. Because almost all living eukaryotes have a mitochondria, it is safe to assume that this event happened before plants and animals split in the evolutionary lineage.After this first evolutionary leap came a

Endosymbiosis is the theory that eukaryotic cells were formed when a prokaryotic cell ingested some aerobic bacteria. The first step of the evolution of a eukaryotic cell is the infolding of the cellular membrane. This process takes place when the plasma membrane folds inwards and develops an envelope around a smaller prokaryotic cell. Once the smaller cell is engulfed, it becomes dependent upon its host cell. It relies on the host cell for organic molecules and inorganic compounds. However, the host cell also benefits because it has an increased output of ATP for cellular activities and becomes more productive. This ATP comes from the mitochondrion (the aerobe) that is engulfed.

Prior to the development of DNA technology and the sequencing of organismal genomes, Charles Darwin suggested that the “tree” of life can be traced back to a single root (Koonin and Wolf, 2012). While Darwin’s theory was primitive, it laid the groundwork for the phylogenetic trees that are currently studied in science classrooms around the world. The three-domain tree, containing Eukarya, Archea, and Bacteria, soon became too simplistic due to the realization that some bacteria possessed the ability to exchange genetic information by horizontal gene transfer (Koonin and Wolf, 2012).

Mitochondria are small organelles found in eukaryotic cells which respire aerobically. They are responsible for generating energy from food to ‘power the cell’. They contain their own DNA, reproducing by dividing in 2. As they closely resemble bacteria, it gave the idea that they were derived from bacteria (which were engulfed by ancestors of the eukaryotes we know today). This idea has since been confirmed from further investigations, and it is now widely accepted. (Alberts et al., 2010a)

7. The theory of endosymbiosis says that mitochondria and plastids used to be small prokaryotes living within larger cells. The prokaryotic ancestors of mitochondria and plastids were bacteria engulfed by a larger cell. Because they both benefited from this situation, the bacteria living inside the cell was passed down from generation to generation. The evidence is that mitochondria reproduce and move independently within the cell.

Mitochondria, dubbed the ‘powerhouse of the cell’, are a type of organelle present in most human cells. Their primary function is to generate Adenosine Triphosphate (ATP), the cell’s principal source of chemical energy. Unlike most other organelles, mitochondria store their own set of genetic material, distinct from the DNA situated in a cell’s nucleus. Although this ‘mitochondrial genome’ represents only 0.1% of a cell’s genetic information, it often plays a significant role in development.

The cells of a eukaryote are different from any other cells because they do not have cell walls, and have organelles. An example of an organelle is the nucleus. Mitochondria are the energy providers for cells. A symbiotic relationship is when to organisms benefit from living together. When one organism lives inside the other, it is known as endosymbiosis. Chloroplasts trap sunlight and make photosynthesis possible. Once two organisms live together for a while, they loose their ability to live

Prokaryotes are ubiquitous, successfully adapting to diverse environments as well as developing symbiotic relationships with host organisms (Lengeler, Drews, & Schlegel, 1999). Prokaryotes may have both autotrophic and heterotrophic characteristics. A cyanobacteria is photosynthetic, commonly called blue-green algae, and may produce toxins (Crayton, 1993). Bacteria are most commonly associated in the general

Microscopic  organisms  known  as  cyanobacteria are  interesting  for  the  following  reasons: [SELECT  ALL  THAT  APPLY] Select  one  or  more: A.  Oxygen  produced  by  their  photosynthesis is  thought  to  be  responsible  for  the  "great oxygenation  event"  about  2.3  billion  years ago.   B.  The  methane  they  produce  is  a greenhouse  gas  that  could  have  helped warm  the  early  Earth,  helping  to  resolve  the Early-­faint-­Sun  paradox. C.  The  chloroplasts  that  carry  out photosynthesis  in  green  plants  are evolutionary  descendents  of  early cyanobacteria.   D.  They  are  known  to  be  the  earliest  forms of  life  on  Earth.

Chloroplasts are important photosynthetic organelles that present in plant cells. It is believed that chloroplasts evolve from an endosymbiotic event; engulfment of a photosynthetic cyanobacterium by a large heterotrophic host cell (1, 2). During this process proteins in the cyanobacterium has been transferred to the nucleus and also the proteins that are essential for organelle biogenesis has been transferred to the cyanobacterium making it dependent on the host. Although chloroplast proteins have estimated to consist of 3500-4000 different types of polypeptides, the protein coding capacity in chloroplast genes is approximately 200 polypeptides (3, 4). This data further suggest that most of the proteins found in chloroplast are encode by nuclear genome and transport to the chloroplast. At least, a few proteins are use secretory pathway in which first targeted to the endoplasmic reticulum and then transfer to the chloroplast through vesicles (5-7).

Bacterial symbionts are predominant among organisms that shape the biological world (Hurst 1993, Wernegreen 2004). Some of the bacterial symbionts that have an inordinate influence on the health and evolution of their hosts are those bacteria that live within their symbiotic hosts, so-called endosymbionts. Bacterial endosymbionts are the focus of current research programs by the National Institutes of Health (Human Microbiome Project, http://www.hmpdacc.org/) and the National Science Foundation (The Symbiosis, Defense, and Self-recognition Program).

The Endosymbiotic theory is an assumption based on experience and/or limited information about the evolution of the cell. Bacteria are one of the oldest single cellular organisms. They began to make their own food using photosynthesis which then produced enough oxygen to reshape Earth 's atmosphere. This change brought upon diverse bacterial life which include clear evidence that chloroplasts and mitochondria were, at one point, crude bacterial cells. Over the years, chloroplasts and mitochondria became dependent on a host cell. After millions of years of evolution, chloroplasts and mitochondria cannot survive outside of the cell. This is the Endosymbiotic theory. Although this is labeled a theory, there is striking evidence that shows similarities in both bacteria, and mitochondria and chloroplasts. They all have their own DNA (separate from the nucleus), and they both use this DNA to produce proteins and enzymes for their functions.

The second important endosymbiotic event occurred as a result, in the acquisition of mitochondria by the earliest eukaryotes (Avissar et al., 2016). Mitochondria are responsible for aerobic cellular respiration in eukaryotic cells, and for a long time it was believed that they were simply organelles. But much like plastids in photoreactive eukaryotes, the evidence points to mitochondria having been absorbed by early eukaryotes, forming a symbiotic relationship in which the larger cell protected the smaller and provided a ready source of nutrients, and in turn the mitochondria allowed the larger cells to process all of the new molecular oxygen as an energy source to promote glycolysis (Cooper, 2000).