In what ways are all living things the same, and in what ways do we differ? All life shares a series of cellular metabolic processes that work so well that, like valuable recipes, they have been handed down almost unchanged since they were invented long ago. But the most numerous living things, the prokaryotes, are all single-celled, and those cells are simpler in design than those of the eukaryotes. So how did the more complex, multi-cellular forms arise? While going it alone is simpler, some organisms must have found it beneficial to team up and become multi-cellular. Groups of cells doing the same job can form specialized tissues, and groups of tissues doing related work can form specialized organs. In certain situations, dividing up the work allows one to get the job done better or faster. It seems to have worked out well for our ancestors. Show We like to think of ourselves as individuals, but each of us is actually a community of living things. The bacteria we carry around with us outnumber our cells by a factor of ten to one, and represent 1-3% of our body mass (remember that prokaryotic cells are tiny compared to eukaryotic cells.) People may not realize that without the symbiotic organisms that live within us, such as our "normal flora" of our gut bacteria, we could not extract most of the nutrients from our food. When we inadvertently knock out these beneficial bacteria while taking antibiotics, which don't discriminate between "good" and "bad" bacteria, we gain an appreciation for what they do for us when things are working right. But what's even more staggering is that there are remnants of once free-living organisms that have been hitchhiking along in our cells for hundreds of millions of years, and that we and they have grown so interdependent that we could no longer live without each other! The Theory of Endosymbiosis explains how eurkaryotic cells arose. Prokaryotic cells didn't always just form colonies of identical units, though this does sometimes occur. Sometimes different prokaryotes merged into one, contributing metabolic recipes from previously separate lineages into one new organism. Were these biological mergers the origin of sexual reproduction? It was certainly one of the most important events in the evolution of life. Comparing the cells of prokaryotes and eukaryotes gives us a number of clues to these ancient unions. Prokaryotes (bacteria) and Eukaryotes (protists, plants, animals, fungi) compared:
Cellular Organelles: Inside the cells of all Eukaryotes are little organelles called mitochondria, commonly referred to as the "powerhouse" of the cell. Plants have additional organelles called chloroplasts, which are more like nature's version of the solar panel. Mitochondria and chloroplasts, two of the most important cellular organelles, have an intriguing origin. They resemble, in many ways, primitive single-celled prokaryotes. They are double membraned structures which contain their own unique DNA; distinct from the DNA of the nucleus. The outer membranes of the mitochondrion and chloroplast resemble those found in eukaryotic or complex cells, while the inner membranes resemble those found in prokaryotic or primitive bacterial cells. (Remember that these inside out membranes are one of the results of endocytosis!) The DNA of the mitochondrion and the chloroplast form a ringed chromosome called a plasmid; a characteristic of bacteria. It is now generally accepted that these organelles, vital to the survival of the cells that house them, originated from the incomplete consumption or absorption of single celled creatures millenia ago. These single celled organisms were adopted by the cell and became an integral part of the multicellular organism, reproducing themselves when the greater cell divides. Without the energy producing mitochondria and the photosynthetic chloroplasts none of the eukaryotes, the "higher" animals, plants, and fungi, would exist. We owe it all to the lowly bacteria that live within us and nourish us.  Endosymbiotic theory
A eukaryotic cell is distinct from a prokaryotic cell by the presence of membrane-bound cellular structures called organelles. And based on this theory, the organelles mitochondria and chloroplasts are supposedly the early prokaryotic endosymbionts that had been taken in. They stayed inside the host cell for so long that they transitioned into those semi-autonomous organelles we know today. Endosymbiotic theory is one of the theories that are still prevalent to this day. It is a presumption that an endosymbiosis occurred between the early life forms. This form of symbiosis involves a larger cell that serves as a host and a smaller cell that is referred to as an endosymbiont. In endosymbiotic theory, it posited that the larger cell engulfed or took in the smaller cell. The larger cell represents the eukaryotic cell of today whereas the smaller cell is the prokaryotic cell. Watch this vid about endosymbiotic theory: EndosymbiosisEndosymbiosis is one of the many forms of symbiotic relationships (symbioses) that occur between or among organisms. In endosymbiosis, the endosymbiont lives within the body of its host. Endosymbiosis naturally occurs to this day. An example is a biological interaction between Rhizobium and the plant legumes. Rhizobium is the endosymbiont that occurs within the roots of legumes and fixes atmospheric nitrogen into a form that is ready for use by the legume. The legume, in turn, provides Rhizobium metabolites such as malate and succinate from photosynthesis. Endosymbiosis is the precept of the Endosymbiotic Theory, which was first conceptualized by botanist Konstantin Mereschkowski (4 August 1855 – 9 January 1921), and then backed up by scientific evidence by Lynn Margulis 1938–2011. According to the Endosymbiotic Theory, endosymbiosis became the means by which organelles such as mitochondria and chloroplasts within eukaryotic cells came about.1 Proponent of this theory posited that about 1.5 billion years ago a larger cell took in smaller free-living prokaryotes (bacteria) and inside the cell the prokaryotes lived as endosymbionts. Research findings that seem to back up this theory implicate that the mitochondria arose from proteobacteria (such as SAR11 clade) 2 whereas the chloroplasts arose from cyanobacteria (particularly the nitrogen-fixing cyanobacteria). 3 The indication that this theory is plausible is based upon the same features shared by these organelles and their prokaryotic ancestors. Some of the characteristics common to them are as follows:
Other Thoughts
The age of the Earth is estimated to be around 4.54 billion years and life eventually existed and began about 3.5 billion years ago or earlier. The modern theory of abiogenesis holds that life on Earth began when the earliest living entities took in non-living materials. They used these organic compounds to produce biomolecules and other building blocks of life. Biochemical processes, e.g. self-replication, self-assembly, autocatalysis, and cell membrane formation, probably led to the emergence of living entities. These processes were believed to be gradual and comprised of multiple events. In the Miller-Urey experiment, the results indicated that the simulated-primitive Earth favored the chemical syntheses of the fundamental structures of the cell membrane. Mixing gases methane, ammonia, hydrogen, and water and then electrically-sparking them resulted in the formation of amino acids.
Around four billion years ago, the Earth was hostile to life. No life forms could exist due to the harsh conditions. Eventually, simple organic compounds formed. The hypothetical model of the early Earth with conditions that led to the synthesis of simple organic compounds is called the prebiotic (primordial) soup. Alexander Oparin 1894–1980 and John Burdon Sanderson Haldane 1892–1964 were the ones to conceive this idea and independently formulated theories that collectively became the heterotrophic origin of life theory. Both of them theorized that the early Earth’s atmosphere was a chemically reducing atmosphere. It aided in producing such organic compounds. As these compounds were produced, they accumulated and formed a so-called prebiotic soup. Through time, these simple organic compounds transformed into more complex organic polymers. In the long run, life came about. The first life entities took in and used organic molecules to thrive and survive in the prebiotic soup. They theorized that the first forms of life were heterotrophic. Recent evidence, though, suggests that autotrophs are likely the first organisms.
The four major biomolecules essential to life are nucleic acids (e.g. RNA, DNA), carbohydrates (various sugars), lipids (fats), and amino acids (constituents of proteins). Primitive life is hypothesized to be RNA-based since RNA could be both genetic material and a catalyst. The transitioning of primitive life forms into single-celled living things occurred gradually for many million years. Take the Quiz on Endosymbiotic Theory! Further Reading
References
© Biology Online. Content provided and moderated by Biology Online Editors How are chloroplasts and mitochondria related to the Endosymbiotic theory?Mitochondria and chloroplasts likely evolved from engulfed prokaryotes that once lived as independent organisms. At some point, a eukaryotic cell engulfed an aerobic prokaryote, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion.
How are chloroplasts and mitochondria similar they both?Both chloroplasts and mitochondria are semi-autonomous organelles having their own DNA and protein-synthesizing mechanisms. Both of them help in the cytoplasmic inheritance of certain specific characters and both depend on nuclear genes for biosynthetic activities. Was this answer helpful?
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How are mitochondria and chloroplasts similar in terms of the endosymbiotic theory )?
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