First Organic Molecules Advanced Douglas Wilkin, Ph.D. Barbara Akre
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AUTHORS Douglas Wilkin, Ph.D. Barbara Akre
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C HAPTER
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Chapter 1. First Organic Molecules - Advanced
1
First Organic Molecules Advanced
Interpret the importance of Miller and Urey’s experiment. Relate the properties of phospholipids to the formation of the first membranes. Compare and contrast the “genes-first” model of the origin of life to the “metabolism first” model. Explain why some scientists believe that RNA was the basis of early life. Evaluate the hypothesis that exogenesis explains the origin of life on Earth.
How do you make large molecules? From smaller ones. The first organic molecules were probably very simple carbon-based molecules made of few atoms. These molecules then combined with other simple molecules to form more complex molecules. Over many years (and probably trillions and trillions of chemical reactions), more complex and more stable molecules formed.
The First Organic Molecules
The Hadean Eon ended 3.8 billion years ago; its timeline is marked by Earth’s oldest known rocks (between 3.8 and 4.2 billion years old) and oldest known minerals (formed 4.4 billion years ago). Scientists use these dates to estimate that the Earth itself is 4.6 billion years old. Evidence of life during the Hadean Eon does not exist, although many scientists push the theoretical origin back that far. How –and when –did life arise? Once again, we will begin with the materials of life –organic molecules this time, made primarily of the element carbon. Most scientists agree that these organic molecules arose before cells, which we now consider essential to the definition of life. Several hypotheses and experiments suggest ways in which organic building blocks may have formed. 1
www.ck12.org Oparin and Haldane
In 1924, the Russian biochemist Aleksandr Oparin proposed that life could have developed through gradual chemical evolution in a “ primordial soup.” This theory was independently proposed by English geneticist J.B.S. Haldane, also in the 1920s. This theory states that a mixture of organic molecules could form in an oxygen-deprived environment through the random interactions of small molecules, with energy provided by sunlight and lightning. This theory is not without controversy. See http://leiwenwu.tripod.com/primordials.htm for additional detailed information.
Miller and Urey
In 1953, Stanley Miller and Harold Urey designed a now-famous test of the hypothesis that the conditions of primitive Earth favored chemical reactions that synthesized organic molecules from inorganic precursors. Their experiment ( Figure 1.1) showed that a mixture of gases, believed to be part of primitive Earth’s atmosphere, when subjected to sparks representing lightning, formed a mixture of monomers representing each of the four major groups of organic molecules. These gases included methane, ammonia, water, and hydrogen. Together, these gases contain carbon, hydrogen, nitrogen, and oxygen atoms, which are all components of organic compounds. Although DNA, RNA, and polymers were absent, 13 of the 22 amino acids (monomers) that make up modern proteins, plus lipids, sugars, and some building blocks of DNA and RNA, were among the products of the experiment. The “leap” from building blocks to polymers and from organic soup to individual replicating units has been more difficult to demonstrate. In the ’50s and ’60s, Sydney Fox showed that early Earth conditions could result in short chains of amino acids, which in turn could form enclosed spheres. Phospholipids can self-organize into membranes in a similar fashion, and cell membranes today consist primarily of a bilayer of these lipids. Phospholipids or polypeptides could have surrounded and protected early metabolic units, forming protocells shown in the Figure 1.2. Simple membrane-enclosed spaces may have led to the later evolution of true cells.
FIGURE 1.1 Miller and Urey’s Experiment.
Miller
and Urey demonstrated that organic molecules could form under simulated conditions of early Earth. The Miller-Urey experiment subjected a mixture of gases thought to be present in Earth’s primitive atmosphere to sparks, representing lightning. After one week, the nonliving system had formed 13 of the 22 amino acids which make up modern proteins, sugars, lipids, and some of the building blocks of DNA and RNA.
See RNA was the first genetic molecule at http://www.dnaftb.org/26/animation.html to see Stanley Miller discuss his work. Recently, the findings of Miller and Urey have come into question due to discrepancies in the composition of the 2
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Chapter 1. First Organic Molecules - Advanced
FIGURE 1.2 Phospholipids,
with hydrophilic phos-
phate "heads" and hydrophobic lipid "tails" self-assemble into membranes (left) and enclosing spheres (right) which could have protected early metabolism from external chemical disturbances.
early atmosphere, allowing a number of other ideas to surface on the formation of the first organic molecules. One idea states that the active volcanoes on early Earth gave the necessary materials for life. Despite the simplified account discussed above, the problem of the origin of the first organic compounds lingers. Despite tremendous advances in biochemical analysis, answers to the problem remain unsolved. But whatever process did result in the first organic molecules was probably a spontaneous process, with elements coming together randomly to form small compounds, and small compounds reacting with other elements and other small compounds to make larger compounds. So, which organic molecule came first?
DNA or RNA?
Which came first, RNA or DNA? Is DNA necessary to order amino acids into a polypeptide? As RNA is the intermediary in the Central Dogma between DNA and protein, does that mean that DNA had to exist prior to RNA? Not necessarily. Is some sort of hereditary molecule, that can be replicated, necessary before "life" can exist? By the very definition of life, the answer is yes. Biologists Walter Gilbert (1980 Nobel Laureate), Carl Woese, and Alexander Rich proposed that RNA, because it can serve both catalytic and replicating functions, was the first informational molecule, and formed the “ RNA World Hypothesis” for the origin of life. Molecular biologist Sol Spiegelman created a short chain of RNA 218 nucleotides long, which was able to replicate itself in the presence of RNA polymerase; the segment is now known as the “Spiegelman monster.” The idea that a successful replicator molecule preceded the evolution of biochemical pathways is the “Genes-First” model. In contrast, the German chemist Günter Wächtershäuser proposed that sulfides of iron and other minerals contain energy which could have polymerized basic building blocks. He argued that extensive evolution of biochemical pathways might have preceded replicator molecules and individualization of life. His ideas formed the basis of William Martin’s and Michael Russell’s 2002 hypothesis that black smokers at seafloor spreading zones, shown in the Figure 1.3, could have provided conditions for extensive chemical and biochemical pathway evolution. Their reasoning suggests that lipid membranes, allowing independent lives away from the smokers, could have been a last step in early evolution. The fact that archaebacteria and eubacteria (and eukaryotes) have completely different membrane lipids but similar metabolism supports the concept of early biochemical pathway evolution. These ideas comprise the “Metabolism-First” model. The discovery of organic molecules in space supports the exogenesis hypothesis which proposes that life could have originated elsewhere –on Mars, or at some distant point in the universe. Comets and meteorites are known to contain organic molecules and could have delivered them to Earth. Exogenesis does not really answer the question of how life originated, but provides a much wider temporal and spatial framework in which it could have happened. 3
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FIGURE 1.3 Black smokers at a mid-ocean ridge hydrothermal vent could have provided conditions suitable for the evolution of early biochemical pathways and much of metabolism, even before lipid membranes formed cells. Martin and Russell proposed that the last universal common ancestor may have emerged from a black smoker.
Vocabulary
• exogenesis: The hypothesis that life originated elsewhere in the universe and was spread to Earth. • genes-first model: The idea that a successful replicator molecule preceded the evolution of biochemical pathways. • metabolism-first model: The proposal that extensive evolution of biochemical pathways might have preceded replicator molecules and individualization of life. • organic molecule: A molecule which contains carbon, made by living organisms; examples include carbohydrates, lipids, proteins, and nucleic acids. • primordial soup: Oceans in which gradual chemical evolution formed life –proposed by Aleksandr Oparin and J.B.S. Haldane. • protocell: Simple, membrane-enclosed early metabolic units surrounded by phospholipids or polypeptides; they were precursors to true cells. • RNA World Hypothesis: The hypothesis that proposes that RNA evolved prior to DNA. Summary
• Miller and Urey showed that a spark igniting a mixture of gases resembling Earth’s primitive atmosphere could produce most of the building blocks for organic molecules of life –thus forming an “organic soup.” • Some lipids and certain polypeptides can spontaneously form into protocells; these early membranes could have self-organized in this way. • The “Genes-first” hypothesis proposes that replicating molecules evolved before biochemical pathways. • Some scientists believe RNA, rather than DNA, was the first replicator. • The “metabolism-first” model suggests that biochemical pathways evolved in an organic soup before selfreplicating molecules. 4
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Chapter 1. First Organic Molecules - Advanced
Practice I
Use the time slider in this resource to answer the questions that follow. • Evolution at http://johnkyrk.com/evolution.swf . 1. 2. 3. 4. 5.
When did the element carbon form? When did the first chemicals appear in Earth’s atmosphere and on its surface? List 5 of these early chemicals. When did the first organic molecules appear? What were these first organic molecules?
Practice II
• Miller-Urey Experiment at http://www.ucsd.tv/miller-urey/ For more information on the Miller-Urey Experiment, explore this site. Review
1. What does the primordial soup theory state? 2. Describe Miller and Urey’s experiment and evaluate its importance to our understanding of the origin of life. 3. Compare and contrast the RNA World, genes first, metabolism first, and exogenesis models of the origin of life. Evaluate the evidence supporting each model.
References 1. User:Carny/He.Wikipedia. http://commons.wikimedia.org/wiki/File:MUexperiment.png . CC BY 2.5 2. OpenStax College; MDougM. http://commons.wikimedia.org/wiki/File:0302_Phospholipid_Bilayer.jpg; ht tp://commons.wikimedia.org/wiki/File:Lipid_vesicles.svg . CC BY 3.0; Public Domain 3. Courtesy of NOAA. http://commons.wikimedia.org/wiki/File:Nur04506.jpg . Public Domain
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