Solomon IM Ch 21

Solomon IM Ch 21 -...

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Chapter 21: The Origin and Evolutionary History of Life 219 21 The Origin and Evolutionary History of Life Lecture Outline I. Early Earth provided the conditions for chemical evolution. A. Life originated under very different conditions than those experienced today. 1. Earth is approximately 4.6 billion years old. 2. The atmosphere of early Earth was composed of CO 2 , H 2 O vapor, CO, H 2 , and N 2 . a) Some NH 3 , H 2 S, and CH 4 may have been present in small quantities. 3. There are 4 requirements for the chemical evolution of life a) No free oxygen was available, as the atmosphere of early Earth was strongly reducing. b) The source of energy was volcanism, thunderstorms, bombardment by extraterrestrial objects, and UV radiation. c) Chemical building blocks included water, ions, and dissolved gases. d) Lots of time was readily available! B. Organic molecules formed on primitive Earth. 1. Two hypotheses suggest the site for this process. a) The prebiotic soup hypothesis states that precursors to life on Earth occurred near Earth’s surface. b) The iron sulfur world hypothesis states that organic molecules formed on or near cracks on the ocean’s floor. 2. Oparin and Haldane independently suggested that simple organic molecules formed from simpler raw materials. 3. The hypothesis was tested in the 1950s by Stanley Miller and Harold Urey in a setup that simulated conditions on early Earth (as was believed at that time). a) An atmosphere rich in H 2 , CH 4 , H 2 O, and NH 3 was exposed to electrical charges. b) Amino acids and other simple organic compounds formed spontaneously. c) More recent experiments using different combinations of gases produced important organic molecules including all 20 amino acids, some sugars, lipids, RNA and DNA nucleotide bases, and ATP. 4. More current hypotheses postulate that the organic polymers didn’t form in shallow seas (“primordial soup”), but rather on the charged surfaces of clay. a) Laboratory experiments have confirmed this possibility. 5. The polymerizations may also have occurred near the oceanic hydrothermal vents. a) The vents are well protected from meteoric bombardment. b) The water emanating from the vents is rich in chemicals that are rich in energy, including CH 4 and H 2 S. 6. A protobiont is the hypothetical ancestor of the cell—it is an assemblage of organic polymers.
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Instructor’s Manual for Solomon, Berg, and Martin’s Biology, 9 th Edition 220 a) Protobionts exhibit division after growth and maintain an internal chemistry different from the external fluids. b) Microspheres are protobionts formed by adding water to polypeptides. (1) Microspheres may show an electrical potential. (2)
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Solomon IM Ch 21 -...

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