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Chapter 2

BIOLOGY Chapter 2.docx

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Department
Biology
Course
BIOL 1000
Professor
Paula Wilson
Semester
Fall

Description
BIOLOGY Chapter 2: Origins of Life What is Life? 9/13/2012 3:19:00 PM The fundamental Unit of Life is the Cell: - three tenets of the cell theory: 1. All organisms are composed of one or more cells. some organisms (e.g. prokaryotes) are composed of a single cell - unicellular organisms, the one cell is a functionally independent organisms capable of carrying out all life activities in multicellular organisms (e.g. plants and animals), major life activities are divided among varying numbers of specialized cells 2. The cell is the smallest unit that has the properties of life. if cells are broken open, the property of life is lost: they are unable to grow, reproduce, or respond to outside stimuli in a coordinated, potentially independent fashion 3. Cells arise only from the growth and division of preexisting cells. although deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) contain the information required to manufacture a vast array of biological molecules, they cannot orchestrate the formation of an entire cell - new cells can arise only from the division of preexisting cells FIGURE 2.4 PAGE 25 shows examples of many different kinds of cells The Origins of Information and Metabolism 9/13/2012 3:19:00 PM - out of the several critical events necessary for the development of life, two stand out: 1. The Origin of the Information System: - all organisms contain DNA - DNA is a large, double-stranded, helical molecule that contains a unique alphabet that provides the instructions for assembling many of the important components of a cell organism from simpler molecules - DNA functions similarly in all organisms the information in DNA is copied onto molecules of a related substance, RNA, which then directs the production of protein molecules (FIGURE 2.11 PAGE 29) - even the simplest of cells contains a thousand of proteins, each coded for by a unique DNA sequence - flow of information from DNA to RNA to protein is common in all forms of life - enzymes are required to catalyze (speed up) the replication of DNA, the transcription of DNA into RNA, and, the translation of the RNA into protein - this information pathway is preserved from generation to generation by the ability of DNA to direct its own replication so that offspring receive the same basic molecular instructions as their parents have - changes in DNA are what contribute to evolutionary change over generations 2. The Development of Energy Harnessing Reaction Pathways: - oxidation-reduction reactions were among the first energy-releasing reactions of the primitive (original) cells - in our cells, we oxidize food molecules (e.g. sugars) and use some of the liberated/released energy (electrons) to reduce other molecules such as, for example, those needed to synthesize proteins - in primitive cells, the elections removed in an oxidation would have been transferred directly to the substances being reduced in a one-step process this is not efficient and leads to a lot of wasted energy - over time, multistep processes would have evolved, whereby the energy from oxidation is slowly released Example of this is cellular respiration (discussed in chapter 6) - greater efficiency of stepwise energy release would have favored development of intermediate carriers and opened the way for primitive electron transport chains - as part of the energy-harnessing reactions, adenosine triphosphate (ATP) become established as the coupling agent that links energy-releasing reactions to those requiring energy - ATP may first have entered early cells as one of many organic molecules absorbed from the primitive environment - initially, it was probably simply hydrolyzed into adenosine diphosphate (ADP) and inorganic phosphate, resulting in the release of energy - later, as cells developed, some of the energy released during electron transfer was probably used to synthesize ATP directly from ADP and inorganic phosphate because of the efficiency and versatility of energy transfer by ATP, it gradually became the primary substance connecting energy-releasing and energy-requiring reactions in early cells Early Life 9/13/2012 3:19:00 PM Earliest Evidence of Life: - earliest evidence of life is found in the fossilized remains of structures called stromatolites, dated to about 3.5 billion years ago - stromatolites: a type of layered rock that is formed when microorganisms bind particles of sediment together, forming thin sheets (FIGURE 2.14 PAGE 32) - sureness that fossil stromatolites were formed by microorganisms comes from the fact that modern day stomatolites still exist in habitats characterized by warm shallow water, and they do harbor microbial life - modern day stromatolites are formed by the action of a group of photosynthetic prokaryotes called cyanobacteria - cyanobacteria posses a metabolism that suggest that earlier life forms must have preceded their evolution Could Life Have Come to Earth from Space? - panspermia: the hypothesis that very simple forms of life are present in outer space and may have seeded early Earth Two points support this: Life arose relatively quickly after the formation of Earth Earth formed 4.6 billion years ago, and evidence of life dates back to 3.5 billion years ago, and chemical evidence to about 3.9 billion years ago primordial Earth had to cool after being formed scientists argue that this window for development of life is very narrow Research in the past decade has shown that life is resilient (able to recover quickly) and could survive for years in space extremophiles (mostly prokaryotes) thrive under very harsh conditions (of temperature, pressure, and nutrients) and might be able to survive in a dormant state in interstellar space prolonged dormancy (inactivity) is a property of the spores of a range of organisms spores are highly resistant to changes in the external environment and can be restored to active growth after exposure to high levels of radiation, water deficiency, and/or exposure to extreme temperatures Given this, one cannot ignore the possibility that simple life forms came to Earth about 4 billion years ago and initiated the evolution of life as we know it
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