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3.1-3.3 - Defining Life & Its Origins.docx

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Biology 1001A
Tom Haffie

3.1: What Is Life 3.1a: Seven Characteristics Shared by All Life-Forms  Life is defined by a list of attributes that all life and forms of life possess:  All life displays order, harnesses and utilizes energy, reproduces, responds to stimuli, exhibits homeostasis, grows and develops, and evolves  Some biological systems straddle the line between the biotic and abiotic worlds  E.g. a virus: small, infectious agents that display many of the properties of life, including the ability to reproduce & evolve over time; however, characteristics are based on its ability to infect cells. They lack the cellular machinery & metabolism to use the DNA and RNA that it contains to synthesize their own proteins  They must infect living cells & hijack their translational machinery & metabolism to reproduce 3.1b: The Characteristics of Life are Emergent  Each of the characteristics of life reflects a complexity resulting from a hierarchy of interactions that begins with atoms and progresses through molecules to macromolecules & cells  Hierarchy may continue to become more complex and include organelles, tissues and organs  7 properties of life:  Display Order: All forms of life are arranged in a highly ordered manner, with the cell being the fundamental unit of life  Harness & Utilize Energy: All forms of life acquire energy from the environment and use it to obtain their highly ordered state  Reproduce: All organisms have the ability to make more of their own kind  Respond to Stimuli: Organisms can make adjustments to their structure, function and behaviour in response to the changes to the external environment  Exhibit Homeostasis: Organisms are able to regulate their internal environment such that conditions remain relatively constant  Growth & Development: Organisms increase their size by increasing the size and/or number of cells. Many organisms also change over time  Evolve: populations of living organisms change over the course of generations to become better adapted to their environment  The properties of life are known as emergent because they “emerge” from simpler interactions that don’t have the properties found at the higher levels  E.g. the ability to harness & utilize energy isn’t a property of molecules or proteins or biological membranes in isolation, but the ability emerges from the interactions of all three as a part of a metabolic process  Not only is the structural or functional complexity of these systems more than the sum of the parts, but it is fundamentally different  Example: termite nest called a cathedral – product of activities of thousands of termites, but there is no master plan that is followed, they just build it cell-by-cell based on local conditions, unaware of the overall structure that emerges 3.2: The Chemical Origins of Life 3.2a: The Earth is 4.6 Billion Years Old  Earth was formed approx. 4.6 billion years ago  Based on chemical evidence, life formed as early as 4.0 billion years ago  First clear fossil evidence of prokaryotic cells occurs approx. 3.5 bya  Fossil evidence of eukaryotes dated back to approx. 2.0 billion years ago  Animals: mid-October 525 MYA  Land plants came afterwards  Extinction of dinosaurs: 65 MYA  Humans around for only 150,000 years 3.2b: Earth lies within the Habitable Zone around the Sun  Widely accepted hypothesis: all components of the solar system were formed at the same time by the gravitational condensation of matter present in an interstellar cloud, initially consisted mostly of hydrogen  Heat & pressure generated in the central region of the cloud formed the sun  Remainder of the spiralling dust & gas condensed into the planets  This activity is typical for the vast majority of the stars and planetary systems in our galaxy  Early Earth history marked by bombardment of rock from the forming solar system & extensive volcanic and seismic activity  Earth radiated away some of its heat & surface layers cooled & solidified into the rocks of the crust  Earth’s gravitational pull was strong enough to hold an atmosphere around the planet  Atmosphere derived from original dust cloud & gases released from Earth’s interior as it cooled  Estimated that it took approx. 500 million years for earth to cool to temperatures suitable for the development of life  Earth is situated at the position where heat from the Sun allows for surface temperatures to be within a range that allows water to exist in a liquid state  Presence of liquid water = fundamental prerequisite for the development of life  Habitable Zone: the region of space around star where temperatures would allow for liquid water  The precise distance from the star that defines the habitable zone will vary depending on the type of star & how much energy it emits 3.2c: Biologically Important Molecules can be Synthesized Outside of Living Cells  All forms of life are composed of the major macromolecules: nucleic acids, proteins, lipids & carbohydrates  All of these macromolecules except for lipids are derived from simpler molecules such as nucleotides, amino acids & sugars, that are the products of metabolic pathways  3 major hypotheses behind how these molecules were formed in the absence of life: 1. Reducing Atmosphere  Atmosphere of 4 billion years ago = vastly different from the one today  Primordial atmosphere probably contained abundance of water vapour from the evaporations of water at the surface, as well as large quantities of hydrogen, carbon dioxide, ammonia and methane; there was almost a complete absence of oxygen  Proposed by Alexander Oparin and John Haldane in 1920s that organic molecules, essential to the formation of life, could have formed in the atmosphere of primordial Earth  Oparin-Haldane Hypothesis: early atmosphere was a reducing atmosphere because of the presence of large concentrations of molecules like hydrogen, methane and ammonia  Molecules contain an abundance of electrons & hydrogen would’ve entered into reactions with one another to create larger and more complex organic molecules  Today’s atmosphere is classified as an oxidizing atmosphere  High levels of oxygen prevents complex, electron-rich molecules from being formed because oxygen is a particularly strong oxidizing molecule and would accept electrons from organic molecules and be reduced to water  L
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