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Chapter 3

Biology Chapter 3

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Wilfrid Laurier University
Holly Smith

Biology chapter 3 Defining life and its origins What is life? - There is very little difference between biotic and abiotic worlds at the atomic/chemical level [relative amounts of elements are different, molecules that are made up of certain atoms will be very different, organisms have organic molecules, and organization is very different] - life is defined most effectively by a list of attributes that all forms of life possess 1. Display order: all forms of life are arranged in a highly order manner with the cell being the fundamental unit of life 2. Harness and utilize energy: all forms of life acquire energy from the environment and use it to maintain their highly ordered state 3. Reproduce: all organisms have the ability to make more of their own kind 4. Respond to stimuli: organisms can make adjustments to their structure, function and behavior in response to changes at the external environment 5. Exhibit homeostasis: organisms are able to regulate their internal environment such that conditions remain relatively constant 6. Growth and development: all organisms increase their size and or number of cells. Many organisms also change over time 7. Evolve: populations of living organisms change over the course of generations to become better adapted to their environment These 7 properties are called emergent because they come about or emerge from many simpler interactions that in themselves do not have the properties found at the higher levels. Is a virus alive? - They have some properties of life. - They have order [even though they’re simple], viruses reproduce, and they also evolve. - In order for a virus to reproduce they rely on a host cell [a virus must infect a cell in order to reproduce/kill host cell] - Viruses do not make their own proteins. They rely on the host cell to make their proteins for them. - Not able to regulate their internal environment very well [Do not have homeostasis] Chemical origins of life: it is believed that over the course of hundreds of millions of years, cells with the characteristics of life arose out of a mixture of molecules that existed on primordial Earth - Earth is 4.6 Billion years old. Earliest prokaryote was formed around 4 billion years ago. First fossil was found 3.5 billion years ago. Oxygen was believed to have increased in the atmosphere 2.5 billion years ago. Earliest Eukaryote was formed 2.2 billion years ago. Animals did not make an appearance until 525 million years ago, following with plants a month later. Earliest humans existed 150,000 years ago. Solar system: According to the most 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, which initially consisted mostly of hydrogen. There was a lot of methane, carbon dioxide, ammonia, hydrogen sulfide but no oxygen. - Intense heat and pressure generated in the central region of the cloud formed the sun, whereas the remainder of the spiraling dust and gas condensed into the planets. - Over time earth radiated away some of its heat, because of its size earth’s gravitational pull was strong enough to hold an atmosphere around the planet - It is estimated that it took around 500 million years for earth to cool down to temperatures that could nurture the development of life. - The reason why it is believed that Earth is the only planet with life is because it is the only planet with a distance from the sun that allows it to have water at its liquid state. [water is the fundamental prerequisite for the development of life] - Habitable zone: region of space around a star where temperatures would allow for liquid water. [depends on the star and how much energy it emits] Biological important molecules: All forms of life are composed of the major macromolecules nucleic acids, proteins, lipids and carbohydrates. [with the exception of lipids, these macromolecules are derived from simpler molecules such as nucleotides, amino acids and sugars that in modern day cells are the products of complex metabolic path ways- polymers] Origins of life on earth: 1. Abiotic production of small organic molecules 2. Polymerization of organic molecules into macromolecules 3. Macromolecules become enclosed in a lipid membrane 4. Origin of self-replicating molecules eventually made inheritance possible (evolution) Why were these molecules formed in the absence of life? There are 3 major hypotheses: 1. reducing atmosphere: the atmosphere today is different from the atmosphere 4 billion years ago. The primordial atmosphere probably contained an abundance of water vapor from the evaporation of water at the surface, as well as large quantities of hydrogen, carbon dioxide, ammonia, and methane. Almost complete absence of oxygen[0 ] 2odays atmosphere is classified as an oxidizing atmosphere, this is because of the presence of oxygen prevents complex, electron- rich molecules from being formed because oxygen is a particularly strong oxidizing molecule and would itself accept the electrons from organic molecules and be reduced to water. Miller Urey apparatus- tried to recreate earth’s atmosphere to test their hypothesis [found a lot of organic compounds, found amino acids and urea then by changing atmospheric conditions to get fatty acids, purines and pyrimidine’s, sugars, and phospholipids- they did not form polymers, they only formed macromolecules] 2. Deep sea vents: the complex organic molecules necessary for life could have originated on the ocean floor at the site of deep-sea vents. These cracks are found around the globe near sites of volcanic or tectonic activity and release the superheated nutrient rich water at temperatures in excess of 300°C as well as reduced molecules. Scientists find interest in these areas because of their ability to live in areas with absence of light and extreme pressure levels 3. Extraterrestrial origins: it is entirely possible that the key organic molecules required for life to begin came from space. Around 500 meteorites impact earth a year. After a specific meteorite hit Australia in 1969, scientists discovered an assortment of biologically important molecules Synthesis of polymers: - Primordial earth contained very little oxygen, because of this organic molecules could have existed for much longer than would be possible in todays energy rich atmosphere. - Polymers are macromolecules formed from the bonding together of individual monomers. Polymers are synthesized by dehydration synthesis - The synthesis of proteins and nucleic acids requires protein based catalysts called enzymes and results in macromolecules that often consist of hundreds to many thousands of monomers linked together. - The question now was how did macromolecules link together? It was very hot so the earth’s surface had a lot of clay and experimentally they showed that if you take these macromolecules and drop them on hot surfaces they can join into polymers From macromolecules to life: we need to explain 3 key attributes of a modern cell 1. A membrane defines compartment- the cell 2. A system to store genetic information and use it to guide the synthesis of specific proteins 3. Energy-transforming pathways to bring in energy from the surroundings and harness it to sustain life. - Protobiont: the term given to a group of abiotically produced organic molecules that are surrounded by a membrane or membranelike structure. [lab experiments show that protobionts could have formed spontaneously, without any input of energy] - An early type of a protobiont could have been similar to a liposome, which is a lipid vesicle in which the lipid molecules form a bilayer very similar to a cell membrane. [can be easily made in the laboratory and are selectively permeable, allowing only some molecules to come in and out] - Would have had early forms of life [in labs they sometimes spontaneously divided on their own] RNA can carry information and catalyze reactions: - the flow of information from DNA to RNA protein is common to all forms of life and is referred to as the central dogma. Each step of the information flow requires the involvement of a group of proteins called enzymes, which catalyze the transcription of DNA into RNA and the translation of RNA into protein. - How did such a system evolve when the product
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