Transcript Search for Life in the Universe Chapter 5 (Part 1)

Search for Life in the Universe
Chapter 5
The Nature of Life on Earth
(Part 1)
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Outline
• What is Life?
– Properties of Life
– Evolution
– Definition of Life
• Cells: Basic Units of Life
– Carbon Base
– Molecular Components
– Prokaryotes & Eukaryotes
– Bacteria, Archaea & Eukarya
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Properties of Life
• Order: molecules organized in at least one cell 
combine to form organs  combine to form a body
• Reproduction: complex, finite life  need offspring(s)
• Growth & development: hereditary instruction set for
body changes during the life cycle
• Energy utilization: required to maintain order, reproduce
& grow
• Immediate response to environment: dictated by needs
of life
• Long-term response to environment: evolutionary
adaptation over multiple generations
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Evolution (1)
• View today: evolution the most important and
unifying property of life
• Anaximander (c. 610547 BC): life arose in
water and gradually became more complex
• Empedocles (c. 492432 BC): survival of the
fittest
• Aristotle (384322 BC): species fixed and
independent of each other  evolution
discarded for 2000 years
• Fossil record: broke down the Aristotelian theory
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Evolution (2)
• Darwin (18091882):
– Malthus (17661834): populations can grow faster
than food sources can support them  struggle for
survival to wipe out competitors
– Individual variations  advantages or disadvantages
in the struggle for survival
 Natural selection: unequal reproductive success
– Evidence:
• Species in the Galápagos Islands, particularly finches
• Artificial breeding of house/farm animals and vegetables
• Deoxyribonucleic acid (DNA): molecular
mechanism of natural selection
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Definition of Life
• “Something that can reproduce and evolve
through natural selection”
• Parasites: need hosts to survive, but not to
reproduce
• Viruses (including computer viruses) need hosts
not just to survive, but also to reproduce
• Extraterrestrial life: given the above subtleties,
are we sure it is based on reproduction and
evolution?
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Carbon Base
• Living mass dominated (96%) by the most
abundant noninert elements: O, C, H & N
(He and Ne are inert)
• Dominance of O due to H2O
• Chemical bond of C: best base for organic
molecules
• Closes rival Si:
– Frail: chemical bonds much weaker
– No double bonds
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Molecular Components
• Carbohydrates:
– Sugars & starches: source of energy
– Cellulose: source of cellular structure
• Lipids:
– Fats: store energy for long-term use
– Membranes: separate cells and regulate transition between them
• Proteins:
– Enzymes: catalysts enabling chemical reactions
– Amino acids: building block of proteins
– Left handed: amino acids on Earth are all left-handed
• Nucleic Acids:
– Deoxyribonucleic acid (DNA): contains the genetic code
– Ribonucleic acid (RNA): helps to carry out the genetic
instructions
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Prokaryotes & Eukaryotes
• Eukaryotes:
– Have a cell nucleus (membrane to protect the DNA)
– Are base of all multi-cell creatures
– Can also be single-cell creatures: amoeba
• Prokaryotes:
–
–
–
–
–
No cell nucleus
Always single-cell creatures: bacteria
Came first
Outnumber and outweigh eukaryotes
Essential to eukaryotes: decompose dead plants &
animals
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Bacteria, Archaea & Eukarya
• Old morphological division into “kingdoms” now
abandoned in favor of cellular and molecular
divisions
• Cellular division:
– Prokaryotes & eukaryotes
– Prokaryotes divide further: bacteria & archaea
• DNA division:
– Archaea more similar to eukaryotes
 Three domains: bacteria, archaea & eukarya
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