1. Homework #6
due Monday, October 25, 10:00 pm

2. Obtaining Energy Living organisms can obtain energy through
“eating”, energy & nutrients from other organisms
extraction from chemical reactions in the environment (black smokers - ocean vents)
extraction from radiative energy (e.g., photosynthesis)

3. Metabolism

4. Metabolism: chemical reactions within living organisms
Metabolism: chemical reactions within living organisms. It takes place within cells.
Why in cells? Chemical reactions much faster than in the open
Collects the raw materials for the chemical reactions
Provides the energy for the reactions
Provides enzymes to catalyze the reactions
Instructions for enzymes encoded in DNA

5. Without enzymes, life as we know it would not exist.
A specialized substance that acts as a catalyst to regulate the speed of the many chemical reactions involved in the metabolism of living organisms.
Without enzymes, life as we know it would not exist.

6. Metabolism and Cells Metabolism: Cells:
Four forms of metabolism defined by:
Sources of carbon (direct or indirect)
Sources of energy (light or chemical)
The four forms of metabolism are quite general and should apply to life anywhere
Cells:
Needed environment for metabolism at acceptable rate
Origin of Life (on Earth and elsewhere):
Look for cells as sites of metabolism

7. Carbon and Energy Sources
Heterotroph: eat other organisms
Autotroph: self-feeding by converting atmospheric CO2
Energy:
Photoautotrophs (plants): photosynthesis: CO2 + H2O + sunlight sugar
Photoheterotrophs (rare prokaryotes): carbon from food but make ATP using sunlight
Chemoheterotrophs (animals): energy from food
Chemoautotrophs (extreme prokaryotes): energy from chemicals and not sunlight

8. Why Carbon based? Can bond to as many as 4 atoms at a time.
Can form skeleton of long chains of atoms (polymers).
The complexity of life requires complex molecules.
Fire – actually has several of these characteristics; viruses have few
All living entities have the ability to reproduce and perpetuate genetic variation

9. Silicon can also form 4 bonds and is relatively abundant, however…
Bonds are weaker than those of carbon (fragile: complex Si-based molecules don’t last long in water)
Does not normally form double-bonds like Carbon; this limits the range of chemical reactions and molecular structures.
Carbon is more mobile in the environment - it can travel in gaseous form, e.g., CO2
Fire – actually has several of these characteristics; viruses have few
All living entities have the ability to reproduce and perpetuate genetic variation

10. Environmental limits to life (as we know it) ?
Is the planet of interest missing any of the key ingredients? (water, energy, nutrients)
Are temperatures below –15 or beyond +115 C?
Is it really cushy? – does it have an atmosphere
>125 protein and carbohydrates and genetic materials break down
Below freezing chemical reactions are too slow to support life
Water can be intermittent – just has to be available as a liquid
Atmosphere – thick enough to block radiation and incoming meteorites; moderate temperatures; gravity issue – planet or moon has to be large enough to hold an atmosphere
Energy – light or chemical
Nutrients – all planets and moons in the solar system have the same general chemical composition. Must have a tectonic or volcanic or water or atmospheric process to make these available.

11. Importance of liquid water
Contact: organic chemicals float in the cell and find each other
Transportation: bring chemicals in and out of cells
Participant in reactions, e.g.,:ATP, photosynthesis
Necessity:
Life on Earth: all use water
Dormant without water: for a limited time only
Elsewhere: need a liquid (are there alternatives?)

12. Water Liquid water plays a fundamental role in life:
Make chemicals available (dissolved)
Transports chemicals
Plays a role in many metabolic reactions

13. Cells
All life on Earth is made of cells - microscopic units in which living matter is separated from the outside world by a membrane.
All cells on Earth share common characteristics (e.g., use of ATP, DNA, …), leading to conclusion that they share a common ancestor
All cellular life is carbon based (organic molecules)

14. Components of Cells
Carbohydrates: energy needs and structures
Lipids: Source of energy & major component of membranes. Lipids can spontaneously form membranes in water.
Proteins: participate in a vast array of functions; structural, enzymes, catalysts. Built from long chains of amino acids.
Nucleic acids: instructions for reproduction

15. 70 amino acids known to exist; only 22 are found in life on Earth.
Only left handed versions are found in living organisms
Both of these traits suggest a common ancestor for life on Earth.

16. Based upon the cellular structure of an organism, living cells come in two types:
Prokaryotes
Eukaryotes

17. The prokaryotes simplest type of cell lack a cell nucleus
Most are unicellular
two domains: bacteria & archaea
asexual reproduction
many do not require free oxygen

18. Eucaryotes
cells are organized into complex structures enclosed within membranes. Have a nucleus.
typically much larger than prokaryotes
May be unicellular, as in amoebae, or multi-cellular, as in plants and humans.
both sexual and asexual reproduction

19. Extremophiles
Life that exists under “extreme” conditions, conditions that until recently were thought to be inhospitable to life.

20. Extremophiles Volcanic vents:
Water temperature reaches 400°C (750°F), possible because of the large pressure
Black smokers: mixed with volcanic chemicals

21. Extremophiles Antarctic dry valleys:
Microbes in small pockets of water in rocks

22. Extremophiles Lithophiles (rock lovers):
Several kilometers below the surface
Chemical energy from rocks
Carbon from CO2 filtering down

23. Extremophiles Endospores (e.g., anthrax)
Can lay dormant for long periods
Can survive lack of water, extreme heat and cold, and poisons
Some can survive in vacuum

24. Implications for Extraterrestrial Life
Oxygen for eukarya on Earth for only ~10% of the age of the Earth
What is the probability that eukarya-like organisms would develop?
We are more likely to find extremophiles elsewhere
Extremophiles may be the norm, not the exception

25. Searching for the Origin of Life
When Did Life Begin?
DNA Molecules as Living Fossils
Where Did Life Begin?
How Did Life Begin?
Early Organic Chemistry
Chemistry to Biology
Migration of Life to Earth?
Early Evolution and the Rise of Oxygen
Early Microbial Evolution
Photosynthesis and Oxygen
Rise of Oxygen

26. Origins of Life

27. Certain chemical processes are energetically favored given specific circumstances, i.e., in the presence of specific elements, energy, liquid(s)
Can the presence of specific elements and energy inevitably lead to the formation of life?
We know it can lead to the building blocks

28. The Miller–Urey experiment
Simulated conditions thought to be exist on the early Earth
Tested for the occurrence of chemical evolution
Considered the classic experiment on the origin of life
After one week, Miller and Urey observed that 10–15% of the carbon was now in the form of organic compounds.
Recent re-analysis of Miller's archived solutions found 22 amino acids

29. Conditions simulated in Miller-Urey experiment do not match conditions now thought to have existed.
Experiment has been redone MANY times, with more realistic conditions.
Results remain consistent:
the presence of specific elements & energy inevitably lead to the building blocks of life

30. When Did Life Begin?

31. Stromatolites Fossil evidence
Living: colonies of bacteria living in outer layer of sedimentary rocks
3.5 Byr old rocks: almost identical layered structure
Inconclusive evidence: sedimentation layering may mimic stromatolites
Fossil evidence
3.5 Byr old Australian rock shows “cells”
Could this form naturally from minerals?
Younger sites: at least two more ( byr old)
Older sites: sedimentary rock too altered to be useful

32. 13C/12C ratio Sterilization Normal abundance ratio 1/89
Living tissue and fossils show less 13C
Some rocks older than 3.85 byr show the low 13C abundance
Sterilization
Last sterilization: byr ago (Late Heavy Bombardment)

33. The evidence indicates life formed quickly after the Earth formed.
Within a few 100 million years, Perhaps as short as 100 million years