Chapter 18: Life On Earth
Chapter 1
How Science Works
- The Scientific Method
- Evidence
- Measurements
- Units and the Metric System
- Measurement Errors
- Estimation
- Dimensions
- Mass, Length, and Time
- Observations and Uncertainty
- Precision and Significant Figures
- Errors and Statistics
- Scientific Notation
- Ways of Representing Data
- Logic
- Mathematics
- Geometry
- Algebra
- Logarithms
- Testing a Hypothesis
- Case Study of Life on Mars
- Theories
- Systems of Knowledge
- The Culture of Science
- Computer Simulations
- Modern Scientific Research
- The Scope of Astronomy
- Astronomy as a Science
- A Scale Model of Space
- A Scale Model of Time
- Questions
Chapter 2
Early Astronomy
- The Night Sky
- Motions in the Sky
- Navigation
- Constellations and Seasons
- Cause of the Seasons
- The Magnitude System
- Angular Size and Linear Size
- Phases of the Moon
- Eclipses
- Auroras
- Dividing Time
- Solar and Lunar Calendars
- History of Astronomy
- Stonehenge
- Ancient Observatories
- Counting and Measurement
- Astrology
- Greek Astronomy
- Aristotle and Geocentric Cosmology
- Aristarchus and Heliocentric Cosmology
- The Dark Ages
- Arab Astronomy
- Indian Astronomy
- Chinese Astronomy
- Mayan Astronomy
- Questions
Chapter 3
The Copernican Revolution
- Ptolemy and the Geocentric Model
- The Renaissance
- Copernicus and the Heliocentric Model
- Tycho Brahe
- Johannes Kepler
- Elliptical Orbits
- Kepler's Laws
- Galileo Galilei
- The Trial of Galileo
- Isaac Newton
- Newton's Law of Gravity
- The Plurality of Worlds
- The Birth of Modern Science
- Layout of the Solar System
- Scale of the Solar System
- The Idea of Space Exploration
- Orbits
- History of Space Exploration
- Moon Landings
- International Space Station
- Manned versus Robotic Missions
- Commercial Space Flight
- Future of Space Exploration
- Living in Space
- Moon, Mars, and Beyond
- Societies in Space
- Questions
Chapter 4
Matter and Energy in the Universe
- Matter and Energy
- Rutherford and Atomic Structure
- Early Greek Physics
- Dalton and Atoms
- The Periodic Table
- Structure of the Atom
- Energy
- Heat and Temperature
- Potential and Kinetic Energy
- Conservation of Energy
- Velocity of Gas Particles
- States of Matter
- Thermodynamics
- Entropy
- Laws of Thermodynamics
- Heat Transfer
- Thermal Radiation
- Wien's Law
- Radiation from Planets and Stars
- Internal Heat in Planets and Stars
- Periodic Processes
- Random Processes
- Questions
Chapter 5
The Earth-Moon System
- Earth and Moon
- Early Estimates of Earth's Age
- How the Earth Cooled
- Ages Using Radioactivity
- Radioactive Half-Life
- Ages of the Earth and Moon
- Geological Activity
- Internal Structure of the Earth and Moon
- Basic Rock Types
- Layers of the Earth and Moon
- Origin of Water on Earth
- The Evolving Earth
- Plate Tectonics
- Volcanoes
- Geological Processes
- Impact Craters
- The Geological Timescale
- Mass Extinctions
- Evolution and the Cosmic Environment
- Earth's Atmosphere and Oceans
- Weather Circulation
- Environmental Change on Earth
- The Earth-Moon System
- Geological History of the Moon
- Tidal Forces
- Effects of Tidal Forces
- Historical Studies of the Moon
- Lunar Surface
- Ice on the Moon
- Origin of the Moon
- Humans on the Moon
- Questions
Chapter 6
The Terrestrial Planets
- Studying Other Planets
- The Planets
- The Terrestrial Planets
- Mercury
- Mercury's Orbit
- Mercury's Surface
- Venus
- Volcanism on Venus
- Venus and the Greenhouse Effect
- Tectonics on Venus
- Exploring Venus
- Mars in Myth and Legend
- Early Studies of Mars
- Mars Close-Up
- Modern Views of Mars
- Missions to Mars
- Geology of Mars
- Water on Mars
- Polar Caps of Mars
- Climate Change on Mars
- Terraforming Mars
- Life on Mars
- The Moons of Mars
- Martian Meteorites
- Comparative Planetology
- Incidence of Craters
- Counting Craters
- Counting Statistics
- Internal Heat and Geological Activity
- Magnetic Fields of the Terrestrial Planets
- Mountains and Rifts
- Radar Studies of Planetary Surfaces
- Laser Ranging and Altimetry
- Gravity and Atmospheres
- Normal Atmospheric Composition
- The Significance of Oxygen
- Questions
Chapter 7
The Giant Planets and Their Moons
- The Gas Giant Planets
- Atmospheres of the Gas Giant Planets
- Clouds and Weather on Gas Giant Planets
- Internal Structure of the Gas Giant Planets
- Thermal Radiation from Gas Giant Planets
- Life on Gas Giant Planets?
- Why Giant Planets are Giant
- Gas Laws
- Ring Systems of the Giant Planets
- Structure Within Ring Systems
- The Origin of Ring Particles
- The Roche Limit
- Resonance and Harmonics
- Tidal Forces in the Solar System
- Moons of Gas Giant Planets
- Geology of Large Moons
- The Voyager Missions
- Jupiter
- Jupiter's Galilean Moons
- Jupiter's Ganymede
- Jupiter's Europa
- Jupiter's Callisto
- Jupiter's Io
- Volcanoes on Io
- Saturn
- Cassini Mission to Saturn
- Saturn's Titan
- Saturn's Enceladus
- Discovery of Uranus and Neptune
- Uranus
- Uranus' Miranda
- Neptune
- Neptune's Triton
- Pluto
- The Discovery of Pluto
- Pluto as a Dwarf Planet
- Dwarf Planets
- Questions
Chapter 8
Interplanetary Bodies
- Interplanetary Bodies
- Comets
- Early Observations of Comets
- Structure of the Comet Nucleus
- Comet Chemistry
- Oort Cloud and Kuiper Belt
- Kuiper Belt
- Comet Orbits
- Life Story of Comets
- The Largest Kuiper Belt Objects
- Meteors and Meteor Showers
- Gravitational Perturbations
- Asteroids
- Surveys for Earth Crossing Asteroids
- Asteroid Shapes
- Composition of Asteroids
- Introduction to Meteorites
- Origin of Meteorites
- Types of Meteorites
- The Tunguska Event
- The Threat from Space
- Probability and Impacts
- Impact on Jupiter
- Interplanetary Opportunity
- Questions
Chapter 9
Planet Formation and Exoplanets
- Formation of the Solar System
- Early History of the Solar System
- Conservation of Angular Momentum
- Angular Momentum in a Collapsing Cloud
- Helmholtz Contraction
- Safronov and Planet Formation
- Collapse of the Solar Nebula
- Why the Solar System Collapsed
- From Planetesimals to Planets
- Accretion and Solar System Bodies
- Differentiation
- Planetary Magnetic Fields
- The Origin of Satellites
- Solar System Debris and Formation
- Gradual Evolution and a Few Catastrophies
- Chaos and Determinism
- Extrasolar Planets
- Discoveries of Exoplanets
- Doppler Detection of Exoplanets
- Transit Detection of Exoplanets
- The Kepler Mission
- Direct Detection of Exoplanets
- Properties of Exoplanets
- Implications of Exoplanet Surveys
- Future Detection of Exoplanets
- Questions
Chapter 10
Detecting Radiation from Space
- Observing the Universe
- Radiation and the Universe
- The Nature of Light
- The Electromagnetic Spectrum
- Properties of Waves
- Waves and Particles
- How Radiation Travels
- Properties of Electromagnetic Radiation
- The Doppler Effect
- Invisible Radiation
- Thermal Spectra
- The Quantum Theory
- The Uncertainty Principle
- Spectral Lines
- Emission Lines and Bands
- Absorption and Emission Spectra
- Kirchoff's Laws
- Astronomical Detection of Radiation
- The Telescope
- Optical Telescopes
- Optical Detectors
- Adaptive Optics
- Image Processing
- Digital Information
- Radio Telescopes
- Telescopes in Space
- Hubble Space Telescope
- Interferometry
- Collecting Area and Resolution
- Frontier Observatories
- Questions
Chapter 11
Our Sun: The Nearest Star
- The Sun
- The Nearest Star
- Properties of the Sun
- Kelvin and the Sun's Age
- The Sun's Composition
- Energy From Atomic Nuclei
- Mass-Energy Conversion
- Examples of Mass-Energy Conversion
- Energy From Nuclear Fission
- Energy From Nuclear Fusion
- Nuclear Reactions in the Sun
- The Sun's Interior
- Energy Flow in the Sun
- Collisions and Opacity
- Solar Neutrinos
- Solar Oscillations
- The Sun's Atmosphere
- Solar Chromosphere and Corona
- Sunspots
- The Solar Cycle
- The Solar Wind
- Effects of the Sun on the Earth
- Cosmic Energy Sources
- Questions
Chapter 12
Properties of Stars
- Stars
- Star Names
- Star Properties
- The Distance to Stars
- Apparent Brightness
- Absolute Brightness
- Measuring Star Distances
- Stellar Parallax
- Spectra of Stars
- Spectral Classification
- Temperature and Spectral Class
- Stellar Composition
- Stellar Motion
- Stellar Luminosity
- The Size of Stars
- Stefan-Boltzmann Law
- Stellar Mass
- Hydrostatic Equilibrium
- Stellar Classification
- The Hertzsprung-Russell Diagram
- Volume and Brightness Selected Samples
- Stars of Different Sizes
- Understanding the Main Sequence
- Stellar Structure
- Stellar Evolution
- Questions
Chapter 13
Star Birth and Death
- Star Birth and Death
- Understanding Star Birth and Death
- Cosmic Abundance of Elements
- Star Formation
- Molecular Clouds
- Young Stars
- T Tauri Stars
- Mass Limits for Stars
- Brown Dwarfs
- Young Star Clusters
- Cauldron of the Elements
- Main Sequence Stars
- Nuclear Reactions in Main Sequence Stars
- Main Sequence Lifetimes
- Evolved Stars
- Cycles of Star Life and Death
- The Creation of Heavy Elements
- Red Giants
- Horizontal Branch and Asymptotic Giant Branch Stars
- Variable Stars
- Magnetic Stars
- Stellar Mass Loss
- White Dwarfs
- Supernovae
- Seeing the Death of a Star
- Supernova 1987A
- Neutron Stars and Pulsars
- Special Theory of Relativity
- General Theory of Relativity
- Black Holes
- Properties of Black Holes
- Questions
Chapter 14
The Milky Way
- The Distribution of Stars in Space
- Stellar Companions
- Binary Star Systems
- Binary and Multiple Stars
- Mass Transfer in Binaries
- Binaries and Stellar Mass
- Nova and Supernova
- Exotic Binary Systems
- Gamma Ray Bursts
- How Multiple Stars Form
- Environments of Stars
- The Interstellar Medium
- Effects of Interstellar Material on Starlight
- Structure of the Interstellar Medium
- Dust Extinction and Reddening
- Groups of Stars
- Open Star Clusters
- Globular Star Clusters
- Distances to Groups of Stars
- Ages of Groups of Stars
- Layout of the Milky Way
- William Herschel
- Isotropy and Anisotropy
- Mapping the Milky Way
- Questions
Chapter 15
Galaxies
- The Milky Way Galaxy
- Mapping the Galaxy Disk
- Spiral Structure in Galaxies
- Mass of the Milky Way
- Dark Matter in the Milky Way
- Galaxy Mass
- The Galactic Center
- Black Hole in the Galactic Center
- Stellar Populations
- Formation of the Milky Way
- Galaxies
- The Shapley-Curtis Debate
- Edwin Hubble
- Distances to Galaxies
- Classifying Galaxies
- Spiral Galaxies
- Elliptical Galaxies
- Lenticular Galaxies
- Dwarf and Irregular Galaxies
- Overview of Galaxy Structures
- The Local Group
- Light Travel Time
- Galaxy Size and Luminosity
- Mass to Light Ratios
- Dark Matter in Galaxies
- Gravity of Many Bodies
- Galaxy Evolution
- Galaxy Interactions
- Galaxy Formation
- Questions
Chapter 16
The Expanding Universe
- Galaxy Redshifts
- The Expanding Universe
- Cosmological Redshifts
- The Hubble Relation
- Relating Redshift and Distance
- Galaxy Distance Indicators
- Size and Age of the Universe
- The Hubble Constant
- Large Scale Structure
- Galaxy Clustering
- Clusters of Galaxies
- Overview of Large Scale Structure
- Dark Matter on the Largest Scales
- The Most Distant Galaxies
- Black Holes in Nearby Galaxies
- Active Galaxies
- Radio Galaxies
- The Discovery of Quasars
- Quasars
- Types of Gravitational Lensing
- Properties of Quasars
- The Quasar Power Source
- Quasars as Probes of the Universe
- Star Formation History of the Universe
- Expansion History of the Universe
- Questions
Chapter 17
Cosmology
- Cosmology
- Early Cosmologies
- Relativity and Cosmology
- The Big Bang Model
- The Cosmological Principle
- Universal Expansion
- Cosmic Nucleosynthesis
- Cosmic Microwave Background Radiation
- Discovery of the Microwave Background Radiation
- Measuring Space Curvature
- Cosmic Evolution
- Evolution of Structure
- Mean Cosmic Density
- Critical Density
- Dark Matter and Dark Energy
- Age of the Universe
- Precision Cosmology
- The Future of the Contents of the Universe
- Fate of the Universe
- Alternatives to the Big Bang Model
- Space-Time
- Particles and Radiation
- The Very Early Universe
- Mass and Energy in the Early Universe
- Matter and Antimatter
- The Forces of Nature
- Fine-Tuning in Cosmology
- The Anthropic Principle in Cosmology
- String Theory and Cosmology
- The Multiverse
- The Limits of Knowledge
- Questions
Chapter 19
Life in the Universe
- Life in the Universe
- Astrobiology
- Life Beyond Earth
- Sites for Life
- Complex Molecules in Space
- Life in the Solar System
- Lowell and Canals on Mars
- Implications of Life on Mars
- Extreme Environments in the Solar System
- Rare Earth Hypothesis
- Are We Alone?
- Unidentified Flying Objects or UFOs
- The Search for Extraterrestrial Intelligence
- The Drake Equation
- The History of SETI
- Recent SETI Projects
- Recognizing a Message
- The Best Way to Communicate
- The Fermi Question
- The Anthropic Principle
- Where Are They?
Molecules of Life
![](http://content.teachastronomy.com/taweb/images/textbook/hrthumbs/2422a82e73a5173db9c2e5c563ea6e77dddd5231.png)
The nature of life on Earth is amazingly complex and diverse. Yet, all life on Earth is composed of the same types of molecules. For instance, all living organisms utilize the same information containing molecules, DNA, to pass on genetic information from generation to generation. So the question of what is the nature of life on Earth can just as easily be posed as: what is the nature of life's molecules on Earth?
We can think of life as being comprised of four main types of biomolecules: carbohydrates, lipids, proteins, and nucleic acids. You may recognize two of these molecules as being listed on the back label of common, everyday grocery store products. This makes sense; after all, you are what you eat! Although we are careful about the amounts of carbohydrates and proteins that we eat, we will begin our look at the biomolecules of life with nucleic acids.
![](http://content.teachastronomy.com/taweb/images/textbook/hrthumbs/d6fb4090a4050a4f71b31534721f194f9d7af816.png)
The two nucleic acids with which we are most familiar are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). However, many people get confused about what these molecules actually are and how they function. In our cells, the genetic information that defines who we are is contained in chromosomes. Chromosomes are just tightly wound molecules consisting of DNA. Therefore, all the information that dictates how living organisms function is encoded within molecules of DNA. A single molecule of DNA is comprised of several nucleic acids. Each nucleic acid is a building block of a larger molecule and can be referred to as a monomer. Several monomers linked together make a polymer. Each of these monomers contains a sugar ring, a phosphate unit, and a nucleotide base. The monomers differ depending on which of four different nucleotide bases they contain: adenine (A), guanine (G), cytosine (C), or thymine (T). These monomers are bonded together to make long linear molecules. The sequence in which the monomers are bonded is what contains our genetic information. Most often in living organisms, two strands of DNA are bonded together to make a double helix. RNA molecules are made in much the same way as DNA molecules. The main difference is that RNA makes use of a different sugar ring and instead of using T as one of its nucleotide bases; RNA uses uracil (U).
DNA plays a vital role in living cells. It contains instructions for the assemblage of proteins, which in turn organize the synthesis and breakdown of other molecules that form parts of the cell, and therefore living organisms. DNA and RNA work together to string together a different set of monomers that comprise proteins. These monomers are called amino acids. On Earth, there are 20 amino acids that are used to produce proteins. The order in which these amino acids are bonded together into a linear molecule is contained in the molecules of DNA. In the DNA molecule, a sequence of 3 nucleotides in a row specifies a particular amino acid. As one "reads" down a strand of DNA, each set of 3 nucleotides would encode a single amino acid. These individual amino acids are bonded together to make a linear molecule known as a peptide. Peptides do not stay linear for long. Within the aquatic environment of the cell, peptides fold up into three-dimensional structures. A protein can be made of a single peptide or several peptides. The key to protein function is its three-dimensional structure. Proteins perform many pivotal functions in the cell. They can span cell membranes and control the import and export of molecules into and out of cells, they can catalyze the reactions necessary for life, and they can serve as important structural elements within the cell.
![](http://content.teachastronomy.com/taweb/images/textbook/hrthumbs/6c681d79d1a8c1969959794af2b03996ce097846.png)
Just as nucleic acids and proteins are important to life, so are carbohydrates and lipids. Because of the roles they play in all forms of life, carbohydrates make up most of the organic matter on Earth. For instance, carbohydrates are important as fuel for living organisms, but also serve as a mechanism to store energy. They are also important constituents of larger molecules; sugar rings are an important element of nucleic acids like DNA. Finally, carbohydrates also function as structural elements within the cell, most commonly for cell walls. Just as proteins are large molecules made up of smaller molecules (amino acids), carbohydrates themselves can be large molecules as well. However, carbohydrates are made of monosaccharides, small monomers that are molecules comprised of three to nine carbon atoms. An example of a large carbohydrate molecule is starch. Starch is made up of several monosaccharides (glucose) linked together and serves as a way for plant cells to store energy. When we eat starch, we break the bonds linking the individual glucose molecules together and use this sugar as fuel. Another example is cellulose. This is one of the most abundant organic compounds in the entire biosphere! Cellulose is also made up of glucose molecules linked together. However, many organisms cannot digest cellulose without the assistance of special proteins. Consequently, cellulose is used as a structural element to provide the rigidity needed for cellular components like cell walls.
Besides cell walls, cellular membranes provide a mechanism to separate what's inside a cell from what's outside. Lipids are a key component, along with proteins, of the cellular membranes that define cells. Without cellular membranes, there would be no way to keep molecules created in a cell from leaking out and prevent unwanted molecules from getting in. In eukaryotic cells, lipids also form the internal membranes that define cellular organelles such as the mitochondria or chloroplasts. Lipids are unusual molecules in that they have components with different characteristics. Part of a lipid molecule is hydrophilic, which means it is attracted to molecules such as water. The other part is hydrophobic, which means it is repelled by water. This part of the lipid is made up of fatty acids which have long chains of carbon atoms. A very famous lipid is cholesterol. We mostly hear about cholesterol as a bad molecule, one that clogs our arteries and shortens the lives of humans. However, cholesterol is an important molecule for life. Cholesterol is one of three main types of lipids often found in cellular membranes. Without it, many membranes would lose their fluidity, which would be detrimental to living cells.
When looking at the amazing diversity of life on our planet, it may be difficult to believe that it could arise from just four types of biomolecules. What is even more amazing is that the similarities between living organisms on Earth are what may enable us to recognize the similarities of life beyond Earth. A central question for astrobiology is whether or not the chemical "toolkit" and architecture of life elsewhere is the same as it is on Earth.