Post on 04-Jan-2016
Chapter 12 Section 1Chapter 12 Section 1How Did Life Begin?
Grade 10 BiologySpring 2011
ObjectivesObjectivesSummarize how radioisotopes can
be used in determining Earth’s ageCompare two models that
describe how the chemicals of life originated
Describe how cellular organization might have begun
Recognize the importance that a mechanism for heredity has to the development of life
The Age of EarthThe Age of EarthEarth formed 4.5 billion years
ago ◦Was a fiery ball of molten rock ◦Eventually cooled and formed a
rocky crust ◦Water vapor cooled and formed
oceans ◦Scientists think life first evolved in
oceans and evolution of life occurred over hundreds of millions of years
Measuring Earth’s AgeMeasuring Earth’s AgeRadiometric dating: estimation
of the age of an object by measuring the content of certain radioactive isotopes◦Scientists have measured the earth
this way
Measuring Earth’s AgeMeasuring Earth’s AgeIsotope: form of an element
whose atomic mass (mass of each individual atom) differs from that of other atoms of the same element
Radioisotopes: unstable isotopes that break down and give off energy in the form of charged particles (radiation)
Measuring Earth’s AgeMeasuring Earth’s AgeRadioactive decay: results in
other isotopes that are smaller and more stable
Half-life: the time it takes for one-half of a given amount of a radioactive to decay
Formation of the Basic Formation of the Basic Chemicals of LifeChemicals of LifeIt is thought that the path to the
development of living things began when molecules of nonliving matter reacted chemically during the first billion years of Earth’s history
Chemical reactions produce many different, simple organic molecules
Formation of the Basic Formation of the Basic Chemicals of LifeChemicals of LifeSun and volcanic heat caused
simple organic molecules to form more complex molecules that became building blocks of first cells
Primordial Soup Model Primordial Soup Model Primordial Soup Model: early
Earth’s ocean contained large amounts of organic molecules ◦Oceans filled with many different
organic molecules like a soup ◦Hypothesized that these molecules
formed spontaneously in chemical reactions activated by energy from solar radiation, volcanic eruptions, and lightning
Primordial Soup Model Primordial Soup Model Scientists proposed Earth’s early
atmosphere lacked oxygen and rich in nitrogen gas, hydrogen gas, and hydrogen containing gases such as water vapor, ammonia, and methane
Primordial Soup Model Primordial Soup Model Electrons in these gases would have
been frequently pushed to higher energy levels by light particles from sun or electrical energy in lightning
Primordial Soup Model Primordial Soup Model Today, high energy electrons are
socked up by oxygen, without oxygen high energy electrons would have been free to react with hydrogen rich molecules, forming organic compounds
Miller-Urey Experiments Miller-Urey Experiments Placed gases they
thought existed on early Earth into a device
Provided electrical sparks to simulate lightning
After a few days, found complex collection of organic molecules ◦Amino acids, fatty acids,
hydrocarbons
Miller-Urey Experiments Miller-Urey Experiments Results support hypothesis that
some basic chemicals of life could have formed spontaneously under conditions like those in experiment
Reevaluating Miller-Urey Reevaluating Miller-Urey Model Model Now know that the reductant
molecules used in Miller’s experiment could not have existed in abundance on early Earth
No ozone present, without ozone UV light would have destroyed any ammonia and methane present
When these gases are absent, key biological molecules are not made
Reevaluating Miller-Urey Reevaluating Miller-Urey Model Model If chemicals needed to form life
were not in the atmosphere, where did they come from?
Could be produced in ocean bubbles, or arose in deep sea vents
Bubble ModelBubble Model
Bubble ModelBubble Model
1. Ammonia, methane, and other gases resulting from numerous eruptions of undersea volcanoes were trapped in underwater bubbles
Bubble ModelBubble Model
2. Inside bubbles, methane and ammonia needed to make amino acids might have been protected from damaging UV radiation. Chemical reactions would take place must faster in bubbles (where reactants would be concentrated) than in primordial soup.
Bubble ModelBubble Model
3. Bubbles rose to surface and burst, releasing simple organic molecules into the air
4. Carried upward by winds, simple organic molecules were exposed to UV radiation and lightning, which provided energy for further reactions
Bubble ModelBubble Model
5. More complex organic molecules that formed by further reactions fell into the ocean with rain, starting another cycle
Precursors of First Cells Precursors of First Cells In the lab, scientists have not
been able to make either proteins or DNA form spontaneously in water
Short chains of RNA, have been made to form on their own in water
Precursors of First Cells Precursors of First Cells
Self-Replication
Possible Role As Catalyst Possible Role As Catalyst RNA molecules can act like
enzymes RNA’s 3-dimensional structure
provides a surface on which chemical reactions can be catalyzed
Possible Role As Catalyst Possible Role As Catalyst RNA was the first self-replicating information storage molecule and it catalyzed the assembly of the first proteins
Would have been capable of changing from one generation to the next
Microspheres and Microspheres and Coacervates Coacervates Lipids which make up cell
membranes, tend to gather together in water
Certain lipids, when combined with other molecules can form a tiny droplet whose surface resembles a cell membrane
Microspheres and Microspheres and Coacervates Coacervates Microspheres: in water, short
chains of amino acids can gather into tiny droplets
Coacervates: composed of molecules of different types, including amino acids and sugars, gather into tiny droplets
Microspheres and Microspheres and Coacervates Coacervates Formation of microspheres may
have been the first step toward cellular organization
Microspheres formed and dispersed Those that could persist longer and
incorporate molecules and energy would become more common than shorter lasting ones
Need to have characteristic of heredity to be considered living things
Origin of Heredity Origin of Heredity Double stranded DNA evolved
after RNA and RNA “enzymes” catalyzed the assembly of earliest proteins
Hypothesis that some microspheres or similar structures that contained RNA developed a means of transferring their characteristics to offspring
Origin of Heredity Origin of Heredity Do not yet understand how DNA,
RNA, and heredity mechanisms first developed