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