Transcript 1.5 The Origin of Cells Lesson

1.5 The Origin of Cells
Essential Idea
• There is an unbroken chain of life from the first cells on
Earth to all cells in organisms alive today.
The First Cell??
• Cells can only be formed by division of pre-existing
cells.
• So Where did the FIRST cell come from??
Formation of Earth
Our knowledge of earth’s history
•Hypotheses about Earth’s early history are based on a
relatively small amount of evidence.
•Gaps and uncertainties make it likely that scientific ideas
about the origin of life will change.
Big Events on Planet Earth
• Scientists infer that about four billion years ago, Earth
cooled and solid rocks formed on its surface.
• Millions of years later, volcanic activity shook Earth’s
crust.
• About 3.8 billion years ago, Earth’s surface cooled
enough for water to remain a liquid, and oceans
covered much of the surface.
Elements/ molecules/ compounds of early Earth
– Earth's early atmosphere probably contained
hydrogen cyanide, carbon dioxide, carbon monoxide,
nitrogen, hydrogen sulfide, and water.
– Notice the early atmosphere had NO oxygen.
– Had frequent storms with lightning
– High ultra-violet light (no ozone layer)
IB LEARNING OBJECTIVE
• Describe four processes needed for the spontaneous
origin of life on Earth.
Requirements for Life as we know it.
• The following steps would have to occur for life as we know it to
evolve:
– The non-living synthesis of simple organic molecules
– The assembly of these molecules into polymers
– The development of self-replication molecules, such as
nucleic acids (DNA, RNA)
– The packaging of these molecules within membrane sacs,
so that an internal chemistry can develop, different from
the surrounding environment.
IB Learning Objectives
Outline the experiments of Miller and Urey into the origin
of organic compounds.
The First Organic Molecules
• The First Organic Molecules
• Could organic molecules have evolved under
conditions on early Earth?
• In the 1950s, Stanley Miller and Harold Urey
tried to answer that question by simulating
conditions on the early Earth in a laboratory
setting.
The First Organic Molecules
• Miller and Urey’s Experiment
Mixture of gases
simulating
atmosphere of
early Earth
Water
vapor
Spark simulating
lightning storms
Condensation
chamber
Cold water cools
chamber, causing
droplets to form.
Liquid containing amino
acids and other organic
compounds
The First Organic Molecules
– Miller and Urey's experiments suggested how
mixtures of the organic compounds necessary for
life could have arisen from simpler compounds
present on a primitive Earth.
– Although their simulations of early Earth were not
accurate, experiments with current knowledge
yielded similar results.
Miller & Urey’s Experiment = The First Organic
Molecules
• In order to discover how organic molecules evolved on
earth, they recreated the conditions of early Earth.
– In their apparatus they mixed the gases:
• Ammonia
• Methane
• Hydrogen
– To form a reducing atmosphere.
Miller Urey’s Experiment = The First Organic
Molecules
• The ran electric charges and the boiling and
condensing of water simulated lightening and rainfall .
Miller & Urey’s Experiment =The First Organic
Molecules
• What they discovered:
– They ran the experiment for one week.
– After one week the water turned murky brown.
– Inside the water they found 15 amino acids
(building blocks of life).
• What they concluded:
– Organic compounds ( the building blocks of life)
could have spontaneously formed in early earth
before there were any living organisms
Miller & Urey Experiment Animations
• http://bcs.whfreeman.com/thelifewire/content/chp03
/0301s.swf
• http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter26/
animation_-_miller-urey_experiment.html
• http://bcs.whfreeman.com/thelifewire/content/chp03
/0302001.html
Polymerization of small organic molecules into BIG
organic molecules
Monomers
Polyomers – Tighly bonded
monomers
Polymerization of small organic molecules into BIG
organic molecules
• Polymerization – forming polyomers from monomers
• Polymerization -may have occurred near deep sea
hydrothermal vents/ underwater volcanoes
Polymerization of small organic molecules into BIG
organic molecules
• Underwater volcanoes provide heat and chemical
energy need to catalyze the polymerization chemical
reactions.
Origin of Cells From Organic Molecules
• To form the first cells, membranes were
needed to separate the cytoplasm and
its metabolism from the surrounding
fluid.
• Phospholipids the molecules that make
up cell membranes, naturally form a lipid
bilayers in water.
• These bilayers form spherical structures,
that enclose a droplet of fluid.
– These structures are similar to
vesicles that are now found in cells.
Origin of Cells From Organic Molecules
• Protobionts are aggregates of abiotically produced
molecules surrounded by a membrane or membranelike structure
• Experiments (like Miller and Urey’s Experiment)
demonstrate that protobionts could have formed
spontaneously from abiotically produced organic
compounds
• For example, small membrane-bounded droplets
called liposomes can form when lipids or other organic
molecules are added to water
LE 26-4
Glucose-phosphate
20 mm
Glucose-phosphate
Phosphorylase
Starch
Amylase
Phosphate
Maltose
Maltose
Simple reproduction
Simple metabolism
Origin of Cells From Organic Molecules
• Hypotheses suggest that structures similar to
Microspheres/ Protobionts might have
acquired more characteristics of living cells.
Origin of Genetic Material:
The “RNA World” and the Dawn of Natural Selection
• The first genetic material was probably RNA, not DNA
• RNA molecules called ribozymes have been found to
catalyze many different reactions, including:
– Self-splicing (cutting itself into smaller parts)
– Self-replicating -Making complementary copies of
short stretches of their own sequence or other
short pieces of RNA
Origin of Genetic Material:
The Puzzle of Life's Origin
– Evolution of RNA and DNA
• Some RNA sequences can help DNA
replicate under the right conditions.
• Some RNA molecules can even grow and
duplicate themselves suggesting RNA might
have existed before DNA.
Origin of Genetic Material:
The Puzzle of Life's Origin
• RNA and the Origin of Life
Proteins build cell
structures and catalyze
chemical reactions
RNA nucleotides
Simple organic
molecules
Abiotic “stew” of
inorganic matter
RNA helps in
protein synthesis
RNA able to replicate itself,
synthesize proteins, and
DNA functions in
function in information
information storage
storage
and retrieval
The First Prokaryotes
• Prokaryotes were Earth’s sole inhabitants from 3.5 to
about 2 billion years ago
Free Oxygen in our atmosphere
Free Oxygen
• Microscopic fossils, or microfossils, of
unicellular prokaryotic organisms resembling
modern bacteria have been found in rocks over
3.5 billion years old.
• These first life-forms evolved without oxygen.
Free Oxygen
• About 3.6 billion years ago, photosynthetic
bacteria began to pump oxygen into the oceans.
• Next, oxygen gas accumulated in the
atmosphere.
Photosynthesis and the Oxygen Revolution
• Oxygenic photosynthesis probably evolved about 3.5
billion years ago in cyanobacteria
The red deposits in rocks that date to 3.6 billion
years ago are Iron Oxides.
This is evidence that oxygen was in the atmosphere
by this time.
The red deposits in rocks that date to
3.6 billion years ago are Iron Oxides.
This is evidence that oxygen was in the
atmosphere by this time.
The red deposits in rocks that date to 3.6
billion years ago are Iron Oxides.
This is evidence that oxygen was in the
atmosphere by this time.
• Effects of oxygen accumulation in the atmosphere
about 2.7 billion years ago:
– Posed a challenge for life
– Provided opportunity to gain energy from light
– Allowed organisms to exploit new ecosystems
– This environmental change selected for organisms
that could do aerobic respiration.
The First Prokaryotes
• Prokaryotes were Earth’s sole inhabitants from 3.5 to
about 2 billion years ago
Eukaryotic cells arose from symbioses and genetic
exchanges between prokaryotes
• Among the most fundamental questions in biology is
how complex eukaryotic cells evolved from much
simpler prokaryotic cells
IB LEARNING OBJECTIVES
Discuss the endosymbiotic theory for the origin of
eukaryotes.
Review our Knowledge Difference between
Prokaryotes & Eukaryotes
Prokaryotes
vs.
Eukaryotes
The First Eukaryotes
• The oldest fossils of eukaryotic cells date back 2.1
billion years
Endosymbiotic Origin of Mitochondria and Plastids
• The theory of endosymbiosis proposes that
mitochondria and chloroplasts were formerly small
prokaryotes living within larger host cells
• The prokaryotic ancestors of mitochondria and
chloroplasts probably gained entry to the host cell as
undigested prey or internal parasites
• In the process of becoming more interdependent, the
host and endosymbionts would have become a single
organism
Aerobic
bacteria
Ancient Prokaryotes
Nuclear
envelope
evolving
Ancient Anaerobic
Prokaryote
LE 26-13
Cytoplasm
DNA
Plasma
membrane
Ancestral
prokaryote
Infolding of
plasma membrane
Endoplasmic reticulum
Nuclear envelope
Nucleus
Engulfing of aerobic
heterotrophic
prokaryote
Cell with nucleus
and endomembrane
system
Mitochondrion
Mitochondrion
Ancestral
heterotrophic
eukaryote
Engulfing of
photosynthetic
prokaryote in
some cells
Plastid
Ancestral
photosynthetic eukaryote
• Key evidence supporting an endosymbiotic origin of
mitochondria and chloroplasts:
– Similarities in inner membrane structures and
functions to bacteria (prokaryotes)
– Both have their own circular DNA like bacteria
(prokaryotes)
• Endosymbiotic Theory
Ancient Prokaryotes
Chloroplast
Aerobic
bacteria
Nuclear
envelope
evolving
Ancient Anaerobic
Prokaryote
Photosynthetic
bacteria
Plants and
plantlike
protists
Mitochondrion
Primitive Aerobic
Eukaryote
Primitive Photosynthetic
Eukaryote
Animals,
fungi, and
non-plantlike
protists
Aerobic
bacteria
Ancient Prokaryotes
Nuclear
envelope
evolving
Ancient Anaerobic Prokaryote
Origin of Eukaryotic Cells
Mitochondrion
Prokaryotes that use
oxygen to generate
energy-rich molecules of
ATP evolved into
mitochondria.
Primitive Aerobic Eukaryote
Origin of Eukaryotic Cells
Prokaryotes that carried out
photosynthesis evolved
into chloroplasts.
Chloroplast
Photosynthetic
bacteria
Primitive Photosynthetic Eukaryote
Video on Endosymbiosis
• http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter26/
animation_-_endosymbiosis.html
(a)Discuss the endosymbiotic theory for the origin of
eukaryotes (6 Point).
• You must clearly and correctly state the theory to
get all other points.
•
The theory states that eukaryotes cells evolved from prokaryotes cells;
•
This theory explains how the organelles in eukaryotes, mitochondria/chloroplasts evolved from (independent)
prokaryotic cells;
•
Prokaryotic cells with similar features to mitochondria/chloroplasts were taken in by larger heterotrophic prokaryote cell
by endocytosis;
•
This theory is supported by characteristics of chloroplasts/mitochondria that are similar to prokaryotes;
•
mitochondria/chloroplasts have naked DNA similar to prokaryotes;
•
mitochondria/chloroplasts divide/carry out fission similar to prokaryotes;
•
mitochondria/chloroplasts have 70S ribosomes / synthezise own proteins similar to prokaryotes;
•
mitochondria/chloroplasts have double membranes similar to prokaryotes;
•
The cristae similar to mesosomes / thylakoid have similar structures in prokaryotes;
•
There are some problems with the theory for example the theory cannot be falsified as it predicts something occurring in
the past;
•
There are some problems with the theory for example the theory does not explain the origins of cilia/flagella/linear
chromosomes/meiosis;
•
There is some weaker evidence that cilia/flagella evolved from attached bacteria/spirochetes;
6 max