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 simulatingatmosphere of early Earth

Condensationchamber

Spark simulatinglightning storms

Watervapor

Liquid containing amino acids and other organiccompounds

Cold water cools chamber, causing droplets to form.

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.mcgraw-hill.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

MonomersPolyomers – Tighly bonded monomers

• 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.

Polymerization of small organic molecules into BIG organic molecules

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 membrane-like 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

Glucose-phosphate

Phosphorylase

Amylase

Starch

Maltose

Maltose

Phosphate

Simple metabolismSimple reproduction

20 m

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

Abiotic “stew” ofinorganic matter

Simple organicmolecules

RNA nucleotides

RNA able to replicate itself, synthesize proteins, andfunction in information storage

DNA functions in information storage and retrieval

RNA helps inprotein synthesis

Proteins build cellstructures and catalyzechemical reactions

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

Ancient Anaerobic Prokaryote

Nuclear envelopeevolving

Aerobicbacteria

Ancient Prokaryotes

LE 26-13

Plasmamembrane

Cytoplasm

DNA

Ancestralprokaryote

Endoplasmic reticulum

Nuclear envelope

Infolding ofplasma membrane

Engulfing of aerobicheterotrophicprokaryote

Nucleus

Cell with nucleusand endomembranesystem

Mitochondrion

Engulfing ofphotosyntheticprokaryote insome cells

Plastid

Mitochondrion

Ancestralheterotrophiceukaryote

Ancestralphotosynthetic 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

Mitochondrion

Aerobicbacteria

Nuclear envelopeevolving

Ancient Prokaryotes

Plants and plantlike protists

Primitive PhotosyntheticEukaryote

Primitive AerobicEukaryote

Ancient AnaerobicProkaryote

Chloroplast

Animals, fungi, and

non-plantlike protists

Photosynthetic bacteria

Ancient Anaerobic Prokaryote

Nuclear envelopeevolving

Aerobicbacteria

Ancient Prokaryotes

Origin of Eukaryotic Cells

Prokaryotes that use oxygen to generate energy-rich molecules of ATP evolved into mitochondria.

Mitochondrion

Primitive Aerobic Eukaryote

Origin of Eukaryotic Cells

Primitive Photosynthetic Eukaryote

Chloroplast

Photosynthetic bacteria

Prokaryotes that carried out photosynthesis evolved into chloroplasts.

Video on Endosymbiosis

• http://highered.mcgraw-hill.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