AP Bio
Ch. – 4
Cells: An Introduction
The Beginning of Life on Earth
The first cells (life) self-assembled from molecules
present in the ancient seas.
Making Cells required
1. Abiotic synthesis of small organic molecules.
2. Joining of these small molecules into
macromolecules.
3. Packaging these molecules into “protobionts”.
4. Self-replication.
Synthesis of Organic
Compounds on Early Earth
Earth’s early atmosphere contained water
vapor and chemicals released by volcanic
eruptions (nitrogen, carbon dioxide, water
and traces of methane, ammonia, and
hydrogen sulfide…. C, H, O, N
The necessary energy for the synthesis of
organic compounds was supplied by
radioactive elements, heat, electrical, and of
course, the Sun’s UV radiation.
The First Testable Hypothesis.
A. I. Oparin hypothesized that the early atmosphere
was a “reducing” environment…..and a
“Chemical Evolution” was responsible for the
formation of the complex molecules required for
life.
No atmospheric O2 could have been present… it
would have destroyed and prevented the
accumulation of any forming organic
compounds!!
In 1951
Stanley Miller and Harold Urey conducted
lab experiments that showed that the
abiotic synthesis of organic molecules
in a reducing atmosphere is possible.
Miller’s experiment only produced 5 amino
acids… both right and left-handed, a racemic
mixture!
Instead of forming in a “reducing” atmosphere,
the first organic compounds formed in the
atmosphere near volcanoes and deep-sea
vents. The “out-gassing” includes large
amounts of CO2, H2O and Nitrogen.
Stanley Miller’s Second Experiment
The vials associated with the original
experiment contained 5 amino acids.
The 11 vials recovered from the unpublished
experiment produced 22 amino acids… all
left-handed.
Simple modifications of the Miller experiment
produce other amino acids, nucleotides and
other organic compounds.
Another Source of Organic Compounds
• Amino acids have
also been found in
meteorites. Along
with complex
organics and
Uracil…. RNA
• The isotopic
abundances verify
extraterrestrial origin.
Building Large Organic Compounds
• Large organic compounds were built through
Dehydration Synthesis from smaller
molecules.
• Structures were modified by Hydrogen
bonds, polarity of individual molecules and
hydrophilic, hydrophobic interactions.
• Natural Selection favored the more stable
compounds.
Comets also contribute Organic
compounds.
The amino acid glycine was found in dust collected
by the Stardust spacecraft, which flew by Comet
Wild 2 in 2004. Other organic compounds included
methanol, ethanol, and acetylene.
Creating Protobionts (microspheres)
Sidney Fox discovered liposomes and proteinoids form when
lipids or other organic molecules are mixed with hot water.
Simple reproduction by liposomes
Building Protobionts
The liposomes grow by absorbing other
lipids and water….. They will eventually
split…forming 2 vesicles…reproduction!
Building Protobionts (Coacervates)
Early protobionts contained abiotically produced
molecules.
These Protobionts, coacervates, exhibited simple
reproduction, by growing and budding, simple
metabolism and maintained an internal chemical
environment.
RNA World: Self-Replicating RNA
• The first genetic material was probably
RNA, not DNA.
RNA molecules can:
1. Store information, and pass it on (heredity)
2. Catalyze its own replication.
3. Act as an enzyme called ribozyme which
can catalyze other metabolic reactions.
How to make RNA?
Simple nucleotides form
in water in the
presence of
phosphates.
Organic compounds,
including nucleotides
polymerize, grow into
chains, when they are
concentrated on hot
sand, clay, or rock.
How “RNA World” Works
A free-floating strands of RNA
gets enclosed in a lipid
vesicle.
The protected RNA can
replicate from other
nucleotides.
RNA forms double strands in
cold water but, unwinds in
warm water…transcription?
External convection currents
drive reproduction.
As the vesicles grow and split,
each carries its replicated RNA.
As amino acids are added….
The ribozymes helped
catalyze protein synthesis.
The proteins folded into more
efficient enzymes which
improved replication and
metabolism.
Proteins, called enzymes,
took over cell metabolism.
RNA World: The Final Step
• The Enzymes catalyze
DNA formation.
• DNA is more stable and
can store more
hereditary information.
• DNA is more efficient at
replication than
RNA….double-stranded.
• DNA takes over the cell’s
protein synthesis.
Why on Earth?
Its distance from the Sun… the
“Goldilocks” planet!
Its mass… its gravity!
Its Atmosphere!
Heterotrophs and Autotrophs
Where do cells get the energy
they need to perform their life
functions?
Heterotrophs
Heterotrophic cells are dependent upon
outside sources of organic
materials…these resources supply
energy and the necessary components
to make various cell structures.
The heterotrophs include fungi, animals,
some bacteria and some protists.
Autotrophs
Autotrophs are capable of making their
own energy-rich organic compounds
from available inorganic materials.
There are 2 types of Autotrophs
1. Photosynthetic Autotrophs – they do not
require organic compounds from the
environment…they can produce them
using inorganic compounds and energy
from the sun…. Plants!
2. Chemosynthetic Autotrophs – capture
energy through a variety of inorganic
reactions… such as:
6CO2 + 6H2O + 3H2S → C6H12O6 + 3H2SO4
…..most are bacteria.
The First Cells
May have been
heterotrophs feeding on
the organic-rich
“Primordial Soup”.
Limited amounts of
organic resources
would have led to
competition and the
Natural Selection of the
“Fittest” cells.
However…
If the heterotrophs were consuming all of the
organic compounds…. How did they get
replenished?
If Autotrophs had not evolved all life would
have stopped.
Maybe the first cells were
Chemosynthetic?….. The “Extremophiles”
Extremophiles are organisms which adapted
to harsh environments much like the
conditions which existed on the early earth.
Most Extremophiles are bacteria ….such as
Methanogens, which live in anaerobic mud
flats, with others living in ice sheets…. near
hot, deep ocean vents and even in toxic
sulfur pools.
Maybe the first cells were Photosynthetic?
Experiments simulating the earth’s early surface
conditions and chemistry produced molecules
which were the precursors of chlorophyll.
When exposed to an oxygen-free environment
and illuminated they produced primitive
photosynthetic reactions.
Today, the sun’s energy is channeled through
photosynthetic autotrophs to all other life
forms.
The Earliest Fossils
Fossils of both
heterotrophs and
autotrophs dating
back about 3.5
billion years have
been found.
The Cell Theory
1665 – Robert Hooke
Observed “little rooms” in cork.
Coined the term "cells” in 1665.
Cork Cells
Anton van Leeuwenhoek (1632-1723) was the
first to adapt lenses to the study of living
organisms.
He discovered microorganisms.
1833 - Robert Brown
• Studied the nucleus. Do all living cells
have a nucleus?
Nucleus
Human Cheek Cell
1838 - M.J. Schleiden
• Botanist….
discovered all
plants are made of
cells.
Typical Plant Cell
1839 - T. Schwann
• Zoologist….discovere
d animals are made
of cells.
• In fact Schwann
believed ALL living
organisms were made
of cells.
Nerve Cells
1855 - Rudolph Virchow
“Omnis cellula e cellula”
“Cells arise from
pre-existing cells.”
Discovered while
researching cancer cells.
The Cell Theory
1.
All living organisms consist of one or more
cells.
2. The chemical reactions of living organisms,
including energy-releasing and biosynthetic
reactions, takes place within cells.
3. Cells arise from pre-existing cells.
4. Cells contain hereditary information and this
information is passed from parent cells to
daughter cells.
Prokaryotes and Eukaryotes
All Cells have:
1.A Cell (Plasma) Membrane – separates the
cell’s internal chemistry from the
environment.
2.Self-replicating, mutable DNA – transfers
genetic information from one generation to
the next.
3.A Cytoplasm filled with cytosol.
4.Ribosomes – which produce structural
proteins and enzymes.
Packaging Your DNA
Prokaryotes have circular forms of DNA,
called Plasmids… or even
chromosomes, which are loosely
associated with structural proteins.
The DNA is located in a central region
called the Nucleoid.
Packaging Your DNA
Eukaryotes have thread-like forms of DNA,
called chromosomes, which are packaged
using structural proteins called histones.
The DNA is located within the Nucleus which
is bound by the double-membrane called
the Nuclear Envelope
Cell Walls
1. All Prokaryotes have cell walls.
2. Some of the Eukaryotes have cell walls…
* some protista have cell walls
* plants cells have cell walls
* fungal cells have cell walls
*** animal cells DO NOT have cell walls.
Fossil records show us the first prokaryotes
appeared approximately 3.5 BYA…
and that
The first eukaryotes appeared approximately
2.0 BYA… the origin of the eukaryotes may
be explained using the “Endosymbiotic”
hypothesis.
Comparing the sizes of
Prokaryotes and Eukaryotes
In general….Eukaryotes (10 – 100 mm) are
larger than prokaryotes ( 1 – 10 mm).
The History of Life on Earth
The geologic record is divided into four eons.
Hadean: 4.5 – 3.8 Gya: Formation, oceans &
atmosphere
Archaean: 3.8 – 2.5 Gya: Stromatolites & fossil
bacteria
Proterozoic: 2.5 Gya – 454 Mya: Eukarya and
Oxygen
Phanerozoic: since 454 Mya: Rise of plant and
animal life
Hadean
HadeanEon
Eon
The Surface begins to cool and solidify.
Formation of the Oceans
Formation of the Atmosphere
Age of Heavy Bombardment….most
events would be “sterilizing”.
The Archaean Eon began with the end of
heavy bombardment ~ 3.8 Gya.
Conditions stabilized.
Oceans, but no O2 in the
atmosphere.
Stromatolites appear in the
geological record ~3.5 Gya
and thrived for >1 Billion
years.
Rise of anaerobic microbes
in the deep ocean & shores
using Chemosynthesis.
Time of rapid diversification of life
driven by Natural Selection.
Stromatolites in Shark Bay, Australia
The Proterozoic Eon (2.5Gya – 450Mya) was
the time of single-celled life and the rise of
Oxygen.
Find microfossils of bacteria.
Emergence of
Photosynthesis:
First using H2S instead of
H2O, so no Oxygen
produced.
Then using CO2+ H2O to
make O2 starting ~2.4 Gya.
Most of the initial
Oxygen made by
photosynthesis was
soaked up by Banded
Iron Formations
(BIF’s)
Major sink for early oxygen production
until ~1.8 Gya.
Rapid rise in atmospheric Oxygen occurred
once the mineral sinks became saturated
~1.8 Gya.
Crisis for anaerobic life:
O2 breaks chemical bonds!
Mass extinction of anaerobic bacteria except in
the deep oceans and deep underground
Good for emerging Aerobic life. The first
Eukaryotes arose ~2.1 Gya.
Biggest change in the late Proterozoic Eon ~ 580 Mya was
the emergence of the multicellular life after the last
Snowball Earth episode.
Dickinsonia costata
Soft-bodied, wormlike
The Phanerozioc Eon: 450 MYA
• The Cambrian Explosion in multicellular life.
• Most major groups of animals and their
body plans appeared rapidly at the start of
the Cambrian Period 450Mya.
• Complex animal and plant life quickly filled
most available ecological niches.