A2.1 Origins of cells Flashcards
A2.1.1—Conditions on early Earth and the pre-biotic formation of carbon compounds
Include the lack of free oxygen and therefore ozone, higher concentrations of carbon dioxide and
methane, resulting in higher temperatures and ultraviolet light penetration. The conditions may have
caused a variety of carbon compounds to form spontaneously by chemical processes that do not now
occur.
. Conditions on early Earth
The Earth was formed about 4.5 billion years ago. Life
appeared at some time during the following billion
years. Conditions on Earth were very different at that
time, compared with those that we experience today.
These were features of the atmosphere:
* little or no oxygen and therefore no stratospheric ozone
* high levels of UV light due to the lack of an ozone layer
* much higher concentrations of CO, and methane
* higher temperatures due to these greenhouse gases
* frequent storms with lightning due to high
temperatures.
These atmospheric conditions probably caused a
variety of carbon compounds to form by chemical
reactions that do not now spontaneously occur. The
substances produced would have dissolved in water
in the atmosphere and been deposited in rainfall,
creating a “soup” of carbon compounds in pools,
rivers and seas.
A2.1.2—Cells as the smallest units of self-sustaining life
Discuss the differences between something that is living and something that is non-living. Include reasons
that viruses are considered to be non-living.
The difference between living and non-living things has
often been debated. Living things are self-sustaining—
they maintain themselves in a highly ordered state
using energy external sources of energy. Damage must
be repaired and there must be periodic regeneration
through reproduction. The ability to maintain a highly
ordered state must be inherited when a living thing
reproduces. Non-living things may have some of these
properties, but not all of them.
ndividual organisms are obviously alive. Within a
multicellular organism, individual cells also have all
the properties of life, but subcellular components do
not. For that reason, a cell is the smallest unit of self-
sustaining life
A2.1.3—Challenge of explaining the spontaneous origin of cells
Cells are highly complex structures that can currently only be produced by division of pre-existing cells.
Students should be aware that catalysis, self-replication of molecules, self-assembly and the emergence of
compartmentalization were necessary requirements for the evolution of the first cells.
NOS: Students should appreciate that claims in science, including hypotheses and theories, must be
testable. In some cases, scientists have to struggle with hypotheses that are difficult to test. In this case the
exact conditions on pre-biotic Earth cannot be replicated and the first protocells did not fossilize
Until the 19th century some biologists believed that
life could develop from non-living material. This was
called “spontaneous generation”. Cells are highly
complex structures and there is no evidence that they
can be formed on Earth today, except by division of
pre-existing cells.
Al the billions of cells in a multicellular organism are
formed by repeated cell division, starting with a single
cell. This cell was either part of one parent or a fusion
of cells from two parents. We can therefore trace the
origins of cells back through generations and billions
of years of evolution.
Eventually we must reach the first cells (protocells) as
life has not always existed on Earth. Before these cells
existed, there was only non-living material.
One of the great challenges in biology is to understand
how the first living cells evolved from non-living matter
and how spontaneous generation could take place
then but not now. These are key features of life that
must have developed:
catalysis-so selected chemical reactions happened
self-replication of molecules-so there could be
inheritance and evolution of useful characteristics
self-assembly-spontaneous building of
macromolecules from small subunits
compartmentalization-to separate the ordered
interior of the cell from the less ordered environment
A2.1.4—Evidence for the origin of carbon compounds
Evaluate the Miller–Urey experiment.
Evaluation of the Miller-Urey experiment:
Strength: repeatable-similar experiments have also
shown that a diversity of carbon compounds can form
spontaneously in an atmosphere lacking oxygen
Limitation: Miller and Urey probably got some
conditions wrong-pre-biotic Earth had more CO, and
N, and less NH and CH, and also less lightning
The hypothesis that conditions on early Earth
allowed the origin of c a r b o n c o m p o u n d s was tested
experimentally in the early 1950s by Miller and Urey.
They simulated a pre-biotic atmosphere by mixing
methane, hydrogen and ammonia in a flask. Water
vapour was added by boiling water in another flask.
Electrical sparks were used to simulate lightning. The
substances produced were condensed and returned
to the flask of boiling water.
After the experiment had been running for a week,
the condensate was dark red and contained a variety
of carbon compounds, including more than 20 amino
acids. This shows that carbon compounds could have
formed spontaneously before life had evolved, if the
conditions on pre-biotic Earth were the same as those
in the Miller-Urey apparatus.
A2.1.5—Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers
Formation of a membrane-bound compartment is needed to allow internal chemistry to become different
from that outside the compartment.
Fatty acids have a hydrophobic hydrocarbon chain
with a hydrophilic carboxyl group at one end. They
naturally form spherical structures in water (micelles)
with the hydrophilic part on the outside. Phospholipids
have two hydrophobic fatty acid tails and a hydrophilic
phosphate head. They naturally form continuous
bilayers in water. Small areas of phospholipid bilayer
tend to become spherical and are then called vesicles.
It seems likely that such vesicles would have formed in
ponds or seas on pre-biotic Earth, assuming that fatty
acids and then phospholipids were spontaneously
generated by chemical processes in air and water.
A vesicle is a membrane-bound compartment that allows
internal chemistry to become different from that outside
the compartment. Spontaneous formation of vesicles
was therefore an essential step in the origin of cells.
A2.1.6—RNA as a presumed first genetic material
RNA can be replicated and has some catalytic activity so it may have acted initially as both the genetic
material and the enzymes of the earliest cells. Ribozymes in the ribosome are still used to catalyse peptide
bond formation during protein synthesis.
Whether DNA or enzymes existed first is a “chicken or
egg” argument-DNA is needed to make enzymes and
enzymes are needed to make DNA. The solution to this
conundrum may be that there was a period when RNA
was the genetic material and also catalysed reactions
inside protocells.
RNA can be copied and passed on to the next
generation. Some viruses still use it instead of DNA as
genetic material.
Some types of RNA can act as catalysts. Ribozymes in
the ribosome are still used to catalyse peptide bond
formation during protein synthesis.
A2.1.7—Evidence for a last universal common ancestor
Include the universal genetic code and shared genes across all organisms. Include the likelihood of other
forms of life having evolved but becoming extinct due to competition from the last universal common
ancestor (LUCA) and descendants of LUCA.
Life may have evolved multiple times on Earth. More
than o n e form may have co-existed for a time, but there
is strong evidence that all organisms alive today evolved
from one life-form. Other forms presumably became
extinct because of competition. Al existing organisms on
Earth are descendants of the same early ancestors. LUCA
is the last universal common ancestor (shared by all).
The strongest evidence for LUCA is genetic. All
organisms use a universal genetic code, with only
minor variation in the meaning of some of the 64
codons. This is almost certainly due to inheritance from
a common ancestor. Also, several hundred genes are
found in all organisms, with relatively minor variations.
The gene for cytochrome c is an example. These genes
give all organisms shared features of their biochemistry
that again are evidence for LUCA.
A2.1.8—Approaches used to estimate dates of the first living cells and the last universal common ancestor
Students should develop an appreciation of the immense length of time over which life has been evolving
on Earth.
- Fossils
Rocks in Western Australia (the Strelley Pool
Formation), contain fossilized stromatolites
dating from 3.4 gigayears ago. (A gigayear
is a billion years.) Stromatolites are rocky
mounds formed by mats of cyanobacteria 3.4 Gya
in shallow seawater which trap sediments
and secrete CaCO,. - Isotope ratios in rock
Carbon originating from living organisms
has a low 3 C / 2 C ratio. B a n d e d iron rock
3.8 Gya
at Akila in West Greenland dating from
3.8 Gya shows this, suggesting life had
evolved by then. Zircon particles found in
deposits at Jack Hills in Western Australia
also have a low, C / * C ratio. They were
eroded in the past from 4.1 Gya rocks. - Genomic information
The number of differences between the
genomes of two species is proportional
to the time since they diverged from a
common ancestor. A recent study using
this approach suggested that LUCA and
4.1 Gya
4.5 Gya
the first living cells existed nearly 4.5 Gya
A2.1.9—Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal
vents
Include fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of
conserved sequences from genomic analysis.
More than 350 types of protein have been found
to occur in a diverse range of bacteria and archaea,
suggesting that genes coding for these proteins were
in LUCA’s genome.
The 350 proteins are needed for anaerobic
metabolism and for fixing carbon dioxide and nitrogen.
This suggests that LUCA lived in an environment with
high concentrations of hydrogen, carbon dioxide
and iron. These conditions are found in and around
hydrothermal vents in the oceans.
Hydrothermal vents are cracks in the Earth’s surface,
characterized by gushing hot water carrying reduced
(unoxidized) inorganic chemicals such as iron sulfide.
Alkaline hydrothermal vents (white smokers) have a
combination of conditions that are needed for the
first cells to form. The white “smoke” that emerges
(see diagram) is fluid at temperatures of 60°C to
90°C and contains high concentrations of hydrogen,
methane, ammonia and sulfides. These chemicals
can be used to generate energy anaerobically, which
protocells could have used to assemble carbon
compounds into polymers. Carbon dioxide, also
needed by evolving protocells would have been
abundant in the pre-biotic ocean water.
ot alkaline
w a t e r with
hydrogen
chimney formed
from precipitated
salts
cold acidic
o c e a n
water with
dissolved
metals
