Origin of cells

Notes for the IB Biology Syllabus 2016

1. Origin of Cells
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2. Origins of Life
There is an unbroken chain of life from the first cells on Earth to all
cells in organisms alive today.
We know this because cells can only be formed by division of pre-
existing cells.
It is estimated that life first emerged at least 3.8 billion years
ago, approximately 750 million years after Earth was formed

4. Can Life Arise Spontaneously?
It was hypothesised in the 1920s that organic
molecules (like amino acids) could arise under the
right conditions
Miller’s experiments in the 1950s supported the
hypothesis
With electricity (i.e. lightning), heat (from the sun),
water and atmospheric molecules like CO
2and NH
3,
the organic molecules began to form over time.

5. From Organic Molecules to Cells
The first cells must have arisen from non-living material.
Miller’s experiments show that the materials to make proteins were
present.
The 64 codons in the genetic code have the
same meanings in nearly all organisms, but that
there are some minor variations that are likely to
have accrued since the common origin of life
on Earth.

6. The RNA World
In the 1980s, Altman and Cechdiscovered that RNA is capable of catalysingsome
chemical reactions, including the polymerization of nucleotides. RNA is uniquely able
both to serve as a templatefor and to catalyse its own replication.
Consequently, RNA is generally believed to have been the initial genetic system, and
an early stage of chemical evolution is thought to have been based on self-
replicating RNA molecules—a period of evolution known as theRNA world.
Ordered interactions between RNA and amino acids
then evolved into the present-day genetic code, and
DNA eventually replaced RNA as the genetic material.

7. Cell Membrane
The first cell is assumed to have had a phospholipid bilayer membrane, enclosing a
cytoplasm and DNA.
The enclosure of self-replicatingRNA and associated molecules in a phospholipid
membrane would thus have maintained them as a unit, capable of self-reproduction
and further evolution.
RNA-directed protein synthesis may already have evolved by this time, in which case the
first cell would have consisted of self-replicating RNA and its encodedproteins.

8. Metabolism
It is believed that cells initially used anaerobic
respiration (in the absence of oxygen) in a process like
glycolysis: the breakdown of glucose to lactic acid
Then two things must have happened:
Origin of photosynthesis
Aerobic respiration

9. Photosynthesis
Photosynthesis allows cells to use light and CO
2to make glucose for
energy
The first photosynthetic bacteria, which evolved about3 billion
years ago, probably used H
2S to convert CO
2to organic
molecules: a pathwaystill used by some bacteria today.
H
2O as an electron donor and hydrogen for the conversion of
CO
2to organic compounds evolved later and led to the
oxygenation of Earth's atmosphere.
The use of H
2O in photosynthetic reactions produces the by-
product free O
2; this mechanism is thought to have been
responsible for making O
2abundant in Earth's atmosphere.

10. Aerobic Respiration
Once oxygen was abundant in the atmosphere, respiration began
to use oxygen: aerobic respiration.
O
2is a highly reactive molecule, and oxidative metabolism, utilizing
this reactivity, has provided a mechanism for generating energy
from organic molecules that is much more efficient than
anaerobicglycolysis.

11. Endosymbiotic Theory
Photosynthesis and respiration initially took place in the cytoplasm.
However, when we look at eukaryotic cells today (like our own), we
see that many have chloroplasts and mitochondria: organelles
where these reactions take place.
They also have a nuclear membrane encasing the DNA
The organelles are thought to have been acquired as a result of
the association ofprokaryotic cellswith the ancestor of eukaryotes.

12. Evidence for Endosymbiosis
The hypothesis thateukaryotic cellsevolved from a symbiotic association of
prokaryotes—endosymbiosis—is particularly well supported by studies
ofmitochondriaand chloroplasts, which are thought to have evolved from
bacteria living in large cells.
Both mitochondria and chloroplasts are similar to bacteria in size, and like bacteria,
they reproduce by dividing in two.
Most importantly, both mitochondria and chloroplasts contain their ownDNA,
which encodes some of their components.

13. What is Endosymbiosis?
Endosymbiotic theory suggests that chloroplasts,
mitochondria and perhaps even other organelles,
were prokaryotic cells which lived inside larger
cells.
Mitochondria and chloroplasts originated from the
endosymbiotic association of aerobic bacteria
andcyanobacteria, respectively, with the
ancestors of eukaryotes.

14. Pasteur’s Experiments
Pasteur’s experiments were evidence that spontaneous generation
of cells and organisms does not now occur on Earth.
In his day, people did not realise that bacterial infections were
transmitted from person to person, so hand washing and treatment
of food was not commonplace
Pasteur showed that bacterial growth on foods like milk, and the
infections in hospitals, were from pre-existing cells replicating and
being transported.
Thus, he developed the germ theory of disease

15. Pasteur’s Work
Using germ theory, Louis Pasteur was then able to
develop a cure for anthrax (in cattle) and rabies.
He developed the process of pasteurisation to
make milk safe to drink