1. 9. THE ORIGIN OF CELLS
1.5 – Cells can only be formed
by division of pre-existing cells
1.5 – The first cell must have
arisen from non-living materials
1.5 – The origin of eukaryotic
cells can be explained by the
endosymbiotic theory
2. REVISION - CELL THEORY
o The development of cell theory has taken hundreds of years
o Much of its development parallels with advances in technology –
including staining techniques and the development of both light and
electron microscopes
o The three main principles of cell theory are:
1. All organisms are composed of one or more cells
2. Cells are the smallest units of life
3. All cells come from pre-existing cells
3. REVISION – PASTEUR’S
EXPERIMENT
o Louis Pasteur performed the famous swan-necked flask experiment
to prove the third main point of cell theory – this experiment
ultimately disproved the long standing idea of ‘spontaneous
generation’
o This provided evidence in support of the third point of cell theory
‘all cells come from pre-existing cells”
4.
5. THE FIRST CELL
o One obvious missing component of cell theory is how the first cell
arose
o The first cells must have arisen from non-living material
o Also what is not explained by cell theory is how cells progressed
from simple, non-compartmentalised prokaryotes to a complex,
highly compartmentalized eukaryote
6. THE FIRST CELL
o If we trace back the ancestry of cells over billions of years, we must
eventually reach the earliest cells to have existed
o The first cells could have arrived on Earth from somewhere else in
the universe (Panspermia theory)
o OR they could have arisen from non-living material and evolved over
hundreds of millions of years. The next slides contain hypotheses for
how some of the main stages in this evolution could have occurred
(taken from Allott).
10. ENDOSYMBIOSIS
o The theory of endosymbiosis helps explain the evolution of eukaryotic cells
by looking at the origin of mitochondria and chloroplasts
o The key points to the theory are:
o About 2 billion years ago a bacterial cell took up residence inside a eukaryotic cell
o The eukaryotic cell acted as a ‘predator’, bringing the bacterial cell inside (by
endocytosis)
o The eukaryotic cell and the bacterial cell formed a symbiotic relationship, in which both
organisms lived in contact with one another
o The bacterial cell then went through a series of changes to ultimately become a
mitochondrion
o In this process the eukaryote helped the bacteria by providing protection
and carbon compounds. The bacteria, after a series of changes, became
specialised in providing the eukaryote with ATP.
o It is thought that mitochondria were originally aerobic heterotrophic
bacteria and chloroplasts were originally cyanobacteria
What is
endosymbiosis?
12. EVIDENCE FOR ENDOSYMBIOSIS
o Mitochondria and chloroplasts are similar in size and structure to
bacteria
o Both organelles are bound by a double membrane – the outer
membrane may be derived from the engulfing vesicle, and the inner
one may be derived from the plasma membrane of the original
prokaryote
o Mitochondria and chloroplasts contain a limited amount of genetic
material and divide by fission (independently of the host cell). Their
DNA is a circular loop like that of prokaryotes.
o Although most of the proteins within mitochondria and chloroplasts
are now produced by the eukaryotic host, they do have their own
ribosomes and they do produce some proteins. Their ribosomes
resemble those of prokaryotes.
13. EVIDENCE FOR ENDOSYMBIOSIS(TAKEN FROM PEARSON)
o “The final bit of evidence for endosymbiotic theory is DNA. DNA
provides a code made up of 64 different ‘words’. Interestingly, this
code has the same meaning in nearly all organisms on Earth and is
said to be ‘universal’. There are only slight variations, which can be
explained by changes since the common origin of life on our planet.
As mentioned, the mitochondria of eukaryotic cells have a DNA code
that more closely resembles bacteria than eukaryotic cells. Most
scientists believe that the more DNA two organisms have in common,
the more closely related they are to one another.”
