1.5 The Origin of cells
Understandings:
Cells can only be formed from pre-existing cells.
Hypothesis: Cells can only be produced by division of a pre-existing cell.
The First Artificial Cell (well, kind of)
In 2010, some biologists had created the first artificial cell. In other words, it was not the result of cell division. BUT, the base sequence of the DNA of a bacterium (Mycoplasma mycoides) was synthesized artificially and was transferred to another pre-existing cell of a different type of bacterium (Mycoplasma capricolum), which was converted into Mycoplasma mycoides.
The first cells must have arisen from non-living material.
Unless cells arrived on Earth from somewhere else in the universe, they must have arisen from non-living material.
1. Production of carbon compounds such as sugars and amino acids
2. Assembly of carbon compounds into polymers
3. Formation of membranes
4. Development of a mechanism for inheritance
The origin of eukaryotic cells can be explained by the endosymbiotic theory.
The Theory of Endosymbiosis:
1. Mitochondria were once free-living prokaryotic organisms that had developed the process of aerobic cell respiration.
2. Then, larger prokaryotes that only had the ability to respire anaerobically took them in by endocytosis. (Figure 1)
Hypothesis: Cells can only be produced by division of a pre-existing cell.
- all the cells in your body were formed when a previously existing cell divided into two
- the origin of the cells in your body are traced back to the first cell - the zygote (produced by the fusion of a sperm and an egg cells)
- sperm and egg cells were produced by cell division in your parents
- the origin of the cells in your parents body come from the zygote of your grandparents
- cells can be traced back to your early human ancestors
- then before human evolution, the cells are traced back to pre-existing ancestral species (chimpanzees)
- then trace those pre-existing ancestral species' cells back to the first cells on Earth millions of years ago
- therefore, there is a continuity of life from its origins on Earth to the cells in our bodies today
The First Artificial Cell (well, kind of)
In 2010, some biologists had created the first artificial cell. In other words, it was not the result of cell division. BUT, the base sequence of the DNA of a bacterium (Mycoplasma mycoides) was synthesized artificially and was transferred to another pre-existing cell of a different type of bacterium (Mycoplasma capricolum), which was converted into Mycoplasma mycoides.
The first cells must have arisen from non-living material.
Unless cells arrived on Earth from somewhere else in the universe, they must have arisen from non-living material.
1. Production of carbon compounds such as sugars and amino acids
- with the mixture of methane, hydrogen and ammonia represents the atmosphere of early Earth and electrical charges simulating lightning
- the production of amino acids and other carbon compounds needed for life happened
2. Assembly of carbon compounds into polymers
- origin of the first carbon compounds is around deep-sea vents (cracks in the Earth)
- characterized by gushing hot water carrying reduced inorganic chemicals such as iron sulphide
- these chemicals represent a supply of energy for the assembly of these carbon compounds into polymers
3. Formation of membranes
- because phospholipids or other amphipathic carbon compounds were among the first carbon compounds, they would have naturally assembled into bilayers
- then these bilayers will form vesicles resembling the plasma membrane of a small cell
4. Development of a mechanism for inheritance
- living organisms have genes made of DNA and use enzymes as catalysts
- early phase in evolution: RNA was the genetic material ---> self-replicates and can act as a catalyst
The origin of eukaryotic cells can be explained by the endosymbiotic theory.
The Theory of Endosymbiosis:
1. Mitochondria were once free-living prokaryotic organisms that had developed the process of aerobic cell respiration.
2. Then, larger prokaryotes that only had the ability to respire anaerobically took them in by endocytosis. (Figure 1)
3. Instead of killing and digesting the smaller prokaryotes, they allowed them to live within their cytoplasm.
4. As long as the smaller prokaryotes grew and divided as fast as the larger ones, they could persist indefinitely.
5. According to this theory, the mitochondria in eukaryotic cells was the result of endosymbiosis and evolution.
The Mutualistic Relationship between the Small and Large Prokaryotes:
The Origin of Chloroplasts:
4. As long as the smaller prokaryotes grew and divided as fast as the larger ones, they could persist indefinitely.
5. According to this theory, the mitochondria in eukaryotic cells was the result of endosymbiosis and evolution.
The Mutualistic Relationship between the Small and Large Prokaryotes:
- the smaller cell is benefited by the supply of food from the prokaryotes
- the larger cell is benefited by the energy that is supplied by the smaller cell's energy efficiently from cellular respiration (aerobic respiration)
- then natural selection favoured cells with this endosymbiotic relationship
The Origin of Chloroplasts:
- if a prokaryote that had developed photosynthesis was taken in (through endosymbiosis), it could have developed into the chloroplasts of photosynthetic eukaryotes (Figure 2)
Evidence for the Evolution from Prokaryotes (Mitochondria & Chloroplasts):
- they have their own genes (on a circular DNA molecule like that of prokaryotes)
- they have their own 70S ribosomes (of a size and shape like that of prokaryotes)
- they transcribe their DNA and use the mRNA to synthesize some of their own proteins
- they can only be produced by division of pre-existing mitochondria/chloroplasts
Nature of science:
Verifying the general principles that underlie the natural world: the principle that cells only come from pre-existing cells need to be verified.
Reasons for this Theory:
- Spontaneous generation is the formation of living organisms from non-living matter (FALSIFIED)
- Universally Accepted: cells only come from pre-existing cells because of Pasteur's and others' experiments
Reasons for this Theory:
- a cell is a complex structure and cannot be produced from simpler subunits (because no such natural mechanism exists)
- no example of increase in number of cells in a population/organism/tissue without cell division occurring
- viruses are produced from smaller subunits but do not consist of cells, and can they only be produced inside the host cells that they have infected
Applications:
Evidence from Pasteur's experiments that spontaneous generation of cells and organisms does not now occur on Earth.
Louis Pasteur's Swan-Necked Flask Experiment (Figure 1.4):
Louis Pasteur's Swan-Necked Flask Experiment (Figure 1.4):
- bent the flasks with long necks by melting the glass
- then boiled one flask with the broth (to kill the organisms present) and left the other flasks unboiled (control group)
- fungi and other organisms soon appeared in the unboiled flasks but not in the boiled flasks
- all the flasks were in contact in air but the dust particles (which microorganisms travelled on) could not travel against gravity so did not come in contact with the broth