1.5 The origin of cells Flashcards
What does the cell theory suggest?
- That living organisms consist of one or more cells, and that cells come from pre-existing cells.
- One of the defining characteristics of life is reproduction.
- So, it follows that cells must come from pre-existing living cells.
Who was Louis Pasteur?
A famous French microbiologist who gave crucial evidence to support the hypothesis that cells must come from pre-existing cells.
Louis Pasteur’s experiment disproved the theory of _______
Spontaneous generation, which stated that life could appear from a combination of dust, air and other factors.
Pasteur hypothesised that ______
Cells must come from cells and designed a falsifiable experiment
Diagram of Pasteur’s experiment
Pasteur’s experiment
1) Pasteur boiled nutrient broth in three swan-neck flasks.
2) He then broke the neck of one flask to allow air to enter, but left the other flask unbroken.
3) The broth in the flask where the swan neck was not broken remained clear (as no microbes were formed and any microbes from the air were trapped in the curve of the swan neck), while the broth in the broken-necked flask became cloudy, as microbes carried to the flask in the air grew and multiplied.
4) In the third flask, Pasteur tilted the flask to expose the broth to the microbes in the curve of the swan neck.
What did Pasteur’s experiment prove?
That spontaneous generation of cells and organisms does not occur on Earth, showing that the present conditions (temperature, pressure, light, radiation level, and so on) do not sustain the process.
Pasteur’s experiments provided evidence that spontaneous generation of cells was not possible on Earth. What evidence supports this?
- Fungi and other organisms did not appear in the boiled broth inside sealed swan-necked flasks after prior exposure to air.
- Pasteur’s experiments involved the use of swan-necked flasks that were filled with nutrient broth and then sealed (no access to the outside air).
- Some flasks were boiled to kill living organisms inside them.
- Some were left unboiled as a control group.
- In the broth that was boiled and sealed, no microorganisms developed after a long period of time.
- The broth in these flasks was in contact with air sealed into the flasks.
- Air was believed responsible for spontaneous generation.
- Since there were no organisms appearing, the spontaneous generation theory was rejected.
One possible explanation for where the first cells came from
- One possible explanation is that life, in the form of cells, was transported to Planet Earth from elsewhere in the universe.
- However, evidence supporting this hypothesis has not been found.
- Therefore, the first cell must have come from non-living material.
- This hypothesis has gained support from evidence collected in the Miller–Urey experiment.
Miller-Urey experiment
- Miller and Urey recreated the conditions of early Earth in a closed system by including a reducing atmosphere (low oxygen) with high radiation levels, high temperatures and electrical storms.
- After running the experiment for a week, some simple amino acids and complex oily hydrocarbons were found in the reaction mixture.
What did the Miller-Urey experiment prove?
That non-living synthesis of simple organic molecules was possible.
Diagram of Miller-Urey experiment
The results of the Miller–Urey experiment helped us to understand what the necessary conditions are for life to occur. What are the four conditions that scientists believe are needed for life to emerge and persist?
- Simple organic molecules, such as amino acids, fatty acids and carbohydrates, must be formed.
- Larger organic molecules, such as phospholipids, RNA and DNA, must be assembled from simpler molecules.
- Organisms reproduce, so replication of nucleic acids must be possible.
- Biochemical reactions require set conditions, such as pH. Therefore, self-contained structures, such as membranes, are necessary.
Where does the proof lie for the evolution of eukaryotic cells?
In the double membrane structure of organelles, such as mitochondria and chloroplasts, and in the comparison of the DNA of those organelles with prokaryotic genomes.
What theory explains the origin of eukaryotic cells?
The endosymbiotic theory.
Diagram showing how the first primitive eukaryote may have come into being through endocytosis. (How the first endosymbionts may have arisen.)
