1.5 The Origin of Cells Flashcards
What is the theory of how cells come to be?
Cells can only be formed from pre-existing cells. We can trace this back to the very first cell.
The idea that cells only come from pre-existing cells needs to be verified, and scientists throughout the years have tried to make a cell from non-living matter.
Why do we think cells can only arise from pre-existing cells?
- A cell is a highly complex structure and no natural mechanism has been suggested for producing cells from smaller subunits.
- No example is known of increases in the number of cells in a population, organism or tissue without cell division occurring.
- Viruses are produced from simpler subunits but they do not consist of cells and they can only be produced inside the host cells that they have infected.
What were Louis Pasteur’s experiments?
He carried out a range of experiments using swan necked flasks.
He placed samples of a nutrient broth in flasks with long necks and then melted the glass of the necks and bent it into a variety of different shapes.
Pasteur boiled the broth to kill any organisms in there but left unboiled as controls. Fungi and other organisms soon appeared in the unboiled controls but not in the boiled, even after long periods of time. But the unboiled flasks had been in contact with air which was suggested was needed for spontaneous generation. Later he snapped the necks of some of the flasks and soon organisms appeared.
He concluded that the swan necks prevented organisms from the air getting into the broth or other liquids and that no organisms appeared spontaneously. His experiments convinced most biologists, both at the time of publication and since then.
How could the first living cell have developed?
Living cells may have evolved over hundreds of millions of years.
1) You can make carbon compounds needed for life from ammonia, hydrogen and methane. Miller and Urey made this mixture, it was supposed to be representative of the atmosphere of the early earth. They passed electrical energy through it and managed to get some carbon compounds needed for life.
2) Then these compounds could be assembled. Deep sea vents, which are cracks in the earth’s surface characterised by hot gushing water carrying reduced inorganic compounds such an iron sulphide, could have supplied the energy to make these compounds into polymers.
3) Phospholipids naturally move into bilayers. If phospholipids were among the first carbon compounds, they would have naturally assembled into bilayers. Then a different internal chemistry from that of the surroundings could have developed.
4) You need enzymes to make genes but genes to make enzymes. This awkward conundrum may be answered by an earlier phase in evolution when RNA was the genetic material. It can store information in the same way as DNA but it is both self-replicating and can itself act as a catalyst.
How could Eukaryotic cells have evolved?
The endosymbiotic theory helps to explain this. It states that mitochondria were once free-living prokaryotic organisms that had developed the process of aerobic cell respiration. Larger prokaryotes that could only respire anaerobically took them in my endocytosis. Instead of killing and digesting the smaller prokaryotes they allowed them to continue to live in their cytoplasm. As long as the smaller prokaryotes grew and divided as fast as the larger ones, they could persist indefinitely inside the larger cells.
The larger prokaryotes and the smaller aerobically respiring ones were in a symbiotic relationship in which both of them benefitted. This is known as a mutualistic relationship. Natural selection therefore favoured these cells.
What is the evidence that mitochondria and chloroplasts were once independent prokaryotes?
- Both have 70S ribosomes
- Both have their own genes on a circular DNA molecule like prokaryotes
- They transcribe DNA and use the mRNA to synthesise proteins.
- They can only be produced by the replication of preexisting mitochondria and chloroplasts.
Do prokaryotes have mitochondria?
NO BC THEY ARE NOT COMPARTMENTALIZED
According to the precautionary principle, what should be done about greenhouse gas emission?
Develop strategies for coping with higher global temperatures
