The origin of cells 1.5

Live has evolved from the first cells to all the cells we find in the huge diversity of todays organisms. This topic covers the origins of cells and the cell theory proposed by Pasteur and Schwaan in an age when spontaneous generation of cells was still believed to occur.

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Revision summary list for topic 1.5 The origin of cells

  • The first cells must have arisen from non-living material.
  • The origin of eukaryotic cells can be explained by the endosymbiotic theory.
  • Evidence from Pasteur’s experiments falsified the theory that spontaneous generation of cells and organisms occurs.

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Pasteur's experiment with 'swan neck' flasks showed that a sterile nutrient medium exposed to the air would not show any signs of bacterial growth under his conditions.

What prevented the growth of bacteria?

Pasteur's famous experiments with swan neck flasks showed that broth kept in a flask where no dust could settle in the nutrient medium, and thus no living cells could get in, would not go mouldy.

This disproved the theory of spontaeous generation.


Spontaneous generation was a popular concept a few hundred years ago.

Which of the following statements best summarises the theory?

The theory of spontaneous generation tried to explain the occurance of organisms like maggots, mould and bacteria in rotting food. Pasteur's experiments falsified this theory.


The 64 codons of mRNA code for the same amino acids in almost all species. A rare exception is found in Paramecium where one of the "stop codons" actually codes for the amino acid glutamine.

What does this suggest about the origin of cells?

The 64 codons in the genetic code give rise to the same amino acids in nearly all organisms, There is very little variaion. If the genetic code had evolved several times in the history off life, there would be many differences.


Cells today come from pre-existing cells. The origin of the fist cell must be different.

Where do biologists think the first cell came from?

The first cell must have come from non-living material. This material must have contained molecules which today we consider as organic, carbon containing molecules.


What is the best definition of endosymbiosis?

Endosymbiosis is where a cell engulfs another cell and it continues to live inside the cell.

The engulfed cell provides something for the host cell, and gets something in return. Both cells benefit.


The image below shows a eukaryotic cell.

Which structure, visible in the image, could be used as evidence supporting endosymbiosis?

The mitochondria provide evidence supporting endosymbiosis because they have:

  • a double membrane
  • 70S ribosomes
  • DNA
What can be concluded from data which shows that the same mRNA codons correspond to the same amino acids in nearly all species, from bacteria to humans?

The 64 codons in the genetic code give rise to the same amino acids in nearly all organisms.
There is very little variaion. This is evidence for a single common origin of life.

Differences in the frequency of amino acid use reflects the different genes in the two organisms.

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