IB Biology - Curriculum Notes 

1.5 The Origin of Cells  

∑ - Understandings:

∑ - Cells can only be formed by the division of pre-existing cells.

- Prokaryotic cells are formed during a process called binary fission.
- Eukaryotic cells form new identical cells by the process called mitosis (genetically identical) and form sex cells through meiosis (haploid cells which not genetically identical to the parent cell and contain half the genetic material).
- Cell division to form new cells from pre-existing cells replaced the concept of spontaneous generation, where cells were formed from inanimate matter.

∑ - The first cells must have arisen from non-living material.

Abiogenesis is the natural process of life arising from non-living matter such as simple organic compounds
If we go back to how the very first living cells were created, we have to conclude they either originated from non-living material, came from somewhere else in the universe or were created by some other unknown entity
These are the hypothesized steps of how living cells possibly developed from non-living material over millions of years

1)  Production of carbon compounds such as amino acids and sugars. Miler and Urey’s experiment showed how this could happen by passing water vapour through Ammonia, methane, and hydrogen (early earth atmosphere). They added electricity to simulate lightning discharge. They found they could create amino acids and carbon compounds

2)  Assembly of carbon compounds into polymers might have occurred at the deep-sea hydrothermal vents, which could have supplied the inorganic compounds such as iron sulphide  and thermal energy for the assembly

3)  Formation of membranes would be possible if phospholipids were some of the first polymers created. These phospholipids would naturally form vesicles allowing for a different environment to exist inside compared to the surrounding water

4)  Development of a mechanism for inheritance would be needed in order for the organism to replicate and pass its DNA on to the next generation. Current organisms need enzymes to replicate DNA; however, enzymes are created by the genes on the DNA. A possible solution to this would be RNA being the first nucleic acid formed because it is self-replicating and can also act as a catalyst.

Abiogenesis Video - https://www.youtube.com/watch?v=nNK3u8uVG7o

∑ - The origin of eukaryotic cells can be explained by the endosymbiotic theory.

- There is compelling evidence that mitochondria and chloroplasts were once primitive free-living bacterial cells.
- Symbiosis occurs when two different species benefit from living and working together. When one organism actually lives inside the other it's called endosymbiosis.
- The endosymbiotic theory describes how a large host cell and the bacteria ingested through endocytosis, could easily become dependent on one another for survival, resulting in a permanent relationship.
- As long as the smaller mitochondria living inside the cytoplasm of the larger cell divided at the same rate, they could persist indefinitely inside those cells
- The smaller cell was provided food and protection by the larger cell and the smaller mitochondria would supply energy through aerobic respiration for the larger cell
- Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell.

Diagram is to the right -->

Applications and skills:

ß - Application: Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.

Pasteur's Experiment

The steps of Pasteur's experiment are outlined below:

First, Pasteur prepared a nutrient broth similar to the broth one would use in soup.
Next, he placed equal amounts of the broth into two long-necked flasks. He left one flask with a straight neck. The other he bent to form an "S" shape.

Picture 1 to the right

Then he boiled the broth in each flask to kill any living matter in the liquid. The sterile broths were then left to sit, at room temperature and exposed to the air, in their open-mouthed flasks.

Picture 2 to the right

After several weeks, Pasteur observed that the broth in the straight-neck flask was discoloured and cloudy, while the broth in the curved-neck flask had not changed.

Picture 3 to right

He concluded that germs in the air were able to fall unobstructed down the straight-necked flask and contaminate the broth. The other flask, however, trapped germs in its curved neck,­ preventing them from reaching the broth, which never changed color or became cloudy.

Picture 4 to the right

Images courtesy William Harris

If spontaneous generation had been a real phenomenon, Pasteur argued, the broth in the curved-neck flask would have eventually become reinfected because the germs would have spontaneously generated. But the curved-neck flask never became infected, indicating that the germs could only come from other germs.

Taken from http://science.howstuffworks.com/innovation/scientific-experiments/scientific-method5.htm.

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