IB Biology - The Origins of Cell

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

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

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

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

• Students should be aware that the 64 codons in the genetic code have the same meanings in nearly all organisms, but that there are some minor variations that are likely to have accrued since the common origin of life on Earth.

• Do you still remember the content in the IB biology chapter 1.1 The Cell Theory? 
• The Cell Theory mentions:
  1. A cell is the smallest unit of life 
  2. All organisms are composed of more than one cells
  3. Cell arises from the pre-existing cell 
• However, how can we trust it? Does it have any evidence to prove it? 
   

• Louis Pasteur is a French biologist. (Yes, Pasteur sterilization in the dairy product is invented by him!)
• He proves "Cell arises from the pre-existing cell" by his experiment 
• In his experiment, there are two swan neck flasks (flask with a long curved neck), the neck prevents the bacteria from entering the flask 

• Both of the flasks contained broth for bacteria growth
• Both of the broths were boiled before the experiment to ensure there was no pre-existing bacteria in the flask
• Pasteur took off one of the flask's neck, so the air and bacteria could move in 

• Result:
  the flask with neck had no bacteria 
  the flask without neck, allowing the bacteria in the air move in had bacteria 
• The experiment proved that cell cannot arise from nothing (the flask with the neck). A cell must come from the pre-existing cell (the bacteria in the air resulted in the presence of bacteria in the flask) 

• The DNA sequence is another proof that all life has a common ancestral cell
• The DNA sequence is made up of 4 bases (A,T,C,G). By using any 3 out of these 4 bases, the DNA can “generate” a codon. Each of the codons represents a specific amino acid.
• A, T, C, G can give 64 codon combinations 
• These 64 codons are universal to all the life in the Earth
• Such “coincidence” suggested that all life in the Earth comes from one common ancestor. Or else, different species should have its own system to generate amino acid.

• Then you may ask where did the very first cell in the world come from? Certainly, there must be the very first cell. 
• The scientists believed the very first cell should be prokaryote since it is much simple than eukaryote. A eukaryote is evolved from a prokaryote. 
• For the arising of the first prokaryote, scientists believed 4 steps must have occurred. 
  1. Cell is made up of organic molecules, so organic molecules (e.g. amino acid, glucose, fatty acid) are the prerequisites of the cell formation
  2. The molecule molecules in the cell are complex, so the organic molecules must combine together to form those much complex organic molecules (e.g. protein, phospholipid, nucleic acid) 
  3. To increase the no. of these molecules, the molecules must need to replicate themselves. This is reproduction. 
  4. To prevent the loss of these valuable molecules, a membrane is required to enclose them. A cell is formed.

• Then how eukaryote evolved from prokaryote? 
• The theory suggested some of the organelles in eukaryotes are prokaryote. A larger prokaryote engulfed those smaller ones for its own good. This is how eukaryote formed. 

Endosymbiotic Theory = eukaryote is formed when the large prokaryote engulfed the small prokaryotes

• mitochondria, chloroplast were great testimonies for the theory 

• Why did scientist believe mitochondria and chloroplast formerly are prokaryotes?

1. It has its own DNA that is independent of the eukaryotic host. The DNA is small circular pieces just like the bacteria plasmid
2. It has its own much smaller ribosome which has similar size as those in prokaryotes
3. Bounded by a double membrane, suggesting that they formerly are independent prokaryotes
4. Can replicate independently by binary fission (a replication method of prokaryote, IB biology chapter 1.2)

Arguments of the theory

• The theory faced criticism and challenges:

• ✋🏻The membrane of mitochondria and chloroplast is derived from the eukaryotes its own light absorption property bearing cell membrane via phagocytosis
  vs 
  🤘🏻Bounded by a double membrane, suggesting that they formerly are independent prokaryotes
   
• ✋🏻The inheritance of these prokaryotes is not guaranteed since its replication is independent of the host. When the host cell replicates and divides, there is a chance the offspring receive no such prokaryotes.
  vs 
  🤘🏻the prokaryotes maximize its number in the cytoplasm via binary fission to ensure each of the offspring receive the prokaryotes
   
• ✋🏻Although the mitochondria and chloroplast are prokaryotes, they cannot survive on its own
  vs 
  🤘🏻The symbiosis has happened for a long time, the mitochondria and chloroplast already lost their ability to survive on its own.

In this IB Biology topic, you have to understand

1. Cell must come from a pre-existing cell and it is proved by Pasteur's experiment 
2. What is the main purpose of Pasteur's experiment? 
3. the very first cell comes from the combination of complex organic molecules 
4. the first cell is believed to be prokaryotes. Eukaryotes arise from prokaryotes 
5. the evidence and arguments of the endosymbiotic theory 

That is the end of the topic

Wanna Boost Up Your IB Biology? Check out our study tips here!

Written by Queenie l IBDP Biology Specialist @ TUTTEE

Photo References:
1. Wnycstudios, https://www.wnycstudios.org/podcasts/undiscovered/episodes/spontaneous-generation

2. Explore Biochemistry World, https://explorebiochemistryworld.wordpress.com/2013/02/03/the-endosymbiotic-theory/