IB Biology A2.2 Notes

Endosymbiotic theory

In the SL syllabus, you learned about the structure of eukaryotic cells. In the HL syllabus, you need to understand how they formed. This is is explained by the endosymbiotic theory. Endosymbiosis refers to the phenomenon of a smaller cell being endocytosed by a larger cell and then living inside the larger cell to perform a function for it. Thus, endosymbiosis is a combination of two words:

1. Endocytosis – we know this term from Topic 1.4 but remember that it is the transport of materials into the cell.
2. Symbiosis – the harmonious living together of two organisms. 

This process is what formed the mitochondria and chloroplasts of animal and plant cells. This means that endosymbiosis must have occurred twice.

1. An anaerobic cell takes in an aerobic bacterium via endocytosis. The bacterium now provides ATP for both cells via aerobic respiration and is protected from the external environment.
2. This now aerobic cell takes in an autotrophic bacterium via endocytosis. The bacterium now provides nutrients for both cells via photosynthesis and is protected from the external environment.

However, we cannot just take this information for granted. The IB expects you to remember the evidence that supports the endosymbiotic theory. There are three main points to remember:

1. Both mitochondria and chloroplasts have a nucleoid and 70S ribosomes. This provides evidence for their origins as prokaryotes, as eukaryotes do not have these.
2. This theory is further supported by the fact that in preparation for mitosis, mitochondria and chloroplasts independently undergo binary fission to replicate themselves, a process unique to prokaryotes.
3. Finally, both mitochondria and chloroplasts have a double membrane. This provides evidence for endocytosis, as this process would envelop a normal bacterium in a second membrane.