IB Biology Sub-topic 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.

Multicellularity

You now know about the origin of eukaryotic cells, but you are also expected how these developed into multicellular organisms. They are thought to have developed when:

1. Single cells clumped together to form colonies.
2. In these colonies, some cells adapted more specialized roles than others depending on their location.
3. This formed a pre-cursor to a multicellular organism.

Whilst many single-celled organisms exist, plants, animals, many algae and fungi have repeatedly evolved after extinction events. The natural tendency for eukaryotic cells to group together into multicellular organisms indicate the advantages of a larger body cell and cell specialization are considered useful.

Cell differentiation

This ability for cells to develop and specialize into specific cell types is called differentiation. As the different cell types grow together, they begin to produce small-scale tissues, organs, and organisms that repeatedly evolve to grow bigger.

However, you are expected understand how cell differentiation occurs:

1. Environmental changes trigger signalling cascades within the cell.
2. This causes a change in the epigenetic patterns of the cell, particularly its methylation pattern.
3. This results in a change of gene expression - turning some genes on and others off and adjusting protein production.
4. This leads to a change in a cell's behavior and function.

Thus, environmental factors cause changes in genetic expression patterns to cause cell differentiation.