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2.11: Origins of Cell Compartmentalization

In Topic 2.10, we discussed the principle of cell compartmentalization and how it allows for the separation of reactions in order to make them efficient. In 2.11 we'll discuss one key part of cell compartmentalization, the origin of organelle membranes.

Vocab List

Written Explanation

Prokaryotic compartmentalization:

While eukaryotes usually get all the credit for compartmentalizing, prokaryotes have their own sneaky way of doing it too. They separate their inner environment into different regions which are each slightly better suited for a set of reactions. In this way, they get some of the benefits of compartmentalization, although those benefits are significantly reduced and weakened.

This is a fairly new idea, so don't worry too much about knowing it. However, the next section is crucial to understand.

The Endosymbiotic Theory:

The Endosymbiotic Theory (aka symbiogenesis) is the idea that some of the organelles within eukaryotic cells originated from what were once prokaryotic cells. This theory was first proposed by the Russian botanist Konstantin Mereschkowski in 1905, and has since gained plenty of credibility and evidence.

The theory goes that there was once (upon a time) an ancestral eukaryote, which we would classify as a prokaryote. This ancestral eukaryotic cell at one point engulfed a cyanobacteria (a photosynthetic prokaryote), and instead of breaking it down, formed a symbiotic relationship. The cyanobacteria would provide energy for its host cell, while the host cell provided a safe environment and nutrients. Over time, this cyanobacteria would evolve into a chloroplast. The same thing may have also happened with mitochondria (which once used to be an aerobic bacteria).

Some strong evidence for this theory is that mitochondria and chloroplasts both have double membranes, suggesting that they originally had one, but when they were engulfed by the ancestral prokaryote they were wrapped in a second. Going back to endocytosis, this could have been a form of phagocytosis which left the original prokaryote wrapped in a vesicle.

Additionally, mitochondria and chloroplasts both contain their own DNA separate from the cell. This DNA is in a loop just like in prokaryotic cells, which further suggests their prokaryotic ancestry.

Lastly, the nucleus may have been a result of a part of the plasma membrane folding in and forming a little bubble around the DNA. This may also have been a vesicle/vacuole surrounding the DNA. Eventually, this would have evolved into a nucleus.

The endosymbiotic theory