Tag Archives: organelles

Eukaryotes and Prokaryotes

I’ve long found it fascinating that every living thing on this planet can be cleanly split into two categories – prokaryotes and eukaryotes.  The prokaryotes consist of all the bacteria while the eukaryotes include animals, plants, fungi, and various protozoa.  The core life processes of the two cells are much the same, being built around the triad of proteins, RNA, and DNA, relying on the ribosome to build the proteins that synthesize everything else, including RNA and DNA, using ATP as the primary energy currency, and using lipid bilayer membranes to compartmentalize.  So what makes the two cell plans so different?

Below is a nice figure that helps you answer this question.

 

As you can see, there are two primary differences: size and level of compartmentalization.  Typical eukaryotic cells are much larger than bacteria and show a much more extensive level of compartmentalization given the numerous membrane-bound organelles and membranous folds.

Yet a question to ponder is why there are two cell types and only two cell types?  The non-telic perspective would explain this (away?) as simply an artifact of a contingent past.  There is no reason to ponder the question “why?”  It just happened that way.  But the telic perspective allows us to think of these two cell plans at a level that runs deeper.

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More ‘Evidence’ of Front-loading Mitochondria

I have previously shared some evidence for the front-loading of mitochondria, along with an emerging picture where symbiogenesis is merged with front-loading. Now there is yet some more evidence in support of the plausibility of front-loading mitochondria.  Those who follow this blog, or have read The Design Matrix, will note how seamlessly this all fits in with my hypothesis of life’s design:

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How to Guide the Blind Watchmaker

In the past, I have provided multiple lines of evidence to establish the plausibility of front-loading evolution.However, we have focused primarily on the transition from a unicellular to a multicellular eukaryotic state.Let’s now take one step back and begin pondering whether eukaryotic cells were themselves front-loaded to appear. For without the eukaryotic cell design, it is unlikely that the planet would possess anything analogous to an animal or plant.

While we still don’t really understand the origin of the eukaryotic cell, there is a strong consensus about the origin of the mitochondria, a very important organelle of eukaryotes.According to the endosymbiotic theory, mitochondria are the descendents of bacteria. The theory postulates that a phagocytic cell engulfed some aerobic bacteria and rather than digest them, a symbiotic relationship was established, where each partner benefited from the new relationship. This relationship then set the stage for the ultimate stream-lining of the bacteria, such that they were transformed into mitochondria through the transfer of much of their gadgetry to the host nucleus.

In a nutshell, the essence of the argument for the endosymbiotic origin of mitochondria is that mitochondria look like they share a common ancestor with bacteria. The argument is quite convincing, as there are numerous mitochondrial genes whose sequences are much more similar to bacterial sequence than that which exists in the nucleus of the same cell. In fact, this is an example where no one piece of evidence carries the day, but instead it’s the cumulative power of multiple lines of evidence.

Since mitochondria were once bacteria, might this transition have been front-loaded to happen?

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Meet Tom

We have just seen that bacterial cells were preadapted for the formation of mitochondria. More specifically, one out of every twenty bacterial proteins harbors the equivalent of the mitochondrial targeting sequence which is needed for entry into the mitochondria. But this story gets more interesting. To appreciate the twist that comes next, let’s step back to make sure we can visualize the process of transporting mitochondrial proteins into the mitochondria.

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Front-loading mitochondria

In the past, I have provided multiple lines of evidence to establish the plausibility of front-loading evolution. However, we have focused primarily on the transition from a unicellular to a multicellular eukaryotic state. Let’s now take one step back and begin pondering whether eukaryotic cells were themselves front-loaded to appear. For without the eukaryotic cell design, it is unlikely that the planet would possess anything analogous to an animal or plant.

While we still don’t really understand the origin of the eukaryotic cell, there is a strong consensus about the origin of the mitochondria, a very important organelle of eukaryotes. According to the endosymbiotic theory, mitochondria are the descendents of bacteria. The theory postulates that a phagocytic cell engulfed some aerobic bacteria and rather than digest them, a symbiotic relationship was established, where each partner benefited from the new relationship. This relationship then set the stage for the ultimate stream-lining of the bacteria, such that they were transformed into mitochondria through the transfer of much of their gadgetry to the host nucleus.

In a nutshell, the essence of the argument for the endosymbiotic origin of mitochondria is that mitochondria look like they share a common ancestor with bacteria. The argument is quite convincing, as there are numerous mitochondrial genes whose sequences are much more similar to bacterial sequence than that which exists in the nucleus of the same cell. In fact, this is an example where no one piece of evidence carries the day, but instead it’s the cumulative power of multiple lines of evidence.

Since mitochondria were once bacteria, might this transition have been front-loaded to happen?

Continue reading