Restrictions on the size of prokaryote molecules

[atay5510]

This question was a lecture discussion topic during my Biophysics course at university. I missed the lecture and the question is bothering me.

"Eukaryotes (multi-cell organisms) evolved into very large sizes whereas prokaryotes (single-cell organisms) remained quite small (about 1 micrometer). What has prevented prokaryotes from growing to larger sizes?"
I think the answer may be grounded in statistical mechanics but since I don't know the answer, that could be rubbish. Any help is appreciated!

 

[chill_factor]

Here's some google results that might help:

http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002173

My guess is that there's information storage and retrieval limits inherent in the prokaryotic's single doublestranded DNA ring that are not present in the eukaryote's multiple chromosomes nicely managed by histones.

 

[Mike H]

I would first note that the distinction between eukaryotes and prokaryotes is rooted in the presence or absence of a cell nucleus - there are unicellular eukaryotes like yeast, after all.

The answer likely involves consideration of the surface area to volume ratio, diffusion of gases through said volume, and that unicellular organisms need to be able to "do everything" - contrast this to multicellular organisms which possesses features which enable them to circulate necessary nutrients and chemicals through their bodies.

 

[atay5510]

Here is the answer I got:

From a survival/evolutionary standpoint, for single celled organisms to survive (such as a bacteria in a pond), it needs to be able to identify a potential source of danger (by some sensing mechanism) and transmit that information to the CPU of that cell in order to take action and propel the cell away (using the flagellum). So there has to be a 'sensing of danger' stage and a 'signal processing' stage.

Because prokaryotes are single-celled organisms there are no specialised cells like neurons that can transmit signals. The only way that information can be transmitted between the cell's sensors and its 'CPU' is by chemical diffusion within the cell. Typical values of diffusion coefficients for a cell's cytoplasm is on the order of D = 10^-9 m^2/s which is very slow.

As a result, prokaryotes with a very small cell size are able to identify danger in order to survive.

Pretty interesting...

 

[LudusRex]

There are a few possible explanations for the size difference between prokaryotes and eukaryotes. One factor could be the surface-to-volume ratio. As a cell grows in size, its volume increases at a faster rate than its surface area. This can lead to difficulties in efficiently exchanging materials and energy with the environment, which may limit the maximum size a prokaryote can reach. Additionally, prokaryotes have a simpler internal structure and lack organelles, which may also limit their size.

Another factor could be the availability of resources. Eukaryotes have the advantage of being multicellular, meaning they can specialize different cells for different functions and better utilize available resources. Prokaryotes, on the other hand, have to carry out all necessary functions within a single cell, which may limit their size due to resource constraints.

There could also be evolutionary reasons for the size difference. Eukaryotes have evolved complex systems for cell division, allowing them to grow and reproduce more efficiently. Prokaryotes, on the other hand, rely on simple binary fission, which may limit their maximum size.

In terms of statistical mechanics, the size limitation of prokaryotes could be related to the thermodynamics of cell growth and division. The physical properties of the cell membrane and the cytoplasm may play a role in determining the maximum size a prokaryote can reach.

Overall, it is likely a combination of these factors that has prevented prokaryotes from growing to larger sizes. As with many biological questions, the answer is likely complex and multifaceted. Further research and study in this area may provide more insights into the specific mechanisms that limit the size of prokaryotes.