Finally we will examine those structures that lie external to the cell wall. Based on planes of division, the coccus shape can appear in several distinct arrangements: diplococcus, streptococcus, tetrad, sarcina, and staphylococcus. The bacillus shape can appear as a single bacillus, a streptobacillus, or a coccobacillus.
The spiral shape can appear in several forms: vibrio, spirillum, and spirochete. The cytoplasmic membrane is semipermeable and determines what molecules enter and leave the bacterial cell. Passive diffusion is the net movement of gases or small uncharged polar molecules such as water across a membrane from an area of higher concentration to an area of lower concentration.
The peptidoglycan cell wall surrounds the cytoplasmic membrane and prevents osmotic lysis. Although the term has carried a phylogenetic burden, meaning that it originally implied a close evolutionary relationship between the Bacteria and the Archaea, no one I know uses it in that sense now. Among biologists, the three domains model is widely accepted, in fact, not even discussed. How come? This usage illustrates the reality that these two groups of microbes, though they likely diverged very early on in evolution, share a large number of common properties.
Their sizes tend to overlap, their overall body plan is generally very similar, they often occupy the same habitats, they share many homologous genes. Presented with an EM thin section, you could not tell a typical bacterium from a typical archaeon.
So, dissimilar as they may be in one sense, they are very similar in a number of important attributes. I agree, there is danger in the P word being misunderstood out in the big wide world, but there is none within the family of biologists. The term is found all over the place, notwithstanding the astonishing campaign waged against it.
They and smaller, self-assembling nanocompartments have a polyhedral structure that looks shockingly like a viral capsid, the protein shell that encloses viral genomic material.
Bacteria seem to have a cornucopia of such organelles, with more waiting to be discovered. Scientists are now starting to explore what that means in the context of eukaryotic evolution. They hope either to establish direct evolutionary relationships among the growing list of structures, or to pinpoint factors that are unique and necessary for compartmentalization and complexity.
Two major milestones defined the origin of eukaryotes. The other was the formation of mitochondria, which are thought to have once been free-living bacteria that were engulfed by an ancestor of the archaea. Experts have disagreed about the most likely order and relative importance of those events. Some posit that gaining mitochondria was the essential change that set off eukaryotic evolution.
Others theorize that eukaryotic evolution was well underway, and that an intricate membrane apparatus which likely included a nucleus was already present and helped to enable the uptake of the mitochondrial precursors. No one knows whether the structures seen in bacteria represent primitive, intermediate steps in the evolution of eukaryotic organelles, or separate innovations that evolved independently of those of eukaryotes.
But even if the bacterial and eukaryotic organelles did evolve completely independently, the prokaryotic structures may be useful for understanding the eukaryotic ones. This possibility was highlighted in April, when a pair of researchers offered an intriguing argument that the nucleus was a much later addition than had generally been believed. Meanwhile, the acquisition of mitochondria would have occurred in parallel — all adding up to a slow, stepwise progression from the archaea to the last eukaryotic common ancestor.
During this process, many intermediate eukaryotes would have lacked a nucleus and other complex features. Moreover, if prokaryotes build and maintain these structures differently than eukaryotes do, then scientists can more confidently determine how and why compartmentalization might arise. When compartmentalization was thought of as a singular feature of eukaryotes, experts were often forced to speculate about how it came about, what the biophysical constraints were, and what selective advantages it might have.
Poole wants to see if this trend extends to the compartmentalization of genomic information. But by examining it in different types of organisms — in the eukaryotic nucleus, in the planctomycete membrane system and even in giant viruses — he thinks researchers can start to outline drivers of compartmentalization and the conditions that might give rise to it.
At the very least, there appear to be certain biophysical constraints: A specific kind of protein fusion, for instance, seems to be required to manipulate membranes. This work not only suggests that compartmentalization is more prevalent among the various branches of the tree of life than people thought; it also indicates that this kind of complexity was not the critical innovation needed to trigger eukaryotic evolution.
Some researchers are taking a different route. Kate Adamala, a synthetic biologist at the University of Minnesota, and her colleagues are building synthetic cells with some basic internal organization.
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