Читать книгу Astrobiology - Charles S. Cockell - Страница 106

The complexity of Gram-positive and -negative membranes should leave you in no doubt that the commonly held idea that prokaryotes are “simple” or “primitive” is likely to be misleading. Clearly, the first cells to have emerged on Earth could not have had the complexity of the modern cell membrane. The question arises – how far away, biochemically, are the most “ancient” prokaryotes on Earth today from the first cells to emerge on Earth? As this chapter progresses, you might like to consider this question. As we discuss microbial motility, the complexity of the genetic machinery, the replication of DNA, and other facets of cells, you will see the exquisite complexity of a whole range of biochemical functions represented in modern prokaryotes. Other questions emerge from this picture: Can we reconstruct the biochemistry of the earliest cells on Earth using modern-day cells, or have they advanced too far for us to truly know the biochemistry of the first living things? What is the minimum set of biochemical machinery needed to build a replicating, evolving cell? Has the evolution of the major fundamental capabilities of prokaryotes finished, or could even more complex machinery and capabilities naturally evolve in future prokaryotes?

The bacterial cell wall is often surrounded by a further layer called the glycocalyx. The glycocalyx is a network of polysaccharides. A distinct, gelatinous glycocalyx is called a capsule, whereas an irregular, diffuse layer is called a slime layer. Sugars attached to the outside of cells are sometimes called extracellular polysaccharide (EPS). The EPS acts as a protective layer of sugars that mediates interactions with surfaces. EPS allows cells to bind minerals and form biofilms. It is the substance that bonds minerals and bacteria together into macroscopic stromatolites or domes of microorganisms. Cyanobacteria are particularly prolific producers of EPS.