Monday, June 3, 2013
Life’s history on Earth is punctuated by some important transitions. One of the more important ones was the development of symbiotic, interdependent relationships between simple prokaryotic cells, such as bacteria. This made possible a new kind of life form: the more complex eukaryotic cells that make up body tissues in complex organisms.
Eukaryotes have nuclei and numerous internal structures. Those structures lie in the cytoplasm, a kind of soup between the outer cell membrane and the cell’s nucleus. The versatility of eukaryotes stems from the variety of the internal structures, called organelles (small organs), they possess.
The development of eukaryotic cells eventually leads to several other fundamentally important transitions, sexual reproduction, processes for better exploitation of energy resources and multi-cellularity. Each of these brought their own significant acceleration in the development of diverse life forms.
Relatively rapid transitions of this sort are called saltation. They stand in contrast to gradualism that drives adaptations of organisms to new environments over longer periods.
How did the more versatile eukaryotic cells arise? We often envision new life forms developing in environments of conflict and competition. However, survival of the fittest is only a part of the story.
The development of complex internal structures in eukaryotic cells illustrates another theme. Most, if not all, internal structures originated with one variety of independent prokaryotic cell engulfing another type of prokaryote within its outer membrane.
The host made available the raw material needs of its captive and commandeered its products. In some instances, some or a portion of the captive’s DNA was transferred to the genome of the host. At that point, the organelle or structure became an indistinguishable part of the host.
This is known as endosymbiosis. “Endo” refers to the combining of the one prokaryote within the other, while “symbiosis” refers to the mutual dependence that developed.
For these fused cells the whole was indeed greater than the sum of its parts. They had enhanced or completely new capabilities, increasing their potential for adapting to new environments or competing for resources.
In some instances, they carved out new ways to support their existence, their own unique niche. Eukaryotes could compete for resources in ways neither free-living prokaryote could alone.
Among the more important organelles that make eukaryotes such robust organisms are mitochondria and plastids. They contain their own DNA and retain their own membranes. That is, while a eukaryotic cell has DNA in its nucleus and that DNA orchestrates the functions of the cell as a whole, the role of mitochondrial DNA is limited to the functions of the mitochondria.
Plastids (chloroplasts), in the cytoplasm of plants, algae and some protists, also have their own membranes and distinct DNA.
These two organelles preform the most fundamentally important metabolic functions required for existence of complex life. Photosynthesis is conducted in the chloroplast of plant cells and algae. It harnesses the sun’s energy, storing it in the chemical bonds of the carbohydrates they fabricate from carbon dioxide. Energy rich carbohydrates are then the food/energy sources for all living things.
Mitochondria are the site of respiration, the metabolic process that extracts energy from carbohydrates and makes it available for cell functions.
A broader definition of organelles might include a variety of structures or cell compartments. These do not contain DNA and are in some cases simply assemblies of macromolecules that carry out special functions. Ribosomes, flagellum, the cytoskeleton, and centrioles exemplify these later structures.
The once independent prokaryotic cells referred to earlier were, in fact, bacteria. Bacteria have no nucleus and very little internal structure. They survive, sometimes in severe environments, by being good at one simple task.
There are countless varieties of bacterial prokaryotes and they make up a very large percentage of the Earth’s biomass. But, by themselves, they have limited potential for increased complexity.
That early life was made up primarily of prokaryotes is demonstrated in 3.5 billion-year-old stromatolite fossils. These formations are made from fused masses of prokaryotic cells (cyanobacteria).
In this early stage of life, cell cooperation was limited to their congregation in colonies. No part of the colony shows signs of specialization for a particular task where its activity might benefit the greater community of cells. The cooperative behavior seen between cells in tissues would have to await the emergence of eukaryotic cells.
This is a perfect example of how ever-more complex life arose over the course of evolution. There was mutual advantage to each when the functions of the captive prokaryote were incorporated within the host cell’s membrane.
This is a recurring theme, new organisms and functions arise by cobbling together what previously existed creating something new. This tinkering leads to enhancements that have survival advantages.
My next article will describe how the advantages of the transition to eukaryotic cells gave rise to organisms capable of sexual reproduction, better exploitation of energy resources and the ability to form the tissues that make up our bodies.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.