|MadSci Network: Cell Biology|
Eubacteria are ~ 1-10 micrometers in diameter. Eukaryotes are roughly 10 times bigger. Why? The answer to the question is evolution. According to the currently accepted theories of evolution, from the primordial soup we got the first living cells, who "soon" (millions of years) became prokaryotic bacteria (appeared at least 3.5 billion years ago). At that time, it is thought the earth had a reduced atmosphere (i.e. no -- or very little -- oxygen, atmospheric composition: CO, CO2, N2, H2O, H2, CH4, NH3) and relatively harsh conditions of living (strong UV radiation and volcanic activity, possibly meteoric activity). Bacteria at that time are thought to have used RNA for replication and had no internal division of specialized organelles. From this original bacteria two branches evolved: the eubacteria (today's common prokaryotes) and a common ancestor of archaebacteria (love extreme environments: thermophilic, methanogenic and halophilic bacteria) and eukaryotes (protists, fungi, plants and animals). About 3.4 billion years ago, photosyntesis (producing O2 by using solar energy) started. Oxygen became a major component of the atmosphere. However, to anaerobic bacteria, oxygen is poisonous. Bacteria had to adapt or die. Purple bacteria evolved a system of reversing the electron flow of molecules through their carbon fixing pathways and modifying their electron transport chains. Cyanobacteria (thought to have evolved from purple bacteria) converted carbon dioxide (CO2) and water (H2O) into glucose, releasing O2 as a pathway biproduct. Given the changing nature of the environment and multiple living organisms competing for resources, natural selection favored organisms that could perform an increasing number of metabolic ativities. The small size and simple internal structure of the original prokaryotic cell impose a number of limits on the number and type of metabolic activities that can be performed. Size allows for bigger genomes (can encode more enzymes for more metabolic activity) and possibility of engulfing smaller organisms (digesting more material, not just small molecules). However, metabolic requirements also impose upper limits on the size of a cell. Increased volume decreases the surface area to volume ratio. Since the transport of molecules across the plasma membrane stays constant, the volume fed by the same surface area is greater. Hence, the cytoplasm is poorer in absorbed molecules per unit of volume, even though the surface area has been increased. Evolution dealt with the size to metabolic activity problem in 3 different ways: 1) becoming multicellular, where different cell type specialize in different activities (filaments of some cyanobacteria). 2) aggregating into communities, where each species benefits from the others' specialities 3) compartmentalizing of different functions within a single cell. The last solution is the evolutionary path the eukaryotes took. According to the endosymbiotic theory, it is thought that the prokaryotic ancestor of today's eukaryotes started invaginating its plasma membrane and creating structures we know today as the Golgi, the endoplasmic reticulum, the nuclear envelope... It also engulfed smaller prokaryotes, the purple bacteria and cyanobacteria, which became the mitochondria and the chloroplasts, respectively. Supporting this evidence, is the mode of replication of mitochondria and chloroplasts (similar to binary fission), as well as RNA analogy to prokaryotic RNA. The eukaryotic evolution allowed not only for more complex metabolic activity, but also for better methods of replication, required for handling an increased genome size (mitosis and meiosis to reproduce large genomes, DNA compacted into strands associated with histones etc.). The larger genome was required in order to synthetize all the enzymes needed for the extra metabolic activities to be performed. Thus, to summarize, the prokaryotes are generally thought to be the original life organisms. Due to evolutionary pressure, bigger and more sophisticated bacteria developed, and then further evolved into today's eukaryotes. References: Campbell's Biology, '96.
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