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Wednesday 21 May 2014

How complex cells arose on earth?

Prokaryotes were the first life on earth. There are fossil bacteria that are 3.8 billion years old, nearly as old as the oceans themselves. Being prokaryotes, these early cells did not have organized organelles enclosed by membranes. Prokaryotic life came on the scene very early in the earth’s history, but the next step took much longer. It was a billion years or more before complex cells with organelles, or eukaryotic cells, appeared. Strangely enough, this advance in cell design, shared by all multicellular life, probably started with a simple meal.

Biologists believe that mitochondria, the “powerhouses” of the cell, were once free-living bacteria. They are 1 to 3 microns (μm) long, about the same size as many bacteria. Like many bacteria, they contain layers of folded membranes. Although most of the cell’s DNA is contained in the nucleus, mitochondria have a small amount of their own DNA. This DNA is even in the form of a single, circular molecule like the chromosome of bacteria. When a cell needs new mitochondria it does not make them from scratch. Instead, the mitochondria within the cell divide in two, much as bacteria divide by cell division. Mitochondria resemble bacteria in several other respects.



Because of these similarities, biologists believe that mitochondria were originally bacteria that came to live inside other cells. The most likely scenario is that relatively large prokaryotic cells that ate smaller ones were unable to digest all of their food. It is also possible that the small cells were disease bacteria that invaded the larger cell but were unable to kill it. Both phenomena have been observed in the laboratory in living cells. Either way, the small cells eventually took up permanent residence in the larger cells, using them as hosts. The living together of different kinds of organisms like this is called symbiosis.

According to this theory, the host cells could not use oxygen in respiration. The smaller, symbiotic, bacteria did use oxygen and gave this ability to their hosts. This gave the host cells an advantage over cells that didn't have symbiotic bacteria.

Over the course of time, the host cells were able to transfer most of the symbiotic bacteria’s genes into their own nuclei, and thus came to control the symbionts. The bacteria and host cells became more and more dependent on each other. Eventually the bacteria became mitochondria.



Chloroplasts, the organelles that perform photosynthesis, are also thought to have arisen from symbiotic bacteria. Like mitochondria, they are about the right size, have a circular DNA molecule, and can reproduce themselves. Chloroplasts also contain folded membranes very similar to those of photosynthetic bacteria that we see today. In fact, biologists have found a possible “missing link” between photosynthetic bacteria and chloroplasts. Some sea squirts and other invertebrates contain symbiotic, photosynthetic bacteria called Prochloron that are similar to the chloroplasts of green algae and higher plants. It is thought that these chloroplasts arose from Prochloron-like bacteria when host cells ate the bacteria, some of which managed to resist being digested and became symbiotic.

The hypothesis that mitochondria and chloroplasts arose from bacteria, once highly controversial, is now almost universally accepted by biologists. Some biologists now even classify mitochondria and chloroplasts as symbiotic bacteria rather than organelles. Several other types of organelles may have similar origins. All multicellular organisms, including humans, carry in their very cells the legacy of these bacterial partnerships, probably the result of an ancient snack.

Complex cell

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