Seminar One
Biology, culture and psychology
Chloroplasts and mitochondria

Now for the stars of the show: the ultimate in mutualists, chloroplasts and mitochondria. The microscopic structures called chloroplasts are tiny organelles which conducts photosynthesis in plant cells. Chloroplasts produce the oxygen which makes up 20.9 percent of our atmosphere. Photosynthesis in plants is a complex series of reactions that use light energy to drive electron transfer from water to carbon dioxide to yield carbohydrates.

These two organelles bear undeniable fingerprints of their origin from bacteria. From bow to stern, they are bacteria in all details. But they have lost most of their genes. Where did the genes go? To the nucleus of their host cell, which has incorporated them in its own chromosomes and reads them out to synthesize the proteins needed by their mutualists.

The collaboration is an ancient truce which has undergone irreversible commitment. And it really works—witness all the animals and plants we know. Without chloroplasts plants could not harness the energy of sunlight and would not produce the oxygen that has transformed Earth’s atmosphere, and without mitochondria eukaryotes could not take a single breath of oxygen.

Lewis Thomas wrote a memorable essay on chloroplasts and mitochondria. These two organelles, he said: “…are, in a fundamental sense, the most important living things on earth. Between them they produce the oxygen and arrange for its use. In effect, they run the place.”

The tight symbiosis between these two organelles and their hosts is called “endosymbiosis.”  The “endo” prefix differentiates the symbiosis from those like lichens, in which the two life forms remain essentially separate from each other.  Endosymbiosis does not fit the Darwinian model of natural selection, as it represents a merger instead of an evolved single entity, unless you consider that natural selection is still working on the two merging entities as they become wedded together.  As with all other mutualisms, each entity is cooperating with the other only if it is selected to do so, i.e., if it benefits from the association and spreads more of its genes into the future as a result.

There is, however, a process that is non-Darwinian:  horizontal gene transfer.  This occurs between microbes such as viruses and bacteria, and is responsible for changes in the influenza A virus each year and for some cases of antibiotic resistance in medicine.  The term “horizontal” means movement directly from one living microbe to another, as opposed to “vertical” inheritance, the usual kind from parent to offspring. A “tree of life” must incorporate horizontal gene transfer throughout its history, mainly in microbes.  We used to think in terms of a “trunk” as the beginnings of the tree of life; now we think more in terms of a soup of microorganisms exchanging genes.