Musings: Internal membranes

What are they? (September 12, 2009) -- follow-up. The post presented two pictures, and asked you what they are. The pictures are repeated here, for your convenience.

Left Left

The picture on the left is a bacterium; the picture on the right is a mitochondrion.

The bacterium is a photosynthetic bacterium, of the genus Ectothiorhodospora. (As you might tell from that name, it is a purple bacterium that deposits sulfur outside the cell.) The mitochondrion is from human lung cells.

The common theme of the two pictures is internal membranes.

For many people, the most familiar internal membrane systems may be those in organelles inside the cells of higher (eukaryotic) organisms: the mitochondria that are found in (almost) all eukaryotes, and the chloroplasts of plant cells. Bacteria (prokaryotes) do not usually have internal organelles or extensive internal membranes. However, a few bacteria do. The example here is of a photosynthetic bacterium, where the internal membranes are serving the same purpose as in chloroplasts. In fact, it is now clear that plant chloroplasts derived from photosynthetic bacteria -- though not the type shown here. (The evidence for that is far more than the simple appearance; there is much evidence from the details of biochemistry and genetics.) Similarly, mitochondria derived from respiring bacteria. In all cases -- bacterial cells or the organelles of eukaryotes -- the internal membrane system allows a greater surface area of the membranes, which host complex biochemical reactions (photosynthesis or respiration).

These pictures came up during a casual discussion with Borislav, not with any attempt to develop a story of relationships. He started it, with the mitochondrion. I retaliated, with the bacterium -- and he realized the ambiguity. We decided they are neat pictures, worth sharing.

Return to original post: What are they? (September 12, 2009).

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