Mighty Mitochondria

I haven't always loved mitochondria. There was once a time, specifically the week before an exam on adenosine triphosphate (ATP) production, when I hated that tiny producer of cellular energy. It wasn't mitochondria's fault that the Krebs Cycle was such an elaborate process, but memorizing the steps involved in aerobic respiration was as tedious as watching plants photosynthesize. I knew that without mitochondria, multicellular organisms couldn't effectively produce energy, but spending that much school time learning how this was achieved seemed excessive.

Then one day, while casually browsing through the texts in my biology classroom, I came across a fact that forever changed the way I felt about this little organelle: mitochondria amount to approximately 10% of an adult human's body weight.¹ When I read this, I found it shocking, even astounding, that something one to ten microns small could amount to one tenth of my bulk. The sheer quantity involved in this calculation made me realize how unfair I had been to mitochondria, and as I revisited this compact wonder, I began to appreciate the practicality of its shape and the intricacy of its molecular alchemy.

The term mitochondrion comes from the Greek words mitos meaning "thread" and chondrion meaning "granule," seemingly how it appeared to German microbiologist Carl Benda who named the organelle in 1898. When they were first identified a few years earlier, they were referred to as "bioblasts" and proposed to be most basic unit of living matter.² In 1949, when respiratory enzymes were discovered in the mitochondria, they were identified as the "powerhouses" of the cell.³

Because of this role in energy production, it's frequently compared to a power plant, but I think transportation metaphors are equally fitting. To me, the cristae resemble the hairpin turns of a cliffside highway or a curvy road like San Francisco's Lombard Street. And the electron transport chain's transit of ions reminds me of a strange ferry service that shuttles charged passengers across a phospholipid river, dropping off parcels on the far shore with all types of tug boat, with the knowledge that the cargo's passage back to the cristae will generate a payload.

This imagery aside, I also find satisfaction in the efficient organization and distribution of mitochondria. The consistency of inhaling oxygen with our lungs so that at the cellular level it can be used to create energy and exhaling CO₂ as a waste for the same reason — it makes sense to me as a system to have organs and organelles working in tandem. Knowing this, I can easily appreciate that cells that require more energy, including those in the heart, liver, and muscle tissues, have increased levels of mitochondria, and that within these high-demand mitochondria, the inner membranes have more cristae folds.

I am even more intrigued by the fact that human mitochondria are inherited through the mother, and that all mitochondria have independent DNA from the nucleus and the ability to manufacture their own RNAs and proteins. The feature has allowed researchers to trace maternal genetic lineages in the hope of finding a Mitochondrial Eve, the most recent common ancestor from whom we all descended, the first Homo sapiens sapiens. Though earlier hypotheses about humankind proposed more separate and widely distributed evolutionary origins, maternal mitochondrial research has supported the Out-of-Africa, or Recent Single-Origin, Hypothesis that maintains humans evolved somewhat recently from a single African species, and later replaced other branches including the Neanderthals living in Europe and members of Homo erectus in Asia.

Mitochondria have their own DNA because they descended from a smaller order of proteobacteria, probably rickettsiales. According to the endosymbiotic theory, sometime about two billion years to around 3.5 billion years ago, prokaryotes accepted mitochondria as endosymbionts that live within the body in a symbiotic relationship. This event allowed for modern eukaryotes with numerous specialized organelles and eventually the emergence of multicellular organisms. Prokaryotes must have realized how valuable mitochondria are, and all these years later, I can't help but agree.

Image Credit: Nature Education and http://wikimedia.org

References:

1. Lane, N. Power, Sex, Suicide: Mitochondria and the Meaning of Life. Oxford: Oxford University Press, 2005.

2. Altmann, R. Die Elementarorganismen und ihre Beziehungen zu den Zellen (The Elementary Organisms and their Relations to the Cells). Leipzig: Verlag Viet & Company, 1894.

3. Kennedy, E.P. & Lehninger, A.L. Oxidation of fatty acids and tricarboxylic acid cycle intermediates by isolated rat liver mitochondria. Journal of Biological Chemistry. 179, 957–972 (1949).