3.15: Mitochondria are the cell’s energy converters.

Cars have it easy: we generally put only a single type of fuel in them, and it is exactly the same fuel every time. With human bodies it’s a different story. During some meals we put in meat and potatoes. Other times, fruit or bread or vegetables, popcorn and gummy bears, pizza and ice cream. Yet no matter what we eat, we expect our body to utilize the energy in these various foods to power all the reactions that make it possible for us to breathe, move, and think. The mitochondria are the organelles that make this possible (FIGURE 3-31).

Mitochondria (sing. mitochondrion) are the cell’s all-purpose energy converters, and they are present in nearly all plant cells, animal cells, and every other eukaryotic cell. Our mitochondria allow us to convert the energy contained in the chemical bonds of carbohydrates, fats, and proteins into carbon dioxide, water, and ATP. Cells use high-energy ATP molecules to fuel all their functions and activities. (ATP and how it works are described in detail in Chapter 4.) Because this energy conversion requires a significant amount of oxygen, mitochondria consume most of the oxygen used by each cell. In humans, for example, our mitochondria consume as much as 80% of the oxygen we breathe. Mitochondria give a significant return on this investment by producing about 90% of the energy our cells need to function.

As with so many aspects of our bodies, form follows function. Cells that are not very metabolically active, such as some fat storage cells in humans, have very few mitochondria. Other cells, such as muscle, liver, and sperm cells in animals and fast-growing root cells in plants—all of which have large energy requirements—are packed densely with mitochondria (FIGURE 3-32). Some of these cells have as many as 2,500 mitochondria! (See Section 3-16 for a description of an experimental approach to documenting how a cell’s structure can change when the cell’s function must change.)

To visualize the structure of a mitochondrion, imagine a plastic sandwich bag. Now take another, bigger plastic bag and stuff it inside the sandwich bag. That’s the structure of mitochondria: there is a smooth outer membrane and a scrunched-up inner membrane. This construction forms two separate compartments within the mitochondrion: a region outside the inner plastic bag (called the intermembrane space) and another region, called the mitochondrial matrix, inside the inner plastic bag. This bag-within-a-bag structure has important implications for energy conversion. And having a heavily folded inner membrane that is much larger than the outer membrane provides a huge amount of surface area on which to conduct chemical reactions. (We discuss the details of mitochondrial energy conversions and the role of mitochondrial structure in Chapter 4.)

As we learned in our earlier discussion of endosymbiosis, mitochondria may very well have existed, billions of years ago, as separate, single-celled, bacteria-like organisms. They are similar to bacteria in size and shape, and may have originated when symbiotic bacteria took up permanent residence within other cells. Perhaps the strongest evidence for this is that mitochondria have their own DNA (see Figure 3-31). Anywhere from 2 to 10 copies of its own little ring-shaped DNA are mixed in among the approximately 3,000 proteins in each mitochondrion. This DNA carries the instructions for making 13 important mitochondrial proteins necessary for metabolism and energy production.

We’re always taught that our mothers and fathers contribute equally to our genetic composition, but this isn’t quite true. The mitochondria in every one of your cells (and the DNA that comes with them) come from the mitochondria that were initially present in your mother’s egg, which, when fertilized by your father’s sperm, developed into you. In other words, all of your mitochondria are descended from your mother’s mitochondria. The tiny sperm contributes DNA, but no cytoplasm and, hence, no mitochondria.

Consequently, mitochondrial DNA is something that we inherit exclusively from our mothers. This is true not only in humans but in most multicellular eukaryotes. As the fertilized egg develops into a two-celled, then four-celled, embryo, the mitochondria split themselves by a process called fission, the same process of division and DNA duplication used by bacteria—so there are always a sufficient number of mitochondria for the newly produced cells. This similarity between mitochondria and bacteria is another characteristic that supports the theory that mitochondria were originally symbiotic bacteria.

Given the central role of mitochondria in converting the energy in food molecules into a form that is usable by cells, you won’t be surprised to learn that mitochondrial malfunctions can have serious consequences. Recent research has focused on possible links between defective mitochondria and diseases characterized by fatigue and muscle pain. It seems, for instance, that many cases of “exercise intolerance”—extreme fatigue or cramps after only slight exertion—may be related to defective mitochondrial DNA.