Mitochondria

Glycolysis takes place in the main body of the cell, the cytoplasm. In the rest of aerobic respiration, the scene shifts to membrane enclosed cellular compartments known as mitochondria. These organelles are often described as the cells power plants because they produce most of our ATP, and therefore, most of our biochemically available free energy. The truth, though, is stranger than that power plant analogy suggests. Mitochondria have their own internal structure. Their own protein making machinery, and even their own genome, separate from the nuclear genome. They are the descendants of a remarkable symbiotic event that happened about two billion years ago, at least a billion years after the glycolytic enzymes had evolved.

A primitive unicellular organism engulfed a much smaller bacterium, but instead of being digested, the bacterium lived on, cooperating with its captor. In return for food, it would produce energy. If mitochondria are the cell’s power plants, then they are the product of an alien technology like dilithium crystals in Star Trek. The oxygen dependent reaction of aerobic respiration all take place in the mitochondria. First, three carbon pyruvate molecules are imported from the cytoplasm and stripped of high energy electrons, which are stored in the form of an NADH and FADH2. As pyruvate is oxidised, its carbon skeleton falls apart and carbon dioxide is formed. Second, the electronic energy is captured in ATP during the process of oxidative phosphorylation.

It is in this second stage that most of our body’s ATP molecules are made in what is known as the electron transport chain, as electrons flow from organic molecules to gaseous oxygen. Mitochondria can handle these high energy oxygen reactions in a way that the rest of our cellular machinery never evolved to do. Mitochondrial symbiosis happened around the same time that oxygen was first building up in the Earth’s atmosphere. The reactivity of oxygen presented both a threat and an opportunity to life as it still does. Gaseous oxygen allowed life access to plentiful energy, but it needed specialised machinery to deal with it, and that is what our imported mitochondria still provide.

One of the most important features of mitochondria is that they have two sets of membranes, an outer membrane and an inner membrane. The space between the membranes is called the intermembrane space, and the internal compartment is known as the matrix. As we will see, the enzymes of the matrix and those embedded in the inner mitochondrial membrane are crucial to respiration.

The remarkable story of the symbiotic mitochondria was accepted in the 1980s, due largely to the efforts of an American biologist called Lynn Margulis. She had been championing the importance of symbiosis in evolution since the 1960s, despite the scepticism of most of her peers. Genetic techniques proved her right in the end, and she is remembered as one of the most important and tenacious biologists of the 20th century.