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How did we discover the functions of the mitochondria?

We know that mitochondria are energy factories, but how did scientists figure that out? Some of the energy that mitochondria make produces heat, much like how a burning match produces heat. Burning matches consume oxygen.  The human body also consumes oxygen when it generates energy. So, scientists measured oxygen consumption in pure preparations of mitochondria and found that they consume much more oxygen and produce more heat than other organelles.

Do you know why the body needs oxygen?

In non-living systems, oxygen can release the trapped energy of chemical bonds (as in wood and paper, for example).  Of course the energy released in burning wood is "lost" as light and heat.  In a living system, oxygen does a similar thing, helping to release the stored chemical bond energy of foods.  Except a living system cannot afford to waste too much of the energy as heat.

Self-quiz

Scientists first discovered that mitochondria were "energy factories" by measuring their oxygen consumption and ______ .

A. glucose
B. membrane surface
C. size
D. heat

How do scientists find out how organisms capture energy?

Scientists knew that the energy release in mitochondria needed to be captured temporarily in some kind of molecule. By testing the chemicals in mitochondria, they found one that could take up or give up large amounts of energy. The molecule that traps and stores energy is known as adenosine triphosphate (ATP). Our body heat comes from the left over food energy that is not captured in ATP. 

When scientists measured oxygen consumption and the build-up of ATP inside of cells, they found two sets of reactions:

  • One set of reactions uses no oxygen and captures only a few molecules of ATP.
     
  • The other set uses lots of oxygen and captures a great deal of energy by producing many molecules  of ATP. 

The first set of reactions breaks glucose into a series of three-carbon molecules. This first step is called glycolysis. The second set, where oxygen is consumed, releases water and carbon dioxide as waste products. These are the reactions that our Story-Time hero discovered and which are named after him as the "Krebs' cycle."

Self-Quiz

Identify the pair that is correct:

A. glycolysis--little energy produced
B. glycolysis--many ATP molecules
C. Krebs cycle--little  energy
D. Krebs cycle--few ATP molecules

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