Muscles do the work for your body. You eat food and, somehow, it turns into energy that your muscles burn to move. But how? How does this thing called metabolism work?
The process of metabolism in human bodies is so important to our understanding of human anatomy that three Nobel prizes have been given to people who contributed to our understanding of it. The third was given to Hans Adolf Krebs, who finally solved the mystery and discovered how our bodies metabolize food into energy. It was one of the great medical discoveries of the twentieth century.
British physiologist Archibald Hill believed that muscles should produce heat when they contracted. By 1913, he had developed ways to measure changes as small as 3/1000 of a degree. To his surprise he discovered that, during muscle contraction, no heat was produced nor was any oxygen consumed.
Five years later, German Otto Meyerhof discovered that, during muscle contraction, the chemical glycogen disappears and lactic acid appears. He named this process anaerobic, from the Greek words meaning “without oxygen.” He later discovered that oxygen was used in the muscle cells later to break down lactic acid. Other researchers found that when they added any of four different carbon-based acids to muscle tissue slices, it stimulated the tissue to absorb oxygen.
Even though these discoveries seemed important, they created as much confusion about the process of how muscles work as they provided answers. Someone had to make sense of these different, seemingly confusing studies.
Hans Adolf Krebs was born in 1900 in Germany, the son of a surgeon. He studied chemistry and medicine and was then hired to conduct research at Cambridge University. He focused this research on the chemical process of muscle metabolism.
Beginning in 1937, Krebs studied pigeon liver and breast muscle tissue. He was able to measure the amounts of certain groups of acids, some that contain four carbon atoms each and other groups of acids that contain six carbon atoms each, that were created when sugars are oxidized (combined with oxygen). He also noted that this process created carbon dioxide, water, and energy.
These results were confusing. What did all these chemicals have to do with simple metabolism of sugars into energy? Krebs saw citric acid being broken down and yet at the same time, citric acid was being produced. The same was true for a number of other acids.
Slowly Krebs realized that the process worked in a circle, a circle with seven separate chemical steps. It started with citric acid. Each step produced the chemicals and acids that were needed for the next step in the cycle. In the last step, citric acid was produced to start the cycle all over again.
The cycle continues endlessly in each of our cells. Along the way, glucose molecules (sugars) supplied by the blood are consumed. Two waste products were produced by this seven-step cycle: carbon dioxide and free hydrogen atoms. These hydrogen atoms then combine with oxygen and a form of high-energy phosphate to create water and ATP, the chemical that stores energy for cells just like a battery.
Sugar molecules enter the cycle, and carbon dioxide, water, and ATP to power the cells exit the cycle.
By 1938 Krebs had unraveled this amazingly complex and yet amazingly efficient seven-step chemical cycle, specifically designed to accomplish a seemingly simple task: convert sugars in the blood into energy for muscle cells. Amazingly, each muscle cell in our bodies creates these seven sequential reactions, each sparked by a different enzyme, every minute of every day. And Hans Krebs discovered how it works.
The average person’s body could theoretically generate 100 watts of electricity using a bio-nano generator, a nano-scale electrochemical fuel cell that draws power from blood glucose much the same way the body generates energy using the Krebs Cycle.
