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Beautiful Science: The Body’s Energy Factories

There are roughly 37,000,000,000,000 (that’s 37 trillion) cells inside the human body, each one hungry for energy to carry out its duties of keeping us alive. Where does that energy come from? The obvious answer is through the food we eat. But that is only the first of many steps in the complex process of converting food fuel (namely, glucose) into energy usable by cells. The key to the other steps lies in “energy factories” within cells called mitochondria, and the remarkable work they do in synthesizing Adenosine Triphosphate (ATP), an organic compound that stores and delivers energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis.

To power a single cell requires a lot of ATP. The more complex a multicellular life form is, the more ATP is required. Humans, for example, requires about 2 milliwatts of energy per gram of body mass. This translates to a need for about 130 watts for a typical adult, which implies that we literally house more than a standard incandescent light bulb of 60 watts. To generate this much energy, we have our body’s mitochondria and its remarkable ability to synthesize ATP to thank.

Kidney-shaped organelles called mitochondria are present in the cells of all higher organisms, and play an extremely important role in producing the huge amount of energy in the form of ATP needed for cells to carry out their work. For this reason, mitochondria are often called the ‘powerhouse of the cell’.

In a nutshell, here are the steps involved in ATP synthesis. The sugars that food provides ultimately goes into tiny kidney-shaped structures called mitochondria. There, they are oxidized and broken up into protons. These sugar protons are in turn churned into ATP molecules by thousands of “rotors” situated on the membrane of each mitochondria called ATP synthase. ATP molecules are the crucially important molecules that does the work of storing the energy that powers all living cells. Making ATP is hard work; It takes the rotors of ATP synthase 150 revolutions of per second to churn out just three ATP molecules for every complete revolution. Therefore, to generate the 130 watts of energy a typical human adult needs requires a ton of work by the mitochondria which have to work without break to generate cellular energy all through our lives. The following chart gives a pictorial summary of this remarkable work of mitochondria and ATP synthesis.

The F1 motor of ATP synthase uses the power of rotational motion to build ATP, or when operating as a motor, it breaks down ATP to spin the axle the opposite direction. The synthesis of ATP requires several steps, including the binding of ADP and phosphate, the formation of the new phosphate-phosphate bond, and release of ATP. As the axle turns, it forces the motor into three different conformations (spatial arrangements) that assist these difficult steps. Two states are shown here. The one on the left shows a conformation that assists the binding of ADP, and the one on the right shows a conformation that has been forced open to release ATP. Notice how the oddly-shaped axle forces the change in conformation. Source: the educational portal of PDB (Protein Database), https://pdb101.rcsb.org/motm/72.

Watch: The Architecture and Catalytic Function of ATP Synthesis

A video on the architecture and catalytic function of ATP synthase, presented by Nobel laureate, John Walker (b. 1941) who, with Paul Boyer (1918-2018). was awarded the 1999 Nobel Prize in Physiology or Medicine for their elucidation of the enzymatic mechanism underlying the synthesis of ATP.


Further study:

Website of John Walker: http://www.mrc-mbu.cam.ac.uk/people/john-walker

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