Chemical Potential Energy
We have learned that when you jump, bend a paper clip, or lift an object you transfer kinetic energy, potential energy, or thermal energy to the objects, but where did that energy come from and what form was it in before? Plants use photosynthesis to convert electromagnetic energy in sunlight to chemical potential energy into organic molecules in the food we eat. During cellular respiration, organic molecules are oxidized with the release of carbon dioxide, water, and energy used to form ATP molecules. The body uses the molecule ATP to power cellular functions, including muscle contraction. To learn more about these processes consider taking courses in human anatomy and physiology, general biology, cell biology, molecular biology, and biochemistry. The release of chemical potential energy during the ATP cycle is shown in the following animation:
Everyday Example: No work and all heat
Hold an object up in the air. Keep holding. Do you eventually get tired? Why? You are certainly applying a force, but the object hasn’t moved any distance, so it would appear that you really done anywork. Why should you get tired when you aren’t doing any work? The animation in the previous video provides the answer, you haven’t done any useful work, but you have work on a microscopic scale to transfer chemical potential energy to thermal energy. The ATP cycle occurs repeatedly just maintain muscle tension, even if the muscle does not actually move a noticeable distance. The ATP cycle continues to use up potential energy even if you aren’t doing any useful work. Where does that energy go? Into thermal energy If you hold the object long enough, you might even begin to sweat! If using stored energy without doing useful work seems pretty inefficient, you’re right. In fact the efficiency of the body in such a situation is zero!
The digestive and metabolic process is essentially oxidation of food so it requires oxygen just like oxidation of fuel in an engine requires oxygen. Therefore, we can determine the the actual chemical potential energy consumed during different activities by measuring oxygen use. The following table shows the oxygen and corresponding energy consumption rates for various activities.
|Energy consumption in watts
|Oxygen consumption in liters O2/min
|Sitting at rest
|Sitting in class
|Walking (5 km/h)
|Cycling (13–18 km/h)
|Ice skating (14.5 km/h)
|Climbing stairs (116/min)
|Cycling (21 km/h)
|Cycling, professional racer
- OpenStax College Biology. OpenStax CNX. http://email@example.com ↵
- OpenStax College Physics. OpenStax CNX. https://openstax.org/books/college-physics/pages/7-6-conservation-of-energy ↵
- "Energy in Natural Processes and Human Consuptions" by ENVIR215 Earth, Air, Water: The Human Context, University of Washington School of Oceanography ↵
kinetic energy stored in the motion of microscopic particles
any interaction that causes objects with mass to change speed and/or direction of motion, except when balanced by other forces. We experience forces as pushes and pulls.
A quantity representing the effect of applying a force to an object or system while it moves some distance.
work done by the body to transfer chemical potential energy to mechanical energy (kinetic energy, gravitational potential energy, elastic potential energy)
energy stored in the bonds between atoms and molecules
ratio of work done to output energy in the desired to energy input required to do that work