When Paul Hopkinsberger swam the English Channel he had to train and he had to gain weight. What is it called when he or any other athlete runs out of the carbohydrates stored in the liver and the muscle and must start burning fat as their energy source?​

Not exactly sure this is right  .Bonking, or “hitting the wall” as it is also known, is a well-known phenomena among marathon and ultramarathon runners which occurs when your body runs out of sugar to burn. Sugar, stored in your liver and muscles bound into large chains called glycogen, is the prime fuel for a distance runner.

Explanation:

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.