In which of the following cells is glycogen most concentrated?

Glycogen, a polysaccharide that serves as a form of energy storage in animals, is most concentrated in the liver and muscle cells. The liver plays a crucial role in maintaining blood glucose levels by storing glycogen and releasing glucose into the bloodstream as needed. This helps ensure a constant supply of energy for the body's needs. Muscle cells, on the other hand, store glycogen for use during physical activity, providing a quick source of energy for muscle contraction.

The liver contains significant amounts of glycogen, typically around 100 grams, which can be mobilized to regulate blood sugar levels. Muscle cells store larger amounts of glycogen as well, which can vary based on physical fitness and dietary carbohydrate intake but generally serves as an immediate energy reserve during exercise.

In other tissues like the brain, while glucose is the primary energy source, glycogen is present in much smaller amounts. Adipose tissue stores energy as fat, and cells in the heart and lungs are primarily focused on their specific functions rather than glycogen storage. Lastly, kidney and pancreatic cells are involved in filtration and hormone production, respectively, with minimal roles in glycogen storage.

Therefore, the liver and muscle are recognized as the primary sites for glycogen concentration and usage.