Even though the cells in our body rely on glucose as a source of energy, they can replace glucose's energy with lipids or proteins' energy. In the brain's case, this is not possible since the blood-brain barrier, which is a highly selective barrier between the blood and the extracellular fluid in the central nervous system, allows the passage of glucose as the only source of energy. This is why maintaining the correct levels of glucose is crucial for the metabolism in the brain.
This is because the basketball is the bigger object here so that should represent something bigger like the solar system. Then the tennis ball would represent earth because it is the smaller object here.
