The molecule that serves as the major source of readily available fuel for neurons and blood cells is glucose.

Here's why:

* Neurons: Neurons have a high energy demand due to their constant communication and signaling functions. Glucose is the primary fuel source for these processes.

* Blood cells: Red blood cells, in particular, rely heavily on glucose for energy production. They lack mitochondria and therefore cannot use other fuel sources like fatty acids.

While other energy sources like fatty acids and ketone bodies can be used in certain situations, glucose remains the preferred and most efficient fuel source for neurons and blood cells.