* Red blood cells lack mitochondria, the organelles responsible for cellular respiration in most cells. Therefore, they cannot utilize other energy sources like fats or proteins. They rely solely on glycolysis, a process that breaks down glucose to produce a small amount of ATP (adenosine triphosphate), the cell's primary energy currency.

* The brain has a high energy demand and primarily uses glucose as its fuel source. It can also utilize ketone bodies during prolonged fasting or starvation, but glucose is its preferred fuel.

While other tissues can utilize different energy sources, red blood cells and the brain are primarily dependent on glucose for their energy needs.