Why are top-level predators like lions and eagles so rare in a community?

This is because the transfer of energy from one trophic level to another is very inefficient. This is why the energy illustration of an ecosystem is more or less a pyramid structure. Only approximately 10% of biomass energy consumed  by one level become biomass in the upper trophic level. The rest is lost as heat energy. This means the higher up the trophic level the fewer the organisms will be.  

b tumor cells have a cell wall instead of a membrane

explanation:

tumor cells different from normal cells because tumor cells have a cell wall instead of a membrane.