Explain why aliens invading earth are not likely to be giant cells the size of humans.

Those would have to be some pretty big cells.

Explanation:

Cell transport and other functions of the cell involving movement of fluids and enzymes throughout, especially in terms of waste disposal, heat dispersion, and energy transfer throughout the cell make it more than unreasonable and wholly impossible for a cell to get to that size, not to mention gravity near a celestial body on a cell larger than a microscopic scale would likely break apart, being they are not particularly strong on their own in their cell membranes.

dna damages cause changes in the structure of the genetic material and prevents the replication mechanism from functioning and performing properly. dna damage and mutation have different biological consequences. while most dna damages can undergo dna repair, such repair is not 100% efficient.

as the gila monster consumes the eggs or young of the desert cottontail and gambel's quail, thus its main food supply is only accessible for a restricted time duration every year. however, for this, the gila monsters exhibit three adaptations, which elevate their survival rate in the desert condition.  

first, is that they possess huge bodies, and can consume greater concentrations of food during a small period. secondly, they require to feed infrequently as they can accumulate fat in their tails. ultimately, their rate of metabolism is very low, so that they can burn their reservoirs of fat gradually.