The upper temperature limit at which cells can function is determined by the point of

The correct answer is that the upper temperature limit at which cells can function is determined by the denaturation point of proteins. Proteins, one of the molecular building blocks of life (along with carbohydrates, fats, and nucleic acids) are the ones responsible in performing various cellular functions. These proteins are polypeptides folded properly to perform specific functions. In the event of high temperatures, these proteins unfold and lose their function thereby having the whole cell lose its function. 

The upper temperature limit at which cells can function is determined by the denaturation point of proteins. Proteins work at specific or optimum temperatures, these including enzymes that catalyze most reactions in the body, without which high temperatures denatures them distorting their secondary and tertiary structures, while low temperatures below the optimum temperatures deactivates the proteins. Therefore, at high temperatures and low temperatures, cells may not be in a position to carry out their activitities as many enzymes or proteins are either denatured or inactive.

Region b indicates the occipital lobe