Cells function best at the body temperature. As the temperature increases, the rate of reaction increases until the optimal temperature is reached. This is the point at which enzyme activity is at its maximum level. Once this is reached the rate of reaction, slows down until it stops all together for the enzyme to function. The heat causes the enzyme to denature, the enzyme changes in structure and shape of the molecule due to excessive heat.
Each enzyme has its own narrow range of pH within which it functions most efficiently. Extreme levels of pH cause the enzyme to denature and becoming ineffective.