The primary factor that affects enzyme activity is temperature. Enzymes are proteins that catalyze biochemical reactions, and their activity is highly sensitive to thermal conditions. Each enzyme has an optimal temperature range where it functions most efficiently. Beyond this range, either low temperatures can reduce activity by slowing molecular movement, or high temperatures can denature enzymes, disrupting their structure and function. Other factors, such as pH and substrate concentration, also influence enzyme activity, but temperature stands out as a key determinant that directly impacts metabolic processes within living organisms.