When is a control variable not needed to make an experiment valid?

For example, if two enzymes' activities were being measured by seeing how much O2 gas was produced over time, would a control of no enzyme added to substrate be needed to ensure it was the enzymes having the effect and not a variable e.g. temperature?
Update: In reply to your answer, but wouldn't a control allow me to validate that it is the enzyme having the effect and not another variable?
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