Because you cannot test it or observe it.
A scientific theory are created through the scientific method, in the scientific method you need to be able observe it and experiment.
The answer is a population.
You measure temperature with a barometer
Cell membrane, Organells, flagella
Enzymes can be denatured by irregular PH. Meaning that they can change shape. Enzymes rely on their shape in order to function so if their shapes change because of the PH then the active sites don't fit anymore and they enzymes wont work.
