We just had an experiment to check enzyme reaction rate according to it's substrate concentration.
In the experiment, we used a varying amount of substrate and the same amount of enzyme:
(1.5mm enz, 4.0mm water, 0.5mm subs)
(1.5mm enz, 3.5mm water, 1.0mm subs)
(1.5mm enz, 2.5mm water, 2.0mm subs)
(1.5mm enz, 1.5mm water, 3.0mm subs)
(1.5mm enz, 0mm water, 4.5mm subs)
We check the rate by looking at the optical density after 3 minutes.
Upon staring at this, I couldn't help but wonder: We don't only change the substrate concentration! We also change it's amount!
While 3.0/4.5 is a higher concentration than 0.5/4.5, it's also a higher amount resulting in longer reaction time for sure. If we check after 3 minutes, there's a chance that the 0.5mm subs would already be gone.
If we really wished to look at the rate according to concentration, shouldn't we have only changed the amount of water? It also changes the concentration of the enzyme but there is there any way to isolate either of them?