# Module 6.3: Substrate

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The Substrate model shows the removal of a substrate's concentration
in an enzymatic reaction. This model is based on the differential
equation where

d[S]/dt = k2[ES] - k1[E][S].

S = substrate

E = enzyme

In this equation, the rate of change of removal of [E] is the same as
the rate of change of removal of [S]. E can be formed from ES by
forward and backward reactions, with a rate of change proportional to
ES.
In this model, ES, S, and E concentration are all represented by
stocks. S formation and S removal are flows going in and out of the S
concentration stock.

The S formation flow contains the term k2, the reverse reaction rate
of ES back to substrate S and enzyme E, multiplied by the ES
concentration.

The S removal flow contains the equation: k1 * E concentration * S
concentration. k1 is the reaction rate of substrate S in the presence
of enzyme E to intermediate enzyme-substrate complex ES.

E concentration, S concentration, ES concentration, k1, the removal
constant and k2, the formation constant, are all controlled by sliders
in this model. The graph shows S concentrations over time