Please see the image. The phases are as follows: A, The substrate is entering the cells and no reaction can occur yet; B, The substrate is inside cells and abundant, leading to peak rate reactions; C, The substrate is becoming exhausted, slowing the rate of reaction; D, The substrate is used up, so no more reactions can occur.
Work Step by Step
A key here is to recall that the substrate is added to the dish of cells at the start of the experiment and that the enzymes are in the cells, requiring the substrate to cross the cells' plasma membranes. Then consider how the substrate will be used up over time to get the phases B-D.