Answer:
The substrate molecules are entering the cells, so no product is made yet.
There is sufficient substrate, so the reaction is proceeding at a maximum rate
As the substrate is used up, the rate decreases (the slope is less steep)
The line is flat because no new substrate remains and thus no new product appears
Step-by-step explanation:
First the substrate molecules enters in the cell and no product is produced because the reaction is not started yet. When the reaction starts, the substrate is in large amount while on the other hand, the amount of product is low. As the reaction proceed forward, concentration of substrate is lowering and concentration of product is increasing. When all the substrate used up, so no new product is formed.