Equations for zero order reactions

A new drug has been developed to treat erectile problems in men. The liver rapidly inactivates this drug, but the mechanism is unknown. To be active the drug has to have a blood concentration of at least 40 nM. After administration of the drug, its blood concentration is measured at certain time intervals and these are the data  

When concentration of drug versus time is plotted, a straight line is obtained, hence it must be a zero order reaction, k = 1.2 nM / min 50 min 83.3 min 

The receptor for a growth factor is produced in the cell with a rate constant of 24 pM per 12 minutes. Assuming that the starting concentration of the receptor is 20 pM and the cell becomes responsive to growth factors if the receptor concentration is at least 0.1 nM, calculate how long it takes to reach this level. How do you know that this is a zero order reaction  

Answer 40 minutes can tell by the unit of the rate constant 

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Plot of integral form of rate equation for zero-order reaction. 

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