# Documentation

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## Create a Rate Rule for a Constant Rate of Change

This example shows how to increase the amount or concentration of a species by a constant value using the zero-order rate rule. For example, suppose species `x` increases by a constant rate `k`. The rate of change is:

Set the initial amount of species `x` to 2, and the value of parameter `k` to 1. Use the following commands to set up a SimBiology model accordingly and simulate it.

```m = sbiomodel('m'); c = addcompartment(m,'comp'); s = addspecies(m,'x','InitialAmount',2); p = addparameter(m,'k','Value',1); r = addrule(m,'x = k','RuleType','rate'); [t,sd,species] = sbiosimulate(m); plot(t,sd); legend(species) xlabel('Time'); ylabel('Species Amount');```

Alternatively, you could model a constant increase in a species using the Mass Action reaction `null` -> `x` with the forward rate constant `k`.

```clear m = sbiomodel('m'); c = addcompartment(m,'comp'); s = addspecies(m,'x','InitialAmount',2); r = addreaction(m,'null -> x'); kl = addkineticlaw(r,'MassAction'); p = addparameter(kl,'k','Value',1); kl.ParameterVariableNames = 'k'; [t,sd,species] = sbiosimulate(m); plot(t,sd); legend(species) xlabel('Time'); ylabel('Species Amount');```