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Specify variable or constant species amount


The ConstantAmount property indicates whether the quantity of the species object can vary during the simulation. ConstantAmount can be either true or false. If ConstantAmount is true, the quantity of the species cannot vary during the simulation. By default, ConstantAmount is false and the quantity of the species can vary during the simulation. If ConstantAmount is false, the quantity of the species can be determined by reactions and rules.

The property ConstantAmount is for species objects; the property ConstantValue is for parameter objects.


When you want the species to participate in a reaction, but do not want any reactions to modify its quantity, set its BoundaryCondition to true, and ConstantAmount to false.

More Information

The following is an example of modeling species as constant amounts:

Modeling the role of nucleotides (GTP, ATP, cAMP) and cofactors (Ca++, NAD+, coenzyme A). Consider the role of GTP in the activation of Ras by receptor tyrosine kinases.

Ras-GDP + GTP  -> Ras-GTP + GDP
Model GTP and GDP with constant amount set to true. In addition, you can set the BoundaryCondition of these species to true, thus making them boundary species.


Applies toObject: species
Data typeboolean
Data valuestrue or false. The default value is false.


  1. Create a model object named my_model.

    modelObj = sbiomodel ('my_model');
  2. Add a species object and verify that the ConstantAmount property setting is 'false' or 0.

    speciesObj = addspecies (modelObj, 'glucose');
    get (speciesObj, 'ConstantAmount')

    MATLAB® returns:

               ans =
  3. Set the constant amount to 'true' and verify.

    set (speciesObj, 'ConstantAmount', true);
    get (speciesObj, 'ConstantAmount')

    MATLAB returns:

    ans =