### Step 2: Reaction Input

There are several ways to define the set of reactions for the model. The fastest one is probably to type them into the reaction overview table. You find this reaction overview in the tree on the left under `Model->Biochemical->Reactions.`

(Click here to go to the reaction overview.)

To create a new reaction just click on the empty cell in the Reaction column and start typing. A reaction equation usually consists of three parts.

First comes the set of substrates. Several substrates can be combined with the "+" sign.

After the substrates you either write "->" if it is a irreversible reaction or "=" if it is a reversible reaction.

The third part consists of the set of products. Just as the substrates, the individual products are combined with the "+" character.

There are several things to watch out for when writing reaction equations. First the individual elements of the reaction equation have to be separated by whitespaces. Since it is perfectly valid to have the "+" sign as part of a species name, this is the only way for COPASI to know where one species name ends and the next one starts. If you actually plan to use whitespaces in the species names themselves, those whitespaces have to be escaped with the "\" character.

Examples of valid reaction equation would be `Gluose + Phosphate -> Glucose-6-phosphate` or `F16BP = F26BP`.

It is also possible to either omit the set of products or the set of substrates, but not both. If the reaction involves so called modifiers, they are added to the end of the reaction equation, separated by a semicolon. Individual mdofiers are separated by spaces (e.g. `A = B ; C D` ).

When you are finished with typing the reaction equation, hit the return key. Now you are in the next row and can type the next reaction.

All reactions are created with a reversible or irreversible mass-action kinetic with a kinetic constant of 0.1, all species have an initial concentration of 0.1 standard concentration units (the ones specified in the Model Settings dialog.) and the single compartment has a volume of 1 standard volume units.