# Units

## Fundamental Units

To describe a spatial model we need to define the following fundamental units:

• `amount` (e.g. Mole)

• `length` (e.g. metre)

• `time` (e.g. second)

Volume is not a fundamental unit. However, for user convenience we treat volume as a fundamental unit

• `volume` (e.g. litre)

This allows the use of units such as cm for length and mMol/mL for concentration, instead of the equivalent but less common mMol/cm^3.

## Derived Units

All quantities in the model have units that can be written as some combination of these fundamental units:

• species concentrations
• have units of `amount` / `volume`

• reactions inside a compartment
• describe the rate of change of the concentration of species

• have units of concentration / time, i.e. `amount` / `volume` / `time`

• reactions between two compartments
• describe that rate at which a unit amount of species crosses a unit area of the membrane

• where the membrane is the area where the two compartments touch each other

• have units of amount / membrane-area / time, i.e. `amount` / `length` / `length` / `time`

• diffusion constants
• have units of area / time, i.e. `length` * `length` / `time`

For more information see `units.pdf`