In heme synthesis in mammalian cells: 1. uroporphyrin III is the immediate precursor of coproporhyrin III. 2. delta-aminolevulinic acid is formed by the condensation of succinyl CoA and valine. 3. ferric iron is inserted into protoporphyrin IX prior to attachment to the globin molecule.
If the linear function f is given, the linear equation of the graph of this function is obtained by defining the variable y to be the functional value f(x), that is, setting y = f(x) = ax + b and suppressing the functional notation in the middle.
In particular, a differential equation is linear if it is linear in terms of the unknown function and its derivatives, even if nonlinear in terms of the other variables appearing in it. As nonlinear equations are difficult to solve, nonlinear systems are commonly approximated by linear equations (linearization).