What is the function of a postsynaptic membrane?

In a chemical synapse, the postsynaptic membrane is the membrane that receives a signal (binds neurotransmitter) from the presynaptic cell and responds via depolarisation or hyperpolarisation.

Why is binding on the postsynaptic membrane important?

The binding of neurotransmitters, either directly or indirectly, causes ion channels in the postsynaptic membrane to open or close (Figure 7.1). Typically, the resulting ion fluxes change the membrane potential of the postsynaptic cell, thus mediating the transfer of information across the synapse.