  The physical limit with which a cell senses external ligand concentration
corresponds to the perfect absorber, where all ligand particles are absorbed
and overcounting of same ligand particles does not occur. Here we analyze how
the lateral diffusion of receptors on the cell membrane affects the accuracy of
sensing ligand concentration. Specifically, we connect our modeling to
neurotransmission in neural synapses where the diffusion of glutamate receptors
is already known to refresh synaptic connections. We find that receptor
diffusion indeed increases the accuracy of sensing for both the glutamate AMPA
and NDMA receptors, although the NMDA receptor is overall much noiser. We
propose that the difference in accuracy of sensing of the two receptors can be
linked to their different roles in neurotransmission. Specifically, the high
accuracy in sensing glutamate is essential for the AMPA receptor to start
membrane depolarization, while the NMDA receptor is believed to work in a
second stage as a coincidence detector, involved in long-term potentiation and
memory.
