  Diffusive molecular communication (DMC) is envisioned as a promising approach
to help realize healthcare applications within bounded biological environments.
In this paper, a DMC system within a biological spherical environment is
considered, inspired by bounded biological sphere-like structures throughout
the body. As a biological environment, it is assumed that the inner surface of
the sphere's boundary is fully covered by biological receptors that may
irreversibly react with hitting molecules. Moreover, information molecules
diffusing in the sphere may undergo a degradation reaction and be transformed
to another molecule type. Concentration Green's function (CGF) of diffusion
inside this environment is analytically obtained in terms of a convergent
infinite series. By employing the obtained CGF, the information channel between
transmitter and transparent receiver of DMC in this environment is
characterized. Interestingly, it is revealed that the information channel is
reciprocal, i.e., interchanging the position of receiver and transmitter does
not change the information channel. Our results indicate that the conventional
simplifying assumption that the environment is unbounded may lead to an
inaccurate characterization in such biological environments.
