Center surround organization is a means by which the visual system is able to detect contrast, particularly in the use of edge detection. At the molecular level, it is a function of the connections of the center photoreceptor on the bipolar cell, the connections of the side photoreceptors on the horizontal cells, and the horizontal cells synapsing onto the center photoreceptor cell.
The important thing to note is that the photoreceptors always respond the same way to light / dark, regardless of whether they are part of an on-center / off-surround bipolar cell network or an off-center / on-surround bipolar cell network. The key point which differentiates the response to light or dark is at the level of the bipolar cell and what type of glutamate receptor is expressed.
For On-center, it is a metabotropic receptor which responds to glutamate by inhibiting the bipolar cell. For example, in dark conditions, the glutamate is released from the center photoreceptor and inhibits the bipolar cell, as expected in a On-center bipolar cell. Conversely, for an off-center, it is an ionotropic receptor which responds to glutamate by exciting the bipolar cell. Thus, in dark conditions, the glutamate is released from the center photoreceptor and excites the bipolar cell, as expected in an Off-center bipolar cell.