Part of the retina of a young rat was removed and kept in the dark for two hours. This allowed the pigment in the rod cells to recover from bleaching caused by exposure to light. Suggest what happens in the rod cells during this two hours of darkness.

Rhodopsin is the pigment found in rod cells, which consists of cis-retinal and the protein opsin. When the rod cells are kept in darkness, the opsin is uncoupled from the cell surface membrane of the rod cells and the trans-retinal is converted back to cis-retinal. These 2 components join to reform rhodopsin, resulting in a process known as dark adaptation. The permeability of the rod cell to sodium ions increases as the sodium channels become unblocked, decreasing the hyperpolarisation within the cell. This leads to the release of greater quantities of neurotransmitter (glutamate).