( It ( ( begins ( when ( ( ( a photon ) ( of light ) ) ( meets ( one ( of ( ( the ( photoreceptor cells ) ) ( of ( ( the retina ) ( -LRB- ( ( either ( ( ( a rod ) or ) ( a ( cone cell ) ) ) ) -RRB- ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ begins) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT a) (NN photon)) (PP (IN of) (NP (NN light)))) (VP (VBZ meets) (NP (NP (CD one)) (PP (IN of) (NP (NP (DT the) (NN photoreceptor) (NNS cells)) (PP (IN of) (NP (NP (DT the) (NN retina)) (PRN (-LRB- -LRB-) (PP (CC either) (NP (NP (DT a) (NN rod)) (CC or) (NP (DT a) (NN cone) (NN cell)))) (-RRB- -RRB-))))))))))) (. .)))
It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell).
(ROOT (S (NP (NP (DT The) (JJ light-sensing) (NNS cells)) (PP (IN in) (NP (DT the) (NN retina)))) (VP (VBP are) (VP (VBN called) (NP (NNS rods) (CC and) (NNS cones)))) (. .)))