The main purpose of photo shock is to protect the
cells DNA from being damaged due to high levels of ultraviolet
radiation. When the eyespot detects to intense a light the cell quickly
changes its direction by altering the way it swims. During normal
motility a Chlamydomonas cell swims with a breast stroke like motion but
after photoshock is initiated, as you can see above, the cell switches from a normal breast
stroke motion to movement more like that of a sperm and propels
itself in the reverse direction effectively keeping its DNA unchanged.