What is the difference between photo tactic and chemo tactic stimulus?

Answer 1

Taxis is an organism's instinctive movement either towards or away from a stimulant. Positive taxis occurs when an organism moves up a gradient, towards a stimulus. Negative taxis occurs when an organism moves down a gradient, away from a stimulus.

Phototaxis occurs when an organism moves either towards or away from a light source. (That is, up or down a light gradient). The phototactic stimulus is the light itself (regardless of whether the organism moves towards or away from it).

For example, a single-celled Euglena (a photosynthetic protist) exhibits positive phototaxis during the daytime: when exposed to sun, it moves from dim areas up the light gradient into bright sunshine. The phototactic stimulus is sunlight.

Chemotaxis occurs when an organism moves either towards or away from a chemical source. (That is, up or down a chemical gradient). Thus the chemotactic stimulus is the source of the chemical (again, regardless of whether the organism moves towards or away from it).

Interestingly, positive chemotaxis facilitates the process of fertilization in mammals. The egg secretes powerful chemical attractants, which the sperm swim towards enthusiastically as they make their way up the fallopian tube, like a hungry traveler following his nose to a bakery. For the sperm, the chemotactic stimulus is the chemical attractant secreted by the egg. (Check out this website for more info on this specifically: http://www.livescience.com/health/060519_sperm_attraction.html)