Phototaxis is a kind of taxis
A taxis is an innate behavioral response by an organism to a directional stimulus or gradient of stimulus intensity. A taxis differs from a tropism in that the organism has motility and demonstrates guided movement towards or away from the stimulus source ...

, or locomotory movement, that occurs when a whole organism moves in response to the stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

 of light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite.

Two types of positive phototaxis are observed in prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s. The first is called scotophobotaxis (from the word "scotophobia"), which is observed only under a microscope. This occurs when a bacterium swims by chance out of the area illuminated by the microscope. Entering darkness signals the cell to reverse flagella
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

 rotation direction and reenter the light. The second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. This is analogous to positive chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...

 except that the attractant is light rather than a chemical.

Phototactic responses are observed in many organisms such as Serratia marcescens
Serratia marcescens
Serratia marcescens is a species of Gram-negative, rod-shaped bacterium in the family Enterobacteriaceae. A human pathogen, S. marcescens is involved in nosocomial infections, particularly catheter-associated bacteremia, urinary tract infections and wound infections, and is responsible for 1.4% of...

, Tetrahymena
Tetrahymena are free-living ciliate protozoa that can also switch from commensalistic to pathogenic modes of survival. They are common in fresh-water. Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.- T...

, and Euglena
Euglena is a genus of unicellular flagellate protists. It is the best known and most widely studied member of the phylum Euglenozoa , a diverse group containing some 44 genera and at least 800 species. Species of Euglena are found in fresh and salt waters...

. Each organism has its own specific biological cause for a phototactic response, many of which are unintended and serve no end purpose.

Further readings

  • Madigan, Michael and John Martinko. Brock Biology of Microorganisms. Prentice Hall 11th Edition.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.