BIOMOLECULES AND ENZYMES

Detailed explanation-1: -The ability of organisms to respond to light is called phototaxis. Motile algae such as Euglena, Chlamydomonas and Volvox show phototaxis.

Detailed explanation-2: -Motile microorganisms such as the green Euglena gracilis use a number of external stimuli to orient in their environment. They respond to light with photophobic responses, photokinesis and phototaxis, all of which can result in accumulations of the organisms in suitable habitats.

Detailed explanation-3: -Euglena rotates about its long axis as it swims, and thus in the presence of light from one side the photoreceptor will be periodically shaded by the eyespot. It has been suggested1 that this shading causes a succession of phobic responses (shock reactions) which act to point the organism towards the light source.

Detailed explanation-4: -Uniquely, Euglena exhibits a positive phototactic response in daylight, but remains unresponsive at night, regardless of a light source.

Detailed explanation-5: -Phototactic orientation is mediated by inverse photophobic reactions which occur when the shadow of the stigma periodically falls upon the photoreceptor proper. Euglena also exhibits shock reactions when an already high light intensity is increased further (direct photophobic responses).