Final answer:
Phototaxis is the directed and immediate movement of organisms towards or away from a light source, exemplified by certain bacteria and protozoa, which can be either positive or negative. In contrast, phototropism is the directional growth of plants in response to light, involving hormonal changes that cause the plant to bend towards the light source.
Step-by-step explanation:
Phototaxis is the directed movement of an organism towards or away from a source of light. This type of movement can either be positive phototaxis, where the organism moves towards the light, or negative phototaxis, where it moves away from the light. For instance, certain bacteria and small protozoans like Tetrahymena thermophila exhibit this behavior by swimming toward the light using their cilia in a positive phototactic response.
In contrast to phototaxis, phototropism is a growth orientation that occurs in plants as they grow towards or away from light, a process influenced by blue wavelengths of light. This response involves the hormone indole acetic acid (IAA), which accumulates on the shaded side of a stem, causing cells there to elongate more rapidly than on the illuminated side, thus bending the plant towards the light.
While phototropism involves growth towards or away from light over time, phototaxis is an immediate movement in response to light stimuli. Organisms that display phototactic behavior can quickly respond to changes in light intensity and direction, which is crucial for their survival and reproductive success.