Final answer:
Taxis is the directed movement towards or away from a stimulus. Positive taxis is when an organism moves towards the stimulus, while negative taxis is when it moves away from it. Phototaxis is the movement in response to light, where organisms can move towards or away from a light source. Chemotaxis is the movement in response to chemicals, where organisms can move towards or away from a chemical source.
Step-by-step explanation:
Taxis is the directed movement towards or away from a stimulus. It can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis). Positive taxis is when an organism moves towards the stimulus, while negative taxis is when it moves away from it.
Phototaxis is the movement in response to light, where organisms can move towards or away from a light source. Chemotaxis is the movement in response to chemicals, where organisms can move towards or away from a chemical source. For example, the unicellular protozoan Tetrahymena thermophila exhibits positive chemotaxis by moving towards a chemical attractant using its cilia.