Final answer:
Chemotaxis is the directed, movement of a living structure in response to a chemical stimulus, such as sperm moving towards an egg or bacteria moving towards nutrients.
Step-by-step explanation:
Chemotaxis is the directional movement in response to a graded chemical stimulus. This biological phenomenon is observed in various organisms, including simple unicellular entities like bacteria, and complex multicellular organisms. It's a type of taxis, where the movement of a living structure is guided by chemical signals. For instance, sperm cells exhibit a positive chemotaxis towards the chemical signals released by an egg. Similarly, the unicellular protozoan Tetrahymena thermophila demonstrates positive chemotaxis by swimming towards increasing concentrations of a chemical attractant, altering its turns and swimming in a more directed manner towards the source.
Organisms exhibit responses to several other stimuli as well. Plants can show movements such as bending towards light or climbing on surfaces, which are responses to different types of stimuli other than chemicals. Overall, the movement towards a stimulus is known as a positive response, while movement away is known as a negative response.