Chemotaxis: Videos & Practice Problems

Chemotaxis is a fundamental biological process defined as the movement of a cell toward a chemoattractant and away from chemorepellents. A chemoattractant is a chemical substance that attracts motile cells, leading to what is known as positive chemotaxis. Conversely, a chemorepellant is a chemical that repels motile cells, resulting in negative chemotaxis as cells move away from these substances.

In addition to chemotaxis, there is a specific type called phototaxis, which refers to the movement of cells toward or away from light. Positive phototaxis occurs when cells move toward light, while negative phototaxis is when they move away from it. This distinction highlights the various stimuli that can influence cellular movement.

When examining the movement patterns of motile cells, it is important to note that their paths are not linear. Instead, cells navigate their environment through a series of movements known as runs and tumbles. This behavior prevents them from moving in a straight line for extended periods, allowing for more dynamic responses to their surroundings.

Understanding chemotaxis and its mechanisms is crucial for comprehending how cells interact with their environment, which can have significant implications in fields such as microbiology and cellular biology.

Cell motility during chemotaxis is a fascinating process where motile cells, such as bacteria, utilize their flagella to navigate their environment through a mechanism known as "run and tumble." This method involves alternating between smooth movements (runs) and directional changes (tumbles). When a cell detects a chemoattractant, it moves toward areas of higher concentration, demonstrating its ability to sense and respond to environmental changes.

The sensing of concentration changes is facilitated by complex cellular mechanisms. As the concentration of the chemoattractant decreases, the length of the runs increases, allowing the cell to explore its surroundings more extensively. Conversely, when the concentration is higher, the runs become shorter, indicating that the cell is approaching the source of the attractant. This behavior is crucial for effective navigation toward favorable conditions.

For instance, in the absence of a chemoattractant, a paratrichous bacterium exhibits a pattern of movement characterized by numerous runs and tumbles, resulting in minimal net displacement. The starting and ending positions are nearly identical, indicating no significant progress. However, in the presence of a chemoattractant, the bacterium's movement changes dramatically. The runs become longer when the attractant concentration is low, and as it approaches higher concentrations, the runs shorten, leading to a clear net movement toward the attractant.

In contrast, when a cell encounters a chemorepellant, the opposite effect occurs, prompting the cell to move away from the repellent source. This intricate balance of runs and tumbles allows bacteria to navigate their environment effectively, ensuring they can locate nutrients or avoid harmful substances.

Understanding these mechanisms of cell motility during chemotaxis not only highlights the adaptability of microorganisms but also provides insights into broader biological processes and potential applications in fields such as microbiology and biotechnology.