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Chemotaxis Definition
The ability of somatic cells, bacteria, other single-celled organisms and multicellular organisms to move in a particular direction in response to a chemical stimulus is known as chemotaxis. In bacteria, this phenomenon is important for finding food or running away from poisonous substances. Similarly, in eukaryotic cells, this phenomenon helps in early development, as well as in regular growth and function.
If the movement of the organism occurs in the direction of a higher concentration of the chemical (in the direction of an attractant), it is referred to as positive chemotaxis. However, if the movement occurs in the opposite direction, it is called negative chemotaxis.
Bacterial Chemotaxis
Biased Random Walk
Flagellated bacteria can rotate in two directions:
- When the bacteria rotates in a counterclockwise direction, the flagella get arranged in a single rotating bundle, thus allowing the bacteria to swim freely in a straight line.
- When the bacteria rotates in a clockwise direction, each flagella points in a different direction, causing the bacteria to tumble in between their normal path.
Thus, bacteria move in a combination of swim and tumble movement, known as the run-and-tumble motion. The biased random walk is, in simple words, a choice of the bacteria to either swim or tumble. A bacteria such as E.coli often forgets its direction of movement and gets reoriented.
When moving along a chemical gradient, a bacteria will swim for a longer time (without tumbling) if it is travelling in the right direction. However, it tends to tumble frequently when it senses that it is moving in the wrong direction. E.coli makes use of temporal sensing to find out whether they are moving in the right direction or not.
Signal Transduction
The chemical gradient of attractants and repellents is sensed by transmembrane receptors that are found in abundance in bacteria. These receptors bind with the chemical present and send a signal to the cytosol via plasma membrane, such that Che proteins are activated. The Che proteins alter the frequency of tumbling in bacteria and also control the receptors.
Eukaryotic Chemotaxis
The process of chemotaxis in eukaryotes is entirely different from prokaryotes. However, the process of sensing the chemical gradient remains the same. Since prokaryotes are smaller in size, they have to employ a number of methods, such as temporal sensing, to sense the chemical gradient. They travel a large distance in random directions to sense the amount of the chemical gradient.
Since eukaryotes are bigger in size, their cell membranes are embedded with receptors that can recognise the chemical gradient. These receptors sense the chemical gradient, and hence, the organism can move in the direction of the attractant or away from the repellant.
The motility of the eukaryotic cells in the direction of the chemical is still unclear. The receptors that recognise the chemical gradient include a signalling pathway, such that actin filaments are polymerised which ultimately gives rise to pseudopods and uropods. In some eukaryotes, the beating of cilia also promotes chemotaxis.
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Also Read:
- MCQs on Morphology of a Bacteria for NEET 2022
- Difference between Cilia and Flagella
- Classification of Bacteria based on the Arrangement and Number of Flagella
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