How do simple bacteria find their food? If you watch a bacterium, especially one with a long whip-like tail (this tail is called a flagellum), you will notice that it rapidly zeroes in on a source of food, even if that source is a distance away, by whipping its tail back and forth to propel it in the right direction. How does it know which way to go? A bacterium doesn't have a brain, but it does have a very primitive sensing organism, which can detect molecules. It works much like your nose, which can, for example, detect very tiny numbers of perfume molecules in the air, a long way from the source of the perfume. When you detect the perfume 'smell', you can often locate the source of the perfume by turning and following the scent ... your nose can sense when the concentration of molecules is increasing, and you head in that direction. The principles that make this detection system work are based on these facts: molecules given off by a source will diffuse through the air. the concentration of molecules in the air is higher close to their source. molecules in the air are farther apart as you get further from their source. your nose can detect tiny numbers of molecules. your nose can sense when there are higher numbers of molecules in the air. By following the molecules in the direction in which they get more dense, you can find the source of the smell. It's like entering a room and locating a dead mouse under a chair, by following the ever-stronger scent trail. Bacteria can follow a trail of molecules in the same way. A bacterium has a receptor on one end, which can trap a molecule of its food and hold it for a short time. When the molecule is released, the bacterium will keep moving, by waving its flagellum.  - If it does not immediately encounter another molecule, it will change direction.  - If it does encounter a molecule, and after releasing it, encounters another one       right away, it keeps moving in the same direction. In this fashion, the bacterium will continually change direction and move toward a higher concentration of food molecules, eventually arriving at the source. This process is called chemotaxis. Resources HTML, graphics & design by Bill Willis 2023