Bacteria swim by rotating their flagella. They alter course by abruptly changing the direction of this rotation. The probability of the occurrence of this event is biased by chemoreception. The bias depends on the way in which the concentration of the attractant or repellent changes with time. Sugars are detected as they bind to specific proteins which also play a role in transport. The way in which the receptors are coupled to the flagella is not known. The coupling may involve changes in membrane potential.