Taxes

A taxis is a movement in the direction of a stimulus.Simple organisms that are motile can move towards favourable environmental stimuli or away from unfavourable stimuli. This increases the chances of survival.Unicellular algae show positive phototaxis. They move towards light. This increases their chances of survival since they need light in order to carry out photosynthesis to make their food.Earthworms show negative phototaxis. They move away from light and into the soil. This increases their chances of finding food in the soil and reduces their chances of being eaten by predators and enables them to reduce water loss by evaporation.Some species of bacteria that use glucose as a food source show positive chemotaxis for glucose. They move against a concentration gradient towards an area of high glucose concentration. Glucose is then used for respiration.