Response to stimuli in living organisms
The ability of living things to respond to influences from the environment is called response to stimuli (irritability). Environmental factors – moisture, light, sound, temperature, pressure, the composition of air, chemicals – are called stimuli. Response helps organisms survive in changing conditions. Humans and animals have sense organs and a nervous system. The nervous system consists of the brain, spinal cord and nerves; the brain and spinal cord form the central nervous system. Sensory nerves bring information to the centre as electrical impulses, and motor nerves carry the instruction to muscles. An organism’s response to a stimulus is called a reflex: pulling your hand away from a hot object is a reflex. Plants have no nervous system or sense organs, yet they also respond: their organs change the direction of growth – this is tropism. For example a shoot growing towards light is phototropism (“photos” – light).
Experiment “Reaction time”: work in pairs. One pupil holds a 30 cm ruler vertically; the other holds open fingers near the ruler’s bottom end. When the ruler is dropped, the second catches it as fast as possible; write down how many cm it fell (5 tries). A shorter distance means a faster response. Phototropism: put a pot with a seedling by a window and watch the shoot bend over 10 days. Safety: sit down for the ruler test; do not hit hands or faces; never test reflexes with hot objects.