Responsiveness, Sensitivity, and Irritability in Living Organisms

Responsiveness in Living Organisms

• All living organisms respond to changes in their internal and external environment to maximize their chances of survival.
• A stimulus (singular) or stimuli (plural) triggers a response in an organism, such as rapid withdrawal of a hand from a hot object or growth towards sunlight in green plants.

Types of Stimuli

• Examples of stimuli include changes in light, sound, heat, chemicals, and mechanical contact.

Detecting Stimuli

• Organisms must have means of receiving information, such as eyes, ears, and taste buds, to detect stimuli.

Coordinating Activities

• A system of nerves and chemical regulators called hormones coordinate activities within an organism.

Responding to Stimuli

• Organisms respond to stimuli by means of effectors such as muscles and glands.
• Energy is generally used in the process of responding to stimuli.

Examples of Responses

• Trees lose their leaves in autumn in response to shortened hours of sunlight.
• Blood vessels in the skin of mammals dilate (increase in diameter) in response to a rise in body temperature, helping to return the body to its optimum temperature.