Predicting How Organisms Will Respond to External Stimuli

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Reacting to stimuli is one of the requirements for something to be considered 'alive'. This lesson will explain what a stimulus is, and how an organism might react to some possible stimuli.

What is a Stimulus?

What does it mean to be alive? It is more than just pointing to a person walking and talking and saying that is being alive. Can you name the characteristics of a living thing? Let's think about it: something that is alive is composed of cells, grows, uses energy, regulates things like temperature and water within itself, and responds to stimuli. Most of these are ideas you are probably familiar with. But what about stimuli?

Let's start by defining the word 'stimulus'. A stimulus (plural: stimuli) is a change that can be detected by a living thing. An external stimulus would be something coming from outside the organism, from the environment. External stimuli are things that we can see, hear, or otherwise detect with our senses. These differ from internal stimuli, which are changes that happen within our own body. Hunger and thirst are two common internal stimuli.

Recognition and Response to Stimuli

Stimuli can be detected by an organism through a variety of ways. We can smell, see, hear, touch, and taste changes in our environment. But what about individual cells, like bacteria? Bacteria don't have noses, ears, or eyes. They do have receptors, though, that can sense specific changes.

An organism will respond to a stimulus by modifying its behavior, or what it's doing. Stimuli can be broken into either good or bad. If something is good, a living thing will behave in a way that increases the stimulus. If something is bad, a living thing will behave in a way that decreases the stimulus.

Examples of Response to Stimuli

Bad Stimuli

A bad or negative stimulus is something an organism doesn't want. An example could be something scary. If an opossum, for example, is suddenly startled, it will respond by 'playing dead'. The opossum has a physical response that causes the animal to basically lose consciousness. This makes predators think the opossum is dead and bad tasting. The negative stimulus of the scary predator is removed by this behavior.

A single-celled bacterium can respond to a negative stimulus as well. If the bacterium is moving, and its receptor picks up the presence of a dangerous chemical, the bacterium will change directions, moving away from the danger.

Humans can respond to bad stimuli as well. If you eat something sour, such as lemon, you probably immediately decide you don't want that taste in your mouth. You'll spit out the lemon, then maybe drink some water to help wash out the taste. Perhaps you'll eat something sweet to get rid of the bitterness, also. You're actively changing your behavior so that you can remove the stimulus.

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