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.

To unlock this lesson you must be a Study.com Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account
Support