What is a stimulus?

Stimulus: any change in an organism’s environment that causes the organism to react. It is a fancy way of saying “cause”.

Stimulus – singular

Stimuli – plural

Example: An animal is cold so it moves into the light.

What is a response?

Response: how the organism reacts to a stimulus and results in a change in behavior. It is a fancy way of saying “effect”

Example: Getting a drink when you are thirsty.

Examples of stimuli and their responses.

• You are hungry so you eat.

• A rabbit gets scared so it runs away.

• You are cold so you put on a jacket.

• A dog is hot so he lies in the shade.

• It starts raining so you take an umbrella.

Write a stimulus and response that you have experienced

recently. Underline the stimulus and circle the response.

2 Types of Stimuli

External Stimuli

Or

Internal Stimuli

External Stimulus

External stimulus: a stimulus that comes from outside an organism.

Examples:

1. You feel cold so you put on a jacket.

2. A snake lunges at a rabbit so it runs away.

3. A dog feels hot so it gets water.

Internal Stimulus

Internal Stimulus: a stimulus that comes from inside an organism.

Examples:

1. You feel hungry so you eat.

2. A cat feels thirsty so it drinks water.

3. You sweat because you are hot.

Internal Stimulus – Wilting in plantsWilting refers to the loss of rigidity of non-

woody parts of plants (leaves). Plants wilt because of 1. Over watering: Most common in indoor

plants.2. Lack of water: Plants need water.3. Root bound plants: move the growing

plant to another container.4. Too much sun5. Not enough sun6. Too much fertilizer7. Disease

Internal Stimulus: Vomiting in humans

Vomiting is• the forceful expulsion of the contents of one's

stomach through the mouth and sometimes the nose.

It is a response to internal stimuli such as:

1. Drugs 2. Pain and Stress 3. Motion sickness 4. Food poisoning 5. Excessive consumption of alcohol 6. Eating disorders (eg: bulimia)

Make a T-Chart on your paper and label the columns internal and external. Put the

letter of each stimulus in the proper column

A. You have a stomach ache and decide to lay

down.

B. A bird is thirsty and drink some water.

C. A squirrel sees a cat and runs up a tree.

D. A lion gets hungry and eats a gazelle.

E. You see a spider and scream.

internal

internal

external

internal

external

Tropism – The growth response of a plant toward or away from a

stimulus• Phototropism – the way a plant grows

or bends in response to light.

• Geotropism – the way a plant grows or bends in response to gravity.

• Hydrotropism – the way a plant grows or bends in response to water.

• Thigmotropism – the way a plant grows or bends in response to touch.

Assessment

• 1. What is the difference between a stimulus and a response?

• 2. Vomiting is what type of stimulus?

• 3. Wilting in plants is what type of stimulus?

Positive vs Negative Tropism

• Positive Tropism: Going toward the stimulus.– Example: If a plant grows toward the stimulus

(light, earth, water, etc.) it shows a positive tropism.

• Negative Tropism: Going away from a stimulus.– Example: Roots grow downwards showing

negative tropism to light.

Phototropism

Positive: Leaves and Stem

because it is going toward the

light source.

Negative: Roots because it

is going away from light

source.

Hydrotropism

What is positive hydrotropism?

Roots

What is negative hydrotropism?

Leaves and stem

GeotropismWhat is the

positive geotropism?

What is the negative geotropism?

Roots

Stem and leaves

Assessment

1. Identify the external response from the following.

A. Your muscle sores when you run a mile

B. You drink water when you are thirsty

C. You run away when you see a tiger

D. After school you eat when you are hungry

2. Which part of the plant shows positive geotropism?

A. Leaves

B. Stems

C. Flower

D. Roots

3. Which of the following is not an external stimulus dealing with plants?

A. Light

B. Touch

C. Gravity

D. None of the above

4. In the word phototropism, what is the stimulus?

A. Touch

B. Light

C. Water

D. None of the above

5. If the plants is placed upside down and allowed to grown, in what direction would the roots grow?

A. Up

B. Down

C. Sideways

D. Not grow

6. If the plants were tilted and allowed to grow sideways, in what directions would the leaves and stem grow?

A. Up

B. Down

C. Sideways

D. None of the above

Assessment

7. Write an example of a stimulus and response.

8. Give an example of positive phototropism.

9. Give an example of positive geotropism.

10. What is thigmotropism?