An Organism’s Responses to

changes in their Environment

StimulusAny change in an organism’s

internal or external environment that causes the

organism to react to the change.

Stimuli – plural (more than one stimulus)

Response

The reaction of an organism to a stimulus.

Examples of stimuli and their responses:

•The dog is hungry, so it eats

some food from the bowl

•A rabbit sees a wolf, so it runs away to safety

Plants also respond to their environment.

When plants grow towards the sun, we call this reaction

phototropism

Plants respond to their environment in other ways too .

Plants roots grow down towards the water due to the effect of

gravity

Two Types of Stimuli

•Internal •External

Internal stimulus

A stimulus that comes from inside the organism.When the body

becomes too warm due to exercise, the skin responds by sweating. The sweat evaporates and cools the body temperature down.

Internal Stimuli & ResponsesThink of all the changing conditions that an organism might need to respond to. Here’s just a few samples:And here are some specific responses to these internal stimuli. Forget shivering…

Where’s my hoodie?!?

When the finger senses that something is dangerously hot, it pulls away.

External stimulus

A stimulus that comes from outside of the organism.

External Stimuli & ResponsesExternal stimuli can be more complicated. Let’s see how.

EXTERNAL STIMULUS RESPONSE

• Bright sunlight • Squinting (ok, this one is pretty simple)

• Shorter days, colder temperatures

• Trees drop their leaves in autumn

• Increasingly hot weather • Dogs shed their thick winter coats in late spring

• Sunlight, motion of the Sun. • Plants grow toward the Sun’s position.

• Difficult survival conditionscoming up

• Hibernation (chipmunks sleep thru cold months)• Migration (geese fly to warm weather & easy food)• Dormancy (grass turns brown in winter instead of

trying to grow & stay green)

There really are countless more examples, from as simple as an itch on your arm to the complicated conditions that cause plants to realize winter is over and spring back into life.

When there is not enough light to see, the pupil of the eye enlarges to allow as

much light in as possible so you can

see.

When the light is too bright, the pupil size becomes smaller to prevent too much light from going in

and temporarily blinding you.

pupil

Internal or External Stimulus?External

When pollen enters your nose, the body responds by sneezing.

External

Internal or External Stimulus?

When growth hormones help you add more muscle & bone tissue during growth spurts

Internal

Internal or External Stimulus?

The End!!