An Organism’s
Responses to changes
in their Environment
Stimulus
Any 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 & Responses
Think 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?!?
External stimulus
A stimulus that comes from
outside of the organism.
When the finger
senses that something
is dangerously hot, it
pulls away.
External Stimuli & Responses
External stimuli can be more complicated. Let’s see how.
EXTERNAL STIMULUS
RESPONSE
•Bright sunlight
•Squinting
•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 conditions
coming 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)
(ok, this one is pretty simple)
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.
Internal or External Stimulus?
External
pupil
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.
Internal or External Stimulus?
External
When pollen
enters your nose,
the body responds
by sneezing.
Internal or External Stimulus?
Internal
When growth
hormones help you
add more muscle &
bone tissue during
growth spurts
The End!!