Stimulus &
Response
TEKS 7.13A
investigate how
organisms respond
to external
stimuli found in
the environment
such as
phototropism and
flight or fight
TEKS 7.13B
describe and relate
responses in
organisms that may
result from internal
stimuli such as
wilting in plants and
fever or vomiting in
animals that allow
them to maintain
balance
Stimulus
A change in organism’s
environment
Types of Stimuli
 A stimulus can either be external or internal.
 External stimuli- come from factors found in the
environment.
Examples- temperature, predators, presence
of water or food, etc
 Internal stimuli- come from factors within an
organism
Examples- germs, dehydration, lack of
energy, pain
Stimulus
Is this stimulus external or internal?
Response
An organisms reaction to change.
Response
Homeostasis
Homeo- same
Stasis- state of balance
The purpose of response to stimuli
in organisms is to maintain
homeostasis, a balance within
it’s internal systems.
Examples
 Stimulus = Lack of water
Is this internal or external?
 Response = Wilting
 Stimulus = Bacteria or Virus
Is this internal or external?
 Response = Fever
Examples
 Stimulus = Stomach virus
Is this internal or external?
 Response = vomiting
 Stimulus = Saber-tooth Tiger
Is this internal or external?
 Response = Fight or flight
Just as animals
respond to the
environment,
plants do as
well.
Tropisms
 Tropism = plant’s growth response
toward or away from a stimulus
Tropisms
 Tropism = plant’s growth response
toward or away from a stimulus
 Positive tropism
 Negative tropism
Tropisms
 Tropism = plant’s growth response
toward or away from a stimulus
 Positive tropism = plant grows toward
a stimulus
 Negative tropism
Tropisms
 Tropism = plant’s growth response
toward or away from a stimulus
 Positive tropism = plant grows toward
a stimulus
 Negative tropism = plant grows away
from a stimulus
Hydrotropism
Hydrotropism = a plant’s
roots grow toward water
Is this positive or negative?
Phototropism
Plants grow toward a source of light
Animation:
http://botanical-online.com/animation8.htm
Examples
Thigmotropism
A plant grows toward or away from pressure
Positive
turns toward
pressure
Negative turns
away from
pressure
Geotropism
 A plant grows with or against gravity
 Also called Gravitropism
Positive- A
Negative- A
plant’s roots
plant’s stems or
grow down,
trunk grow up,
with the pull of
against the pull
gravity
of gravity.
Animation:
http://botanical-online.com/animation4.htm
Regardless of the
position a seed is
planted in, the roots
grow down with
gravity and the stem
up against gravity.
Regardless of the slope
of the land, the tree
grows up against gravity.
Working Together
 Plant tropisms work
together to maintain
homeostasis within the
plant
+ phototropism
+ geotropism
- geotropism
Plants in Motion Video
+ hydrotropism