Sensory Receptors
Information Processing
External and Internal Stimuli
(Lexile 1100L)
1  Suppose you and your family take a camping trip to Colorado.
Your father is trying to build a fire, and he asks you to collect
some sticks for a fire. While you are searching, you encounter
a huge bear. Your immediate reaction would be to run away!
This is a result of external stimulus. An external stimulus is a
stimulus that comes from outside an organism and causes a
reaction. When you see the bear, your brain immediately
registers fear and activates the sympathetic nervous system,
also known as fight or flight response. Knowing that you
cannot fight the bear and win, your brain opts to go with the
“flight mode” option, so that you can flee danger immediately.
2  Another example of external stimulus is temperature. When it is cold outside, your response is
to get warm by putting on a jacket. If there is an external stimulus, there must also be an
internal. An internal stimulus is a stimulus that comes from inside an organism. You may have
experienced an internal stimulus of hunger after a long day at school, and this prompts you to
eat some food in order to regain needed energy.
3  An animal's response to stimuli is due to two different behaviors: learned and instinctual.
Learned behavior is a response to a stimulus that an animal was taught. Being able to read
and write are examples of learned behavior, because it is not something you are born able to
do. On the other end of the spectrum, instinct is an animal's natural reaction to a stimulus that
is automatic instead of taught. This can be seen especially in wildlife. All animals are capable
of searching for food and water because it is an instinctual habit.
4  Learned and instinctual behaviors are responses that enable the body to survive externally.
Without external and internal stimuli, your body would not be able to maintain homeostasis.
Homeostasis is a property of systems that regulates their internal environment (blood levels,
temperature, nutrient levels, etc.) and tends to maintain a stable, constant condition despite
changing external conditions. Essentially, it keeps the balance between the body and
environment. A good example of homeostasis can be seen in your own household—your
thermostat. In the winter, you probably set your thermostat to about 75 degrees Fahrenheit to
stay warm inside. Whenever the house temperature falls below 75 degrees, the thermostat will
turn on the furnace to heat the house until it reaches the target temperature. Your body uses a
similar process to regulate the internal temperature.
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Sensory Receptors
Information Processing
5  Homeostasis is regulated by a variety of receptors that monitor and respond to changes in a
system, like your body. While playing a game of basketball, you may become very hot and
produce sweat. This is a response to thermoregulation receptors that sustain a constant body
temperature, not allowing it to become too hot or too cold. If your body did not have these
receptors and homeostatic mechanisms, it could overheat, destroying cells in your body, which
could lead to death.
6  All organisms must be able to maintain homeostasis, but do not assume they use the same
homeostatic mechanisms as humans do. Plants cannot run away from bears because they do
not have the flight or fight response that humans and animals have. The main requirement that
plants have is getting enough water and sunlight, which is equivalent to a human's need for
water and food. A special homeostatic mechanism that plants have is tropism, which is a
growth response of a plant to an external stimulus. Plants respond to many kinds of stimulus
such as weather, touch, time, gravity, or light. There are two basic responses, positive and
negative. A positive response is indicated by a plant's growth toward the stimulus, while a
negative response is when it grows away from the stimulus.
7  Two examples of tropism are gravitropism and phototropism. Gravitropism is a plant's growth
response to gravity. Ideally, roots grow down in the ground to absorb water for a plant, and its
stems and leaves grow upward above the ground to absorb light. If a plant were to lose its
position upward, then this could alter its ability to gain required nourishment. When this occurs,
a plant will begin to bend so that it is upright once again. This is possible because plants have
a special plant hormone called auxin that plays an important part in gravitropism. If the plant
fell over, auxin levels would increase on the lower side of the stem, stimulating cell elongation
on that side, and causing the stem to bend toward the sky. At the same time, auxin also
prohibits cell elongation in roots, which results in the roots bending down toward the soil. This
special homeostatic mechanism gives the plant access to water and light no matter what its
position.
8  Phototropism is a plant's response to light stimulus and is also regulated by the plant hormone
auxin. When the Sun shines down on a plant, auxins will move to the darker side of the plant.
This causes the cells on the dark side of the plant to elongate toward the Sun, while the other
side of the plant remains the same. When the plant bends, it allows a larger amount of light to
hit the leaves' surface. The leaves are then responsible for absorbing this light to begin
conducting photosynthesis for food.
9  The human body is built to survive, and with the aid of homeostasis, our bodies can adjust
effectively to the variable circumstances of our complex environment. Without homeostasis,
there would be no internal stimuli to prompt you to eat when your body is lacking energy. In the
face of danger, you would not know how to respond without the external stimuli alerting your
body to fight or flight. Plants would not be able to produce their own food without phototropism
or gravitropism. Instinctual and learned behaviors all contribute to our essential homeostatic
functions.
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Sensory Receptors
Information Processing
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Which action describes a response to internal stimuli?
A
catching a ball at a baseball game
B
drinking when you are thirsty
C
running at the sight of a wild lion
D
writing a thank you note
Based on the context, stimulus refers to _____.
A
outside reactions that monitor your bodily systems
B
the temperature of the body
C
food that gives your body energy
D
a thing or event that causes a reaction
A houseplant is placed on a window sill that gets direct, bright sunlight
every afternoon. The plant starts to grow toward the window. What kind
of response is described by situation?
A
A negative response to a stimulus
B
A learned behavior
C
Gravitropism
D
Phototropism
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Sensory Receptors
Information Processing
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Which homeostasis mechanism is the plant version of an animal drinking
when it gets thirsty?
A
A plant's growth response toward light
B
A plant's growth response to gravitation
C
A plant wilting when the soil dries out
D
A plant making sugar through photosynthesis
What can the reader conclude about homeostasis?
A
Only multicellular organisms maintain homeostasis.
B
Without receptors, an organism cannot maintain homeostasis.
C
Learned behaviors are not important for an organism to maintain
homeostasis.
D
Plants only need to respond to internal stimuli to maintain
homeostasis.
What is the best summary of this passage?
A
An external stimulus comes from outside the organism. Animals
respond to stimuli through learned behaviors and instinct.
B
Stimuli trigger responses in organisms that allow them to maintain
constant internal conditions through homeostasis.
C
Homeostasis is a set of mechanisms that work to maintain stable
internal conditions as external conditions change.
D
Phototropism is a plant's response to turn toward light. Gravitropism
is a plant's response to grow upward against gravity.
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