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Responding
to stimuli
Adrian
Whyte and Joanna Johnson, Contributor
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| Tennis
presentation of Tennis equipment
to Boys Town school on February
1, 2006. - Norman Grindley Photo |
SENSITIVITY
IS a characteristic of all living organisms.
It is the ability to detect and respond
to changes in the environment (stimuli).
When dealing with sensitivity, there
are a few terms we first need to know:
STIMULUS
(THE SINGULAR OF STIMULI)
This
is any change in the environment of
an organism (whether external or internal)
that causes a response in the organism.
RESPONSE
The
change that is produced in an organism
that results from a stimulus or stimuli.
RECEPTOR
A
cell or group of specialised cells
that detect a particular stimulus.
EFFECTOR
A
cell or organ that produces a response
when stimulated by a nerve impulse.
QUESTION
Why
is the ability to detect and respond
to stimuli important for the survival
of an organism?
Organisms
respond to stimuli for various reasons
but all fall under a common banner,
which is for the survival of the organism.
For example:
1)
When an organism goes in search of
food it is responding to the detection
of hunger that is felt. In this case,
hunger pains would be the stimulus
and the response is to search for
food.
2)
Escaping from danger. In this instance
an organism will detect some imminent
threat to safety. For example, on
seeing a predator the organism will
try and find a safe place to evade
a possible attack.
PLANT
RESPONSE TO STIMULI
Different
organisms respond to stimuli in different
ways. How does a plant respond to
stimuli? Plants use hormones to allow
them to respond to stimuli and one
of the main hormones used by plants
is a growth hormone known as auxin.
Auxin plays a major role in how plants
respond to the stimulus of light,
(phototropism) and gravity (geotropism).
Auxins
are produced in the very tips of the
shoots or roots (meristems) of plants.
The meristem is the section of the
plant where growth occurs. Auxins
can make the shoot of a plant grow
towards the sun.
TROPISM
(GROWTH RESPONSE SHOWN BY A PLANT)
A
growth response that is towards the
stimulus is called a 'positive' tropism,
whereas one that is away from it is
described as a 'negative' tropism.
In
plants, the responses to stimuli are
observed in the form of growth. A
response can involve all of the plant
or just a small part of it.
PHOTOTROPISM
A
stimulus could be changing the direction
of the light a plant receives. The
plant's response would be to grow
towards the light. Such a response
involving light is phototropism.
Auxins
play an important part in phototropism.
The shoot tip (which is a meristem)
is responsible for directional movement
by the plant in response to sunlight,
as this is the area where auxins can
be found. Sunlight eradicates auxin,
meaning that the part of the shoot
tip of the plant which is receiving
direct sunlight will have the least
amount of auxin.
The
extra auxin present on the shaded
side promotes more cell division and
elongation, causing the plant to bend
towards the sunlight after this lop-sided
growth.
GEOTROPISM
This
is a similar occurrence to phototropism
where the plant exhibits directional
growth in response to gravity. The
shoot tip illustrates negative geotropism
(grows against force of gravity),
while the root tip exhibits positive
geotropism (grows in the same direction
as gravity).
IN
THE ROOT
The
plant hormone auxin works in the similar
way to get roots to bend down towards
gravity or water in the soil.
DIAGRAM
SHOWING THE RESPONSE OF A TYPICAL
SHOOT TIP TO LIGHT
*
Adrian Whyte and Joanna Johnson
teach Biology at Ardenne High School
masterbio@gmail.com.
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