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Organisms use feedback mechanisms to
maintain their internal environments
and respond to external environmental
changes.
http://www.youtube.com/watch?v=CLv3SkF_Ea
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 Negative
feedback mechanisms maintain
dynamic homeostasis for a particular
condition (variable).
 They do this by regulating physiological
processes, returning the changing
condition back to its target set point.
 is
the mechanism by which the body maintains
conditions within particular limits. The body
will do this by opposing a change that deviates
from the normal.
 Example 1: Body Temperature Regulation
 Body temperature in mammals is regulated by
a sensor that consists of cells within the
hypothalamus of the brain.
 The
amount of glucose in your blood is
carefully controlled.
 After you have eaten a meal, the blood
glucose levels will begin to rise because
the carbohydrates in the food are digested
and absorbed. This rise is detected by
beta cells, which then will produce more
insulin. This insulin then binds to receptor
proteins in cell membranes (particularly in
the liver). This causes more protein
channels to open so that more glucose can
enter the cell.
Also, insulin encourages enzymes to convert
glucose to glycogen for storage.
 If however, you have been doing a lot of exercise,
and glucose is being used up, then alpha cells
will produce glucagon, this causes the release of
an enzyme that breaks glycogen to glucose

 The
trp operon is a repressible operon
 A repressible operon is one that is
usually on; binding of a repressor to the
operator shuts off transcription
 b. Positive
feedback mechanisms amplify
responses and processes in biological
organisms.
 The variable initiating the response is
moved farther away from the initial setpoint.
 Amplification occurs when the stimulus is
further activated which, in turn, initiates
an additional response that produces
system change.
 Lactation
involves positive feedback in
that as the baby suckles on the nipple
there is a nerve response into the spinal
cord and up into the hypothalamus of the
brain, which then stimulates the pituitary
gland to produce more prolactin to
produce more milk.
 when
a contraction occurs, the hormone oxytocin
causes a nerve stimulus, which stimulates the
hypothalamus to produce more oxytocin, which
increases uterine contractions. This results in
contractions increasing in amplitude and
frequency.
 the
first fruit that begins to ripen emits
ethylene triggering the surrounding fruit
to ripen”One bad apple…”
 Alteration
in the mechanisms of feedback
often results in deleterious
consequences.
 Example 1: Diabetes: a person has high
blood sugar, either because the pancreas
does not produce enough insulin, or
because cells do not respond to the
insulin that is produced
 Example
2: Dehydration in response to
decreased antidiuretic hormone (ADH) In
the kidneys, water is first filtered from the
blood (along with waste products). Much of
this water is reabsorbed and delivered back
to the blood. ADH stimulates this
reabsorption of water.
 alcohol
inhibits the release of ADH. This is
why excess alcohol consumption can lead to
frequent urination and dehydration.
 Example
3: Graves’ disease
(hyperthyroidism)
 affects the thyroid causing it to grow 2 to
3X its size (goiter); leads to
increased heartbeat, muscle
weakness, disturbed sleep,
and irritability
Organisms respond to changes in
their external environments.
http://www.youtube.com/watch?v=BUlBwe8mi
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•
Photoperiodism
 Phototropism
 Gravitropism
 Thigmotropism
 Kinesis
– random movement of organisms
 Taxix – movement toward or away from a
stimulus
• Chemotaxis in bacteria, sexual
reproduction in fungi
 Hibernation
and migration in animals
 Nocturnal and diurnal activity: circadian
rhythms (roughly 24 hour cycle in the
physiological processes of living beings,
including plants, animals, fungi and
cyanobacteria)
 Shivering and sweating in humans