- Homeostasis: (from Greek: "to
stay the same")
All the characteristics of life are interrelated...this
is certainly the case when it comes to evolution and homeostasis. (Remember:
natural selection x genetic variation = adaptation. Many adaptations have
homeostatic implications; that is, many adaptations relate to an organism's
ability to maintain a viable internal environment.
- Maintenance of constancy or near-uniformity
internally in a cell or an organism
Certain conditions must be maintained to sustain
life--at whatever level (cell, organism, community).
- Adaptations, homeostasis, and evolution
- Adaptations:
(What's the formula?--above) Adaptations can be either
simple survival mechanisms of homeostatic mechanisms. Examples: simple
survival mechanism: camoflauge; homeostatic mechanism: a hot-desert lizards
minimizing contact with the hot sand (thermoregulation). Remeber the fox-ears
example? How is that a homeostatic adaption/mechanism?
- Homeostasis
- Evolution
- Nature of homeostatic mechanisms
Categories of homeostatic mechanisms: (They are very
interrelated.)
- Morphological
(Structural) Example: the leaves of a cactus, better
know as the spines: their structure restricts surface area which preserves
water (photosynthesis occurs at the stem surface).
- Physiological
Example: Homeostasis in plants: deal with seasonal
changes: in winter plants go into dormancy: to conserve their resources,
the trees remove the photosynthetic green chlorophyll pigments, leaving
other pigments (red, yellow, orange) that were previously masked. Eventually
even these are removed and what remains is basic cellulose skeletons of
the leaves, which fall.
- Behavioral
Example: penguins are flightless birds that must
incubate their eggs. They live in a cold place and must keep the eggs warm
for them to survive. They have a brood patch that, when stimulated by contact
with the egg, triggers the pleasure center in the penguin's brain...so
they literally line up to incubate the eggs.
- Thermoregulation: an example of
homeostatic control
 21.1
Skin
Thermo- refers to heat. We must maintain our internal temperature at
about 98.6 F. (Ever had a fever? Maybe you can appreciate the benefits
of thermoregulation.) A slight increase in body temperature can help fight
infection--that's a part of our natural immune system.
Perspiration is a homeostatic mechanism: has a cooling effect because it
dissipates heat when evaporated from the body's surface.
How does the body react to a changing temperature in the external environment?
21.2 Hormone
Interactions
- Hormone interaction in humans
21.3
Hormones
- General characteristics of hormones
Hormones are part of a communication system that
works in connection with nervous system (electrical). Hormones are chemical
communicators that are produced in one tissue and then transported to another
tissue (target tissue) where they solicit a specific response (via the
blood stream--circulatory system). (Example: puberty) There are two basic
categories of hormones:
- Steroid hormones
Steroid hormones are lipid-soluble, so they are able
to move directly through the cell membranes in the target tissue.
- Protein hormones
Protein hormones, or nonsteroid hormones, are not
lipids and cannot pass directly though cell membranes. Instead, they attach
to receptor sites where a second messenger is triggered within the cell,
effectuating the hormone-induced response.
- Significance of chemical communication:
integration of the whole system by feedback control
Internal conditions are monitored with feedback.
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