Organismal Physiology Lecture No. 7: Physiological Plasticity
Tuesday October 2 , 2012
-Homeostasis can be defined in a variety of ways. In terms of animals, homeostasis is the internal
constancy and the physiological regulatory systems that automatically make adjustments to maintain it.
In terms of plants, homeostasis is the condition of a relatively stable internal physiological environment,
usually involving extensive feedback mechanisms.
-Homeostasis works by having bodily sensors constantly monitoring body temperature with reference to
a set point (the standard for body temperature), working almost exclusively as a household thermostat.
When body temperature deviates from the set point, the sensors signal physiological processes to
commence and regulate body temperature. When the preferred body temperature is reached, sensors
stop sending signals and the negative feedback loop comes to a halt.
Negative & Positive Feedback:
-Homeostasis of body temperature is an excellent example of a negative feedback loop where the
process works to return the value to the set point. When body temperature is too hot, there is an
attempt at increasing heat loss (such as vasodilation and sweating) in order to cool the body towards the
set point. When body temperature is too cold, there is an attempt at decreasing heat loss
(vasoconstriction) and increasing heat production in order to heat the body towards the set point.
-Positive feedback loops result in an amplification of the deviation from the set point and are a rare
occurrence in biology. In examining the ice albedo effect, there is evidence of a positive feedback loop
as the low albedo of the surrounding soil absorbs the progressively warmer effects of climate change
and continuously melts the glaciers.
Control In Homeostasis:
-Homeostasis is present at many levels of control in the body: Hormonal (insulin and glucagon regulating
mammalian blood sugar), molecular (cell-signalling pathways that regulate cytoplasm composition),
nerve-mediated (vasoconstriction and dilation regulating heat loss in vertebrates), biochemical
(maintaining rates of reactions by altering pathways and enzymes).
-Compensation maintains an organism’s physiological performance in the face of varying conditions.
Compensation requires a shift away from the acute response. Some species of ectotherms settle for no
compensation and thus, their body temperature is dictated by the environment. In comparing polar fish
and temperate fish, we see that polar fish have a very narrow temperature range for optimal enzyme
function, while temperate fish have a much more stable temperature range for proper enzyme function. -After compensation, the rate of metabolism of a given organism will be lower than normal and
graphically expressed as a less steep (smaller slope) curve. This is seen in the compensatory response of
warm-grown vs. cool-grown black spruce as the warm-grown trees have a lower rate of metabolism
than the cool grown. This down-regulation of warm-grown trees is fairly homeostatic.
Origin Of Acute Perturbations:
-Acute distresses may come from mismatches in biochemical pathways. This is seen with the rates of
pumping in vs. the rates of diffusing out ions as well as with the rates of production vs. the use of
reducing equivalents (temperature effects on photosynthesis). In the electron transport chain of
photosynthesis there are three main types of processes in relation to temperature: fast physical
processes that are largely temperature independent (very fast absorbance of light), diffusions that are a
bit temperature-dependent (electron shuttling) and enzyme-catalyzed processes, which are quite
temperature-dependent (NADP reduction and oxidative phosphorylation).
-Also, the light-driven reactions of photosynthesis (the production of reducing equivalents) are less
temperature-dependent than the carbon (stroma) reactions (consumption of reducing equivalents),
since the Calvin cycle is largely enzymatic.