Organismal Physiology Lecture No. 3: Thermoregulation In Ectotherms
Tuesday September 18 , 2012
-An ectotherm does not generate internal heat and a poikilotherm has its body temperature determined
by equilibration with the thermal conditions of the environment.
-How on Earth does an organism not producing heat manage to temperature changes within its
environment? From the moderate conditions of tropical regions to the extreme conditions of dry
deserts, are ectotherms merely the passive victims of their thermal environment? We will explore ways
in which this statement is irrelevant.
Plants Affecting Leaf Temperature (Leaf Colour):
-Encelia farinosa is a species of plant that grows in extremely hot, arid environments and therefore must
be white to increase its albedo (the capacity to reflect solar radiation). The black pearl pepper on the
other hand, absorbs far more radiation because of its low albedo. In general, leaf colour alters provides
a long-term adaptive response to altering the amount of solar radiation absorbed by a plant. Most
plants don’t change their colour and utilize short term strategies for maintaining body temperature.
Plants Affecting Temperature (Convection):
-With regards to convection (the heat exchange with air molecules that brings organisms closer to air
temperature), plants will often manipulate the shape of their leaves in order to exploit convection
currents for their benefit.
-In the case of the Northern Red Oak, it possesses sun leaves (more exposed to sunlight) that are small
and have less surface area, which reduces the amount of exposure to the sun and wind (air molecules
skim off the edge and cool the leaf down). A shade leaf is large, with greater surface area, which
increases the amount of area exposed to the sun (as plants need the proper balance of sunlight).
Plants Affecting Temperature (Evapotranspiration):
- Evapotranspiration is the movement of water from the roots to the leaves of the tree with the
eventual loss of water through the stomata of the leaves. For a plant with water readily available, this is
an extremely effective way of dissipating heat. Under hot conditions, water transpiring from the plant’s
stomata cools it down considerably (as water’s latent heat of vaporisation is 2270 kJ/kg). Under cooler
temperatures, the plant’s stomata are tightly closed to preserve water. As a result of transpiration,
some desert plants are at much cooler temperatures than the environment would usually allow. Behavioural Thermoregulation In Plants & Animals:
-Organisms may engage in certain behaviour to adjust the thermal settings of their environment. For
example, when corn plants are under extreme drought stress they will roll their leaves up (pointing
them vertically towards the sky) so that less sun makes contact with their leaves, saving water. These
behaviours are quite common among ectothermic animals such as lizard species that will often bask in
the sun to regulate their metabolism.
-The minor amount of heat that comes from ectotherms derives from futile cycles of cellular respiration
(in plants) and from involuntary muscle contractions (in animals).
Endothermy In The Skunk Cabbage:
-The spadix is a physiological appendage common to certain plants (like the skunk cabbage and other
endothermic plants) which is notable as the site for considerable thermoregulation. The skunk cabbage
in particular exploits the ability of the spadix to generate heat in order to melt the surrounding ice and
commence growth in early spring. Metabolism is however, an extremely energetically costly process.
Importance of Metabolic Heat To Plants:
-Thermoregulation is important to plants as it provides protection against col