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9 Bioenergetics
9.6 Life Under Extremes of Temperature
Life forms have found a wide variety of strategies to keep their essential systems
from being irreversibly terminated from extremes of temperatures (hyper- and
hypothermia). At present, life systems on Earth are dependent on liquid water where
organic reactions can occur. Thus, the temperature range for the activity of organic
life is just below 0 ◦ C to just above 100 ◦ C. At elevated temperatures above 100 ◦ C,
proteins first become ‘denatured’ (unraveling and losing their structural integrity),
and then chemically disassociated. Some bacteria living near hydrothermal vents
can survive temperatures up to 121 ◦ C.
For temperatures much below 0 ◦ C, cellular life forms can be destroyed by the
growth of ice crystals which can break through cell walls. (Very rapid freezing
reduces this risk.) For nanoscale life, such as viruses, water molecules can diffuse
into the virus’ capsid structure, but this water will not become crystalline ice, and so
viruses are not threatened by very low temperatures. Although they become inactive
at low temperatures, viruses are revived when returned to liquid in host cells.
Some life forms use a strategy of active control over the size of any ice crystals
that may form, and become inanimate during below-freezing temperatures. If
the temperature is not far below freezing, some plants seeds survive intemperate
environments by first expelling water from cells (directed desiccation), binding most
of the remaining water in cells, producing osmolytes, such as ethylene glycol, to
protect protein structures, and secreting ‘antifreeze’ compounds (‘cryoprotectants’
into intercellular regions. Some animals use ‘suspended animation’, during which
they become dormant, but with a minimum of metabolic activity (‘metabolic
depression’). Some warm-blooded animals with stored fat and insulation use
hibernation. A low level of metabolic heat keeps the animal from freezing.
9.7 Temperature Regulation in Humans
Humans are warm-blooded animals. Our body attempts to maintain a temperature
of about 37 ◦ C so that our activity and metabolism can consistently function at a rate
compatible with our life style and survival. The temperature regulation mechanisms
are shown in the Fig. 9.2. Whether cold or warm blooded, life activity requires
the release of heat. This follows from the fact that life orders molecules in cyclic
processes. We will elaborate on this kind of metabolic and other microscopic activity
in life systems later.
Warm-blooded animals tend not to be as efficient ‘heat engines’ as coldblooded ones, sacrificing some efficiency in favor of power and activity. Humans
release from about 35 kcal/m 2 of skin surface per hour (sleeping) to 600 kcal/m 2 /h
(running). For an adult, the power output in heat ranges from about 80 W to about
1400 W. Our bodies selectively control the flow of blood in the skin, the extremities,
internal organs, and the more critical organs. Our temperature is also controlled by
9 Bioenergetics
9.6 Life Under Extremes of Temperature
Life forms have found a wide variety of strategies to keep their essential systems
from being irreversibly terminated from extremes of temperatures (hyper- and
hypothermia). At present, life systems on Earth are dependent on liquid water where
organic reactions can occur. Thus, the temperature range for the activity of organic
life is just below 0 ◦ C to just above 100 ◦ C. At elevated temperatures above 100 ◦ C,
proteins first become ‘denatured’ (unraveling and losing their structural integrity),
and then chemically disassociated. Some bacteria living near hydrothermal vents
can survive temperatures up to 121 ◦ C.
For temperatures much below 0 ◦ C, cellular life forms can be destroyed by the
growth of ice crystals which can break through cell walls. (Very rapid freezing
reduces this risk.) For nanoscale life, such as viruses, water molecules can diffuse
into the virus’ capsid structure, but this water will not become crystalline ice, and so
viruses are not threatened by very low temperatures. Although they become inactive
at low temperatures, viruses are revived when returned to liquid in host cells.
Some life forms use a strategy of active control over the size of any ice crystals
that may form, and become inanimate during below-freezing temperatures. If
the temperature is not far below freezing, some plants seeds survive intemperate
environments by first expelling water from cells (directed desiccation), binding most
of the remaining water in cells, producing osmolytes, such as ethylene glycol, to
protect protein structures, and secreting ‘antifreeze’ compounds (‘cryoprotectants’
into intercellular regions. Some animals use ‘suspended animation’, during which
they become dormant, but with a minimum of metabolic activity (‘metabolic
depression’). Some warm-blooded animals with stored fat and insulation use
hibernation. A low level of metabolic heat keeps the animal from freezing.
9.7 Temperature Regulation in Humans
Humans are warm-blooded animals. Our body attempts to maintain a temperature
of about 37 ◦ C so that our activity and metabolism can consistently function at a rate
compatible with our life style and survival. The temperature regulation mechanisms
are shown in the Fig. 9.2. Whether cold or warm blooded, life activity requires
the release of heat. This follows from the fact that life orders molecules in cyclic
processes. We will elaborate on this kind of metabolic and other microscopic activity
in life systems later.
Warm-blooded animals tend not to be as efficient ‘heat engines’ as coldblooded ones, sacrificing some efficiency in favor of power and activity. Humans
release from about 35 kcal/m 2 of skin surface per hour (sleeping) to 600 kcal/m 2 /h
(running). For an adult, the power output in heat ranges from about 80 W to about
1400 W. Our bodies selectively control the flow of blood in the skin, the extremities,
internal organs, and the more critical organs. Our temperature is also controlled by