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Chapter 6


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6.1 All Organisms live in a thermal environment

A world of heat and cold

absorb solar radiation

organisms produce heat during metabolic processes

maintain heat - gain and lose it

plants and photosynthesis

excess heat is dissipated to the environment

Heat transfer:




thermal radiation


Heat balance summarized

Heat gain = Heat loss

heat gain ( solar radiation + thermal radiation + food energy storage + conduction + convection ) equals heat loss ( thermal radiation + conduction + convection + evaporation )


6.2 Plant-temperature interactions can be complex

Plants are fixed, subject to varied temperatures

Plant leaves, buds and twigs exposed to varied temperatures - shade / sunlight

Plant metabolism and internal temperature

Leaves absorb short waves and long wave radiation

Net energy balance ( R n )

Sunny day temperature raise internal leaf 10 to 20 degrees C

leaf orientation angle to sun. leaf size and shape

terrestrial plants lose absorbed heat - convection + evaporation

aquatic plants by convection


6.3 Photosynthesis is temperature-sensitive

T minimum

T optimal

T maximum

C 3 and C 4 photosynthesis pathways


6.4 Plants have metabolic adaptations to heat and cold

Extremes - kills plant tissues, inactivating enzymes, denaturing proteins

sub zero temps. - dehydration

cold tolerance is genetic


heat damage


6.5 Animals maintain temperature differently

Animals and plants differ

Boundary layer

thermal stress


6.6 Animals fall into three physiological groups

1. Endothermy ( heat from within ) - birds and mamals: stored energy to keep a constant internal temperatures independent of external temperatures

Homeotherms ( the same ) - warm blooded

2. Ectothermy ( heat from without ) - gain heat through exposure to environmental sources and dissipate heat through conduction, convection, evaporation

Poikilotherms ( variegated ) - cold blooded, cool to the touch, invertebrates, amphibians, fish, reptiles

3. Heterotherms ( different ) - sometimes are endothermy and sometimes Ectothermy. Depends on environmental situations and metabolic needs. E.g. Bats, bees, humming-birds


6.7 Poikilotherms depend on environmental temperatures

Gain and lose heat fast from environment

environmental controls the rates of metabolism

rise in temp increase enzymatic rate

controls metabolism and respiration

10 0 C rise in temp metabolism doubles

Operative temperature range


aquatic poikilotherms ( fish ) and body temperature changes


6.8 Homeotherms escape the thermal constraints of the environment

Endothermic birds and mammals

oxidizing glucose and other energy - rich molecules

insulation: fur feathers, layer of fat

evaporation cooling, increasing or decreasing metabolic heat production -

body temperature maintenance - specific enzyme systems - at 40 0 C

high metabolic rate and a low thermal conductance

oxygen to tissue - high level of aerobic energy production

high level of physical activity

exhaustion - heat from activity for long durations

escaping from predators

disadvantage - high cost of metabolism


6.9 Heterotherms may or may not regulate body temperature


Flying insects

take in oxygen by demand

temperature critical in flight

below 30 0 C above 44 0 C

wings to the sun

shivering muscles in thorax

when not in flight cool down to ambient temperatures


6.10 Torpor helps some animals conserve energy

Dropping body temperature to approximately ambient temperature for a part of the day. Regardless of season.

Evolved as means to reduce energy demands - day or night depends on animal

bat, hummingbirds

Hibernation - poikilotherms changes

decreased blood sugar, increased liver glycogen, altered concentration of blood hemoglobin, altered carbon dioxide and oxygen content in blood, altered muscle tone, darkened skin

groundhog, chipmunk

deep sleep - black bear, grizzly bear, polar bear


6.11 Animals exploit microclimates to regulate temperature

Behavior means - seek out appropriate microclimates

butterflies, moths, dragonflies, damselflies,

salamanders, frogs, reptiles ( snakes ), lizards, birds, mammals,desert mammals, rabbits, deer, flying squirrels, penguins, quail


6.12 Insulation reduces heat exchange

Keep heat in and out

Covering of fur, feathers, or body fat

small, large animal fur

thickness change by season

Arctic - aquatic mammals - thick layer of fat

feathered feet

scaled feet thermal windows


nonshivering thermogenesis


6.13 Evaporate cooling in animals is important

Birds and mammals - evaporative cooling

heat loss through skin

sweating and panting

sweat glands - humans and horses

pigs - wallow in water and mud


6.14 Some animals use unique physiological means for thermal balance

Camel - store body heat by day, dissipates at night ( water limited ) 34 C in am to 41 C in pm

Oryx, Some gazelles

reduce need for evaporative cooling - reduce water loss

Supercooling - body fluids , body temperature falls below freezing without freezing

Intertidal invertebrates of high latitudes - freezes and then thaws out

arctic and antarctic fish resist freezing - glycerol, increasing degree of supercooling


6.15 Countercurrent circulation conserves or reduces body heat

Countercurrent heat exchangers

porpoise ( fig. 10-11 )

cold - insulated with blubber

heat - uninsulated flukes and flippers

arterial and venous blood

rete - discrete vascular bundles or net

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