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Lecture 16

BIOL 1070 Lecture Notes - Lecture 16: Adipocyte, Aptenodytes, White Adipose Tissue

Course Code
BIOL 1070
Shoshanah Jacobs

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Learning Outcomes for Unit 10
Discuss how endotherms and ectotherms respond to environmental temperature change and
evaluate the costs and benefits of different thermal strategies.//////Identify the processes involved in
heat transfer./////////Interpret data on metabolic rate and temperature, including Q10.//////////Practise
integrative thinking with respect to the plight of polar bears in the face of climate change.
Surviving the Arctic Winter
This unit, we will explore metabolism and body size as well as how and why some endotherms
hibernate while others remain active during the harsh Arctic winter.
Body Size and Surface Area
Exchange with the environment and the amount of material that must be exchanged are both strongly
influenced by body size and surface area. For example, the larger the animal, the greater its absolute
requirements are for gases and nutrients. An elephant needs more food and oxygen each day than a
mouse does. Thus, absolute requirements go up with body size. However, gram for gram, a mouse
uses up more oxygen per unit of body mass than an elephant. Put another way, one elephant would
use much less oxygen than a group of mice weighing the same total amount as the elephant. Relative
(per gram or kilogram) metabolic requirements go down with body size.
Relationships between body size and metabolism. On the left, the positive relationship between body
size and absolute metabolic requirements: bigger animals need more food, oxygen, etc. in total
amount. On the right, the negative relationship between body size and relative metabolic
requirements: per gram of tissue, smaller animals require more food, oxygen, etc.
One of the important characteristics that changes according to body size is surface area to volume
ratio. This is the amount of surface area exposed to the environment relative to the total volume of the
object. Smaller objects have higher surface area to volume ratios than larger objects.
What this means is that exchange with the environment is not very efficient for large animals. To
compensate for this, many organs involved in exchange do not have simple flat membranes, but
folded or convoluted ones that have very high total surface areas. Examples include membranes in
the digestive, respiratory, and circulatory systems — in humans, each of these has 25X greater
overall surface area than the skin. For example, the small intestine (digestive system) and lungs
(respiratory system) have a much higher surface area by increased folding or pouches of the
epithelial surface.
The key points are:
1) Bigger body size means lower surface area to volume ratio, which means less efficient exchange
with the environment.
2) When more surface area is needed, organs may have specialized structures that increase total
area such as extensive folding.
Metabolic Rate and Temperature
To understand whole animal changes with temperature, researchers often measure metabolic rate, on
overall indication of how much energy or O2 is being consumed per unit time. This is usually done in a
controlled lab setting in a metabolic chamber or respirometer. Metabolic rate can be measured in
endotherms or ectotherms in a respirometer, as shown below.
In ectotherms, such as the woolly caterpillar, a decrease in environmental temperature decreases
body temperature and in turn, the overall metabolic rate. Therefore, less O2 will be consumed per unit
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time by the caterpillar at lower temperatures.
With these data, researchers can calculate the change in metabolic rate with temperature. This is
most often done by calculating the temperature quotient or “Q10” which describes the change in rate
with an increase in temperature of 10 degrees:
Q10 = Rate(T°C)Rate(T°C−10) or Q10 = O2 Consumption at 20°CO2 Con
sumption at 10°C or Q10 = 6633 Q10 = 2
For most physiological processes, Q10 values are between 2 and 3. One exception is seen when
animals go into hibernation; small decreases in temperature may result in more profound decreases
in metabolic rate. Although metabolic rate is often used to calculate Q10, researchers can use other
rate processes such as swimming rate, enzyme activity rate, or breathing rate to calculate Q10.
Another example uses reaction rate rather than O2 consumption. If you wanted to
understand the temperature effects on lactate dehydrogenase (LDH) rate or
activity, you could order this particular enzyme
from a biochemical supplier. Using a series of test
tubes you could incubate LDH with its substrates
at various temperatures, then measure the rate of
product (lactate) formation in each test tube.
Based on previously published data of LDH activity, you predict that Q10 is 3. Using this Q10 value,
calculate the LDH activity in the table below.
LDH activity
5 1.3
15 ?
25 ?
You then go ahead and measure LDH activity at 5, 15 and 25°C. Your measured values are 4 µmol/g/min
at 15°C and 12 µmol/g/min at 25°C. Do these values conform with your expectation? Is the Q10 of LDH =
3 or some other value?
Why Hibernate?
For an endotherm, once ambient temperatures start to fall, there is a larger temperature gradient between
internal and external temperature and greater heat loss.
To thermoregulate at normal set point (~37°C), active mammals in the Arctic require lots of energy – food
or fat stores. Alternatively, animals can hibernate to reduce metabolic rate and body temperature and this
circumvents the need for a lot of energy to fuel active metabolism. Hibernation usually lasts for several
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