Review Sheet 40 Animal Structure and Function
1) Anatomy is the study of organism structure. Physiology is the study of organism function. Since
structure and function are highly related, anatomy and physiology are often studied together.
2) Organisms are constrained by physical laws (e.g. properties of elements, geometrical properties, etc.).
These physical constraints partly explain the occurrence of convergent evolution.
3) One major physical constraint is surface area to volume ratio. When a linear dimension (e.g. height,
width, length) increases by x, surface area (e.g., total body surface area, cross-sectional surface area,
etc.) increases by x2, and volume increases by x3. Another way of saying this is that whenever height
doubles, surface area quadruples, and volume increases eight-fold.
“Size” measured in three
different ways
One dimensional (size of a line)
Scale (A = a linear
measurement, e.g. height)
A
Two dimensional (size of a
square)
AxA
Three dimensional (size of a
cube)
AxAxA
Examples
Height, length, width, length of
limbs, length of a nerve axon
Surface area of skin; thickness of
muscles; inner surface area of
intestine
Volume; body weight
4) Body size is constrained by surface area to volume ratio in a number of ways. Cell size is limited
because as cell mass and consequent nutrient and gas exchange requirements increase by a cube, its
membrane exchange surface only increase by a square. Thus, a giant cell would not be able to support
itself.
5) The strength of support structures and muscles depends on the thickness of those structures. The
thickness of a structure is best measured by its cross-sectional surface area. If an increase in body size
results in strength increasing by a factor of x, then body weight will increase by a factor of x2. This
means that when body size increases, body weight increases more quickly than body strength. For this
reason, larger animals must be built with heavier muscles and skeletons (e.g. compare an ant, a dog,
and an elephant). An ant scaled up to cow size would be too weak or too heavy to stand up.
6) The surface area to volume ratio also affects climbing ability. Small animals like insects or squirrels
can crawl on vertical surfaces in part because the surface area of their feet relative to their body weight
is very high. It’s easier for a small animal to get a “grip” on a surface that is strong enough to support
their weight. It also helps that falling is less risky for smaller animals!
7) Organisms cannot ignore physical constraints, but they can work around them. For example, body size
can be increased without reducing the surface area to volume ratio if an organism by changing the
organism’s shape (e.g. sac body plan of hydra). A flat or elongated body will have more surface area
than a sphere, allowing more exchange of materials. Another way to increase the exchange surface
area is elongated projections (e.g. villi in the small intestine, or root hairs of plant roots). An
adaptation for increasing the rate of exchange found in virtually all organisms are branched networks
of transport tubes (e.g. roots, blood vessels, alveolar passages in lungs, slime mold plasmodium, etc.).
8) Bioenergetics is the study of energy flow in living systems. Machines, by definition, use energy to do
work. Organisms also use energy to do work. What make organisms very distinct from machines is
that organisms use energy for self-organization and self-maintenance. Organisms starve if deprived
of food because they need energy for self-maintenance. Machines without fuel do not suffer damage
from starvation because they do not use energy for self-maintenance. Of course, organisms also use
energy for purposes not directly related to self-maintenance, like movement, or synthesizing
chemicals, processing information, etc.
9) Metabolism is the sum total of the chemical reactions that occur in a living organism’s body. Since all
biological functions are fueled by chemical energy, metabolism also represents the sum total of an
organism’s energy use. Metabolic rate (MR) is the rate of these reactions. A higher metabolic rate
means that the organism’s need for food and oxygen will be higher. A high metabolic rate also allows
for a more active life style.
10) Basal metabolic rate (BMR) is the MR of an endotherm that is not growing, at rest but not sleeping,
has an empty digestive tract, and is not stressed. It is roughly the same as the minimum metabolic
reactions necessary for self-maintenance and continued life. Standard metabolic rate (SMR) is like
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basal metabolic rate, except it applies to ectotherms. Since ectotherm body temperatures vary, and
chemical reaction rates depend on temperature, SMR must specify a temperature as well (e.g. SMR of
an anole lizard at 25oC).
Normally, animals adjust their metabolic rate, raising it to meet more energetically demanding
activities. Maximal metabolic rate is the highest metabolic rate an organism can sustain using
aerobic means. E.g. you would be at your maximal metabolic rate if you were to run a marathon at
your fastest possible pace WITHOUT being overwhelmed by fatigue. Your metabolic rate during a
sprint (running as fast as possible and quickly gaining fatigue) would no be maximal metabolic rate,
since your metabolism would be mostly anaerobic. Anaerobic metabolism allows you to have a
metabolic rate higher than maximal, but only for a short time before becoming fatigued.
Maximal metabolic rate is usually proportional to basal or standard metabolic rate. Most animals can
maintain a maximal metabolic rate about ten times their “baseline” metabolic rate.
All organisms have energy budgets, with certain portions of their energy intake going towards
circulation, breathing, neural functions, courting behavior, chasing food, etc.
Two major bioenergetic strategies are endothermy and ectothermy. An ectotherm has a lower
metabolic rate and is less active. It can survive on a much lower rate of food and oxygen intake than
an endotherm. An endotherm has a metabolic rate about ten times greater than that of an ectotherm of
similar size. Their body temperatures are elevated by internal heat production. Endothermy demands
a high rate of food and oxygen intake, but also allows for a much higher maximal metabolic rate,
making sustained physical exercise possible (such as would be necessary for an extended chase after
prey, or away from a predator).
Larger organisms have more tissue and, not surprisingly, more metabolic activity than smaller
organism of similar type. However, metabolic rate is not directly proportional to body size (i.e.
metabolic rate ~ body size is not true). The actual proportionality measured among related species of
organisms is metabolic rate ~ body size0.7. To illustrate what this means, a mammals ten times larger
than another mammal does not have an MR ten times higher, but instead an MR only about five times
higher (100.7 = 5.0). This is called metabolic rate allometry. Allometry means that some
characteristic varies disproportionately with body size. An example of allometry is that a human’s
head does not grow as much as his body as he matures. This is why babies have large heads compared
to their bodies.
Metabolic rate allometry implies that larger animals have tissues that are less metabolically intense.
This means that larger animal need less food per unit body mass than smaller animals. Larger animals
also have slower heart rates since their tissues need less oxygen and nutrients per unit time. Larger
animals also metabolize drugs more slowly. If a dose of anesthetic is scaled directly from humans to
elephants and mice, it would kill the elephant and be insufficient to knock out a mouse.
Smaller animals lose heat more quickly because of their higher surface area to volume ratio. Why is
this not a sufficient explanation for metabolic rate allometry?