Lecture 18: Animal
Structure and Function
1) organisms
have the capacity to adjust to environmental change over the long term and short
term by physiological responses (these traits evolved).
Over the long term, morphological traits have
evolved; for example, the long, tonguelike proboscis
of a hawkmoth is a structural adaptation for feeding.
For a physiological example, while most insects
are inactive when cold, the hawkmoth, Manduca sexta, can
forage for nectar when air temperatures are as low as 5oC.
The moth uses a shivering-like mechanism for
preflight warm up of its flight muscles.
Once in flight, the waste heat of metabolic
activity in the flight muscles and other adaptations maintain a muscle
temperature of 30oC, even when the external environment is close to freezing.
2) Tissues
are groups of cell with a common structure and function.
Different types of tissues have different
structures that are especially suited to their functions.
A tissue may be held together by a sticky extracellular matrix that coats the cells or weaves them
together in a fabric of fibers.
The term tissue is from a Latin word
meaning weave.
Tissues are classified into four main
categories: epithelial tissue, connective tissue, nervous tissue, and muscle
tissue.
Epithelia are classified by the number of cell
layers and the shape of the cells on the free surface.
A simple epithelium has a single layer of
cells, and a stratified epithelium has multiple tiers of cells.
The shapes of cells may be cuboidal
(like dice), columnar (like bricks on end), or squamous
(flat like floor tiles).
The major types of connective tissues in
vertebrates are loose connective tissue, adipose tissue, fibrous connective
tissue, cartilage, bone, and blood.
Adipose tissue is a specialized form of
loose connective tissues that store fat in adipose cells distributed throughout
the matrix.
Adipose tissue pads and insulates the body and
stores fuel as fat molecules.
Each adipose cell contains a large fat droplet
that swells when fat is stored and shrinks when the body uses fat as fuel.
Blood functions differently from other
connective tissues, but it does have an extensive extracellular
matrix.
Muscle tissue is composed of long cells
called muscle fibers that are capable of contracting when stimulated by nerve
impulses.
3) Homeostasis
In a negative-feedback system, a change in the
variable being monitored triggers the control mechanism to counteract further
change in the same direction.
Owing to a time lag between receptor and
response, the variable drifts slightly above and below the set point, but the
fluctuations are moderate.
Negative-feedback mechanisms prevent small
changes from becoming too large.
Most homeostatic mechanisms in animals operate
on this principle of negative feedback.
In contrast to negative feedback, positive
feedback involves a change in some variable that trigger mechanisms that
amplify rather than reverse the change.
For example, during childbirth, the pressure of
the babys head against sensors near the opening of the uterus stimulates
uterine contractions.
These cause greater pressure against the uterine
opening, heightening the contractions, which cause still greater pressure.
Positive feedback brings childbirth to completion,
a very different sort of process from maintaining a steady state.
Over the short term, homeostatic mechanisms can
keep a process, such a body temperature, close to a set point, whatever it is
at that particular time.
But over the longer term, homeostasis allows
regulated change in the bodys internal environment.
Internal regulation is expensive and animals use
a considerable portion of their energy from the food they eat to maintain favorable internal conditions.
4) Energetics
- Food
is digested by enzymatic hydrolysis, and energy-containing food
molecules are absorbed by body cells.
- Most
fuel molecules are used to generate ATP by the catabolic processes of
cellular respiration and fermentation.
- The
chemical energy of ATP powers cellular work, enabling cells, organs, and
organ systems to perform the many functions that keep an animal alive.
- Since
the production and use of ATP generates heat, an animal must
continuously loose heat to its surroundings.
- After
basic energetic needs are met, any remaining food molecules can be used
in biosynthesis.
- This
includes body growth and repair, synthesis of storage material such as
fat, and production of
reproductive structures, including gametes.
- Biosynthesis
requires both carbon skeletons for new structures and ATP to power their
assembly.
- The
amount of energy an animal uses in a unit of time is called its metabolic
rate - the sum of all the energy-requiring biochemical reactions
occurring over a given time interval.
- Energy
is measured in calories (cal) or kilocalories (kcal).
- A
kilocalorie is 1,000 calories.
- The
term Calorie, with a capital C, as used by many nutritionists, is
actually a kilocalorie.
- Metabolic
rate can be determined several ways.
- Because
nearly all the chemical energy used in cellular respiration eventually
appears as heat, metabolic rate can be measured by monitoring an
animals heat loss.
- A
small animal can be placed in a calorimeter, which is a closed insulated
chamber equipped with a device that records the animals heat loss.
- A
more indirect way to measure metabolic rate is to determine the amount
of oxygen consumed or carbon dioxide produced by an animals cellular
respiration.
- There
are two basic bioenergetic strategies used
by animals.
- Birds
and mammals are mainly endothermic, maintaining their body
temperature at a certain level with heat generated by metabolism.
- Endothermy is a high-energy strategy that permits
intense, long-duration activity of a wide range of environmental
temperatures.
- Most
fishes, amphibians, reptiles, and invertebrates are ectothermic,
meaning they do not produce enough metabolic heat to have much effect on
body temperature.
- The
ectothermic strategy requires much less energy
than is needed by endotherms, because of the energy cost of heating (or
cooling) an endothermic body.
- However,
ectotherms are generally incapable of intense activity over long periods.
- One
of animal biologys most intriguing, but largely unanswered questions
has to do with the relationship between body size and metabolic rate.
- Physiologists
have shown that the amount of energy it takes to maintain each gram of
body weight is inversely related to body size.
- For
example, each gram of a mouse consumes about 20 times more calories than
a gram of an elephant.
- The
higher metabolic rate of a smaller animal demands a proportionately
greater delivery rate of oxygen.
- A
smaller animal also has a higher breathing rate, blood volume (relative
to size), and heart rate (pulse) and must eat much more food per unit of
body mass.
- Every
animal has a range of metabolic rates.
- Minimal
rates power the basic functions that support life, such as cell
maintenance, breathing, and heartbeat.
- The
metabolic rate of a nongrowing endotherm at
rest, with an empty stomach, and experiencing no stress is called the basal
metabolic rate (BMR).
- The
BMR for humans averages abut 1,600 to 1,800
kcal per day for adult males and about 1,300 to 1,500 kcal per day for
adult females.
- In
ectotherms, body temperature changes with temperature of the
surroundings, and so does metabolic rate.
- Therefore,
the minimal metabolic rate of an ectotherm must be determined at a
specific temperature.
- The
metabolic rate of a resting, fasting, nonstressed
ectotherm is called its standard metabolic rate (SMR).
- For
both ectotherms and endotherms, activity has a large effect on metabolic
rate.
- Any
behavior consumes energy beyond the BMR or SMR.
- Maximal
metabolic rates (the highest rates of ATP utilization) occur during peak
activity, such as lifting heavy weights, all-out running, or high-speed
swimming.
- Different
species of animals use the energy and materials in food in different
ways, depending on their environment, behavior, size, and basic energy
strategy of endothermy or ectothermy.
- For
most animals, the majority of food is devoted to the production of ATP,
and relatively little goes to growth or reproduction.
- However,
the amount of energy used for BMR (or SMR), activity, and temperature
control varies considerably between species.
- For
example, the typical annual energy budget of four vertebrates reinforces
two important concepts in bioenergetics.
- First,
a small animal has a much greater energy demand per kg than does a large
animal of the same class.
- Second,
an ectotherm requires much less energy per kg than does an endotherm of
equivalent size.
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