|Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; firstname.lastname@example.org; http://www.employees.csbsju.edu/ssaupe/|
Animal Structure & Function
(including temperature regulation & size)
I. Central Themes of Biology. There are several major themes that we have stressed throughout the year. Some of these themes that are featured in this unit include:
II. Revisiting the hierarchy of life. Last semester, during one of our first classes we discussed the hierarchical organization of matter in the universe, from atoms to the biosphere? Let's focus in on just a few levels:
general: cell � tissue � organ � organ systems
example: neuron � nervous tissue � brain � nervous system
We will take a reductionist approach in our studies, in other words, we will usually break our systems into small units to make our studies more feasible. Although this approach can be quite productive, we can (and do) loose some information/perspective. Aristotle said it well, "the whole is greater than the sum of its parts".
III. Animal Tissues.
They are derived from the embryo, which in turn, is derived from the zygote. In other words, thank your mom and dad who provided your DNA which ultimately is the product of our evolutionary history.
B. Tissues are aggregates of cells.
Obvious, yes. But, how do these aggregates stick together? Some mechanisms include: (1) many cells have "sticky" surfaces (recall the glycocalyx); (2) woven together (note: the Latin word for tissue is weave); and (3) chemical interactions between membranes. As an aside, the failure of tumor cells to stick together is what causes some cancers to spread (metastatic) while others dont (benign).
C. Tissue Types.
There are four major types of animal tissues; nervous, muscle, connective, and epithelial.
IV. Nervous Tissue.
Duh, yup. Dis is da stuff en yur brayn. The function of nervous tissue is to sense stimuli and transmit signals. The functional unit is the neuron (axon, dendrite, cell body). Glial cells surround and support the neurons.
V. Muscle Tissue.
These tissues are made of elongated cells, are excitable and can contract. There are three major types of muscle tissue:
VI. Epithelial Tissue - sheets of cells
VII. Connective Tissue
A. Serves to bind and support other tissues. Connective tissues are comprised of an assortment of cells that are sparsely scattered through a gel-like matrix containing protein fibers.
B. Cell Types. Some of the cells found in connective tissues include:
C. Protein Fibers - There are several types:
D. Types of connective tissues. There are a variety of types of connective tissues, some obviously "connective" and the others not so. These include:
VIII. Homeostasis - "Life Exhibits Homeostasis"
An animal is separated from its external environment by some sort of boundary. The external and internal conditions are usually different. Thus, this is evidence that an organism is able to exert control over its internal conditions. Further, the internal conditions are usually remarkably stable and predictable. For example, blood pH (7.4) and body temperature (37 C) vary little. The ability/result of an individual to maintain a constant internal environment within tolerable limits is termed homeostasis. Or in other words, maintaining a "constant internal milieu" in light of environmental fluctuations.
IX. Temperature Regulation
A. Life and Temperature.
- Q10 = rate at temperature T / rate at temperature T - 10
- biological processes (enzymatic reactions, biochemical reactions, metabolism, etc.) have a Q10 between 2-3
- physical processes (water uptake, heat loss/gain) have a Q10 about 1
insert: graphs of body temp vs external temp and
metabolic rate vs temp for mouse and lizard
Take Home lessons from the Graphs:
C. Mechanism of Thermoregulation
Organisms modify behavior to cool/heat body as necessary (eg. lizards bask in sun, hide in burrow; elephants shower with water, wallow in mud/water)
2. Body Morphology.
Environmental temperature has directed evolution of body shape/size. Two major concerns:
- surface - to - volume ratios. (Click here for more on s/v ratios). effect of size, shape (flattening, stretching, cube vs. sphere vs. filament)
- adaptations - fur, feathers, fat
3. Physiological Adaptations
- iguana - brings blood to surface, adjusts heart rate
- cold vs hot fish - countercurrent mechanism
- increase respiratory activities (shivering, fluttering wings, muscles)
- brown fat - rich in blood vessels & mitochondria; thermogenin uncouples ATP production from electron transport
D. Vertebrate Temperature Regulation.
The basic regulatory system that allows animals to respond to their environment, which is essentially the same as in plants, is:
signal � receptor � control center or integrator � effector � response
To summarize, an environmental signal of some sort is received by a receptor which in turn relays the message to a control center that selects an appropriate response by the effector.
Lets use temperature as an example. Imagine the thermostat in your home that is set for a comfortable 21 C. A thermostat (receptor) monitors the temperature (signal). The thermostat has a mechanism (control center) for turning off/on the air conditioner or furnace (effectors) as needed. Thus, if it is a hot day and the temperature in your home increases above the set point (i.e., 21 C), then the thermostat switches on the air conditioner. Once the temperature drops, the thermostat turns off the air conditioner. This results in a relatively constant temperature that will fluctuate slightly around the set point since it takes the system a brief period to respond to the temperature changes.
If we get too hot, blood temperature (signal) is monitored by the hypothalamus (receptor), which triggers the anterior pituitary (control center) to send a signal to the sweat glands (effector) to "chill out" (i.e., start sweating and dilate blood vessels).
If we get too cold, blood temperature triggers the hypothalamus to send a signal to the nerves to tell the blood vessels to constrict, increase shivering, and increase the rate of metabolism (done through the pituitary and thyroid glands).
Note, feedback mechanisms are important aspects of regulatory control. Feedback can be
positive (increase in the signal causes an increased response) or negative (increase in
the signal causes a decrease in response).
X. Animal shape - the correlation of structure and function
A. Animals and motility.
Did you ever wonder why animals can move around but plants cannot? The answer is simple - to obtain food. Recall that animals are heterotrophic and must obtain organic compounds (food) from their environment. Since their food is scattered around in the environment, it was necessary to move to get it. In contrast, plants never had an evolutionary pressure for "motility" since their essential nutrients (water, ions and carbon dioxide) are "omnipotent". To support this idea consider some non-motile animals like coral, hydras, and sea fans. They all live in aquatic environments which enables them to "feed like a plant" - their food is essentially brought to them via water currents. Thus, they never had any pressure for motility - and interestingly, they have very similar lifestyles/forms as plants.
B. Animals are square, or, Body Shape as a consequence of motility.
Remember our surface-to-volume ratio studies when we learned that a plant can be considered a cluster of filaments whereas the body plan of an animal is like a square? The reason - for a given volume, a square has less surface area than a comparable-sized filament. Animals evolved to have a minimal s/v which reduces frictional resistance and is easier to move around.
X. Constraints on animal size.
A. Animals have a mechanical design.
In other words, they are constructed like a machine, made of numerous, different parts that function together. The parts are highly integrated. Parts cannot be added or removed without reducing the efficiency of the operation of the whole. This makes for a more streamlined body design for motility.
In contrast, plants have an architectural design. In other words, the plant body is constructed like a building. Essentially a plant is a modular unit made of a limited number of parts, each of which is relatively independent from the others but are united into a single structure. Thus, just like a building is made of rooms, the leaves, stems and roots of a plant are analogous to the rooms in a building. Each room is somewhat independent, yet they all function together to make an integrated whole. You can seal off a room in a building, or remove a leaf or fruit, with little harm to the overall integrity of the structure. This is critical for plants to be able to add or remove parts (leaves, stems, flowers, fruits) as necessary.
As a result of body design is animals are limited by size and cannot change shape, in contrast to plants. These abilities are important to non-motile organisms like plants to be able to colonize and exploit new areas for resources, but are disadvantageous to an animal because they will make motility more difficult and less predictable.
B. Animals exhibit determinate growth.
This is the process by which an organism or part reaches a certain size and then stops growing. In contrast plants exhibit indeterminate growth and continue to grow and get larger throughout its life cycle. Again, its no surprise that animals are limited by size but plants are not.
XI. What determines the upper size limit for an animal?
Most of the biggest animals are herbivores, or at least feed low on the food chain because of the larger amount of energy available.
No matter what size, an animal must support its body. In water this is less of a problem than on land because of the bouyancy of water. Thus, its no surprise that most of the earths largest animals are aquatic or semi-aquatic (i.e, whales, hippos, brontosaurus).
Terrestrial animals must support their bodies and must fight the force of gravity. One function of leg bones is support. Mechanical engineers know that the height/mass of material that can be supported is a function of the cross sectional area of the support. Thus, the greater the support area, the greater the height/mass that can be supported. Hence, elephants have fatter bones than shrews.
However, once again recall our discussion of s/v ratios - we concluded that as an object gets larger, its surface area increases as the square of the linear dimension whereas the volume increases as the cube. Thus, volume increases more rapidly than surface area. Consider a typical adult male (6 foot tall and 200 pounds). If we assume the femur of this guy is 5 cm in diameter, then the cross sectional area is roughly 20 cm2 (= pi r2). Now, lets compare this to a 60 foot tall Brobdingnagian from Gullivers Travels. Since the Brobdingnagian is 10x taller than a typical man, then his mass is 103 times, or 1000x, greater. Thus, although Jonathan Swift doesnt tell us, a Brobdingnagian would weigh about 200 tons (200 pounds x 1000 = 200,000 pounds). Since the cross sectional area increases by the square, we expect the area of the femur to be 102 or 100x larger. Or in other words about 2,000 cm2 (20 cm2 x 100). Thus, the femur of these giants will have to support 10x more weight than a typical male - which, a bone cant do. Oops honey, our enlarged kid has broken bones!
In short, it is impossible to have normal human proportions carry such a heavy mass. But, then how do some animals get large? (1) shorten the length of the leg bones (think hippos and elephants), (2) smaller head (less mass to support), (3) shorter and fatter neck (compare gazelle, horse, elephant); (4) live in water for support (see above); (5) adapt their posture - a more upright position allows a greater weight to be supported
XII. Size and structural organization.
To get larger an organism can either: (a) increase the size of its cells; or (b) increase the number of its cells. Recall our s/v class where we learned that cells are small (about 100 μm in diameter) and why they are small? Cells must remain small in order to function. Thus, our take-home-lesson was that bigger organisms have more cells than smaller ones. Or, stated another way, an increase in size is accompanied by an increase in the number of cells in the organism. Thus, elephants have more cells than a shrew. In evolutionary terms, getting larger permitted (and required) an increase in complexity (specialization) of organisms.
Again recall that increasing the size of an object decreases it s/v ratio. Thus, among the changes in form that accompany an increase in size are mechanisms to increase s/v ratios. Some examples: (1) lungs increase surface area for gas absorption; (2) guts are long and thin tubes for increased surface area; (3) the brain is convoluted to get rid of excess heat (an overheated brain is an ugly sight); (4) one way that Neanderthals differ from modern humans in that their skulls had bony projections into their nasal cavity. These apparently functioned to increase the surface area to allow for more rapid warming of air before it reached the lungs (Discover March 1997).
XIII. What determines the lower size limit for an animal?
Obviously, the smallest animal possible can be no smaller than a single cell. However, lets consider mammals, and more specifically small humans like the Lilliputians in Gulliver's Travels. Can such small humans exist? To answer this question lets consider intelligence. As we mentioned above, a smaller animal has fewer cells than a larger one. Thus, a Lilliputian would have a brain made of many fewer cells than Gullivers or our own. At a certain point there would be too few cells to allow for human-like intelligence. Further, Lilliputian vision wouldnt be too good either. As the eye gets smaller the number of rods/cones also declines decreasing visual acuity. This explains why cute little mice have such large eyes compared to their body size. But, why do elephants have such small eyes in comparison to size? As the eye gets larger it will let in more light. At a certain point, so much light will enter that it "blinds" the individual.
Oops honey, I shrunk my brain.
XIV. Metabolism as a function of size.
The metabolic rate of an animal refers to the amount of energy an animal uses in a given period of time. It can be measured by monitoring the: (a) amount of oxygen consumed by the animal in a respirometer; or (b) heat lost by an animal in a calorimeter. These studies show that there is an inverse correlation between animal size and metabolism. Or, more simply stated, larger animals have a lower rate of metabolism than smaller ones. Why?
At least for endotherms, the answer relies on s/v ratios. Remember that during our s/v studies we concluded that small objects have a larger s/v than larger objects of the same size. Thus, a smaller animal will loose heat more for its size than a larger one (remember Goldilocks? block vs. cube ice?). Thus, it is necessary to metabolize faster to make up for the heat that is lost through the surface.
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