40 Vert structure_function 2009

40 Vert structure_function 2009 - Biol 61 Animal Structure...

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(Chapter 40) 3/30/09-4/1/09 For this chapter be sure to focus on the topics that I discussed in class and here in the notes (for example, we will not discuss circulatory adaptations here or some of the details of thermoregulation) Animal Body Plan Why are organisms built the way they are and work the way they do? The laws of physics and chemistry set limits to body plans, and force organisms to evolve organs and organ systems to deal with some of the limitations imposed on them by these laws. For example, the shape of fast swimmers such as fish, dolphins, icthyosaurs, seals and penguins is similar (called fusiform) even though the organisms are all quite unrelated. This is due to the effects of water (drag) on movement, and how organisms have evolved to minimize drag (fig. 40.2). The size of an organism is also affected by physical laws. There is a lower limit to the size of an organism that is dependent on Brownian motion (too small and you would just be torn apart by random molecular movement). The upper limits to an organism’s size are very dependent on the relationship between the surface area of an organism and its total volume. The volume always increases more dramatically as a cell increases in size. This matters because the surface area of a cell – its plasma membrane – is the site where compounds need to enter and exit the cell. Oxygen, carbon dioxide, sugars, and waste products enter and exit cells through their surfaces. The demand for these compounds increses as volume increases, but the surface area that is needed does not increase at the same rate (and isn’t large enough to supply the cell). Once a compound is inside a cell, how fast the compound gets to where it is needed is limited by diffusion. If the cell’s volume is too large, it can take too much time for oxygen to efficiently get to the mitochondria, or for the first enzyme in glycolysis to bump into a glucose molecule, especially if the surface area in not large enough to let in sufficient quantites. Whole multicellular organisms work under the same principles. Even though they are made up of small individual units (cells), a layer of cells that is 10 cells thick still has some of the same problems with diffusion that a single large cell would have. Compounds may have trouble diffusing into and out of the inner cells of a thick layer of cells, since they are far away from their environment – where oxygen and sugars are and where waste needs to be dumped. To get around this, The simplest multicellular animals tend to have fairly thin tissues, while other animals try to have thin layers of tissue and to increase the surface area of internal organs and organ systems that are specially designed to interact with the environment (digestive & respiratory systems, most especially – see Fig. 40.4).
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This note was uploaded on 01/15/2011 for the course BIOL 61 taught by Professor Vierra during the Spring '08 term at Pacific.

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40 Vert structure_function 2009 - Biol 61 Animal Structure...

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