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bioa02 chapter 48

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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

Chapter 48 48.1 What physical factors govern respiratory gas exchange? - the respiratory gases that animals must exchange are oxygen and carbon dioxide - cells need to obtain oxygen from the environment to produce an adequate supply of ATP by cellular respiration - there are no active transport mechanism to move respiratory gases across biological membranes o since diffusion is a physical process, knowing the physical factors that influence rates of diffusion helps us understand the diverse adaptations of gas exchange systems - since diffusion results from the random motion of molecules, the net movement of a molecule is always down its concentration gradient - barometer pressure at sea level is 760 mmHg - the partial pressure of oxygen at sea level is 20.9% of 760 mmHg 159 mmHg - the actual amount of a gas in a liquid depends on the partial pressure of that gas in the gas phase in contact with the liquid as well as on the solubility of that gas in that liquid - Ficks law of diffusion all environmental variables that limit respiratory gas exchange and all adaptations that maximize respiratory gas exchange are reflected in one or more components of this equation - Oxygen can be obtained more easily from air than from water for several reasons o The O2 content of air is must higher than the O2 content of an equal volume of water o Oxygen diffuses about 8000 times more rapidly in air than in water thats why the O2 content of a stagnant pond can be zero only a few millimeters below the surface o When an animal breathes, it does work to move water or air over its specialized gas exchange surfaces. More energy is required to move water than to move air since water is 800 times more dense - Diffusion of O2 in water is so slow that even animal cells with low rates of metabolism can be no more than a couple of millimeters away from a good source of environmental O2 o Therefore there are severe size and shape limits on the many species of invertebrates o A critical factor enabling larger, more complex animal bodies has been the evolution of specialized respiratory systems with large surface areas for enhancing respiratory gas exchange - Most water breathers are ectotherms their body temperatures are closely tied to the temperature of the water around them - As temperature gets warmer, their body temperature and metabolic rate rise, making them need more O2
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