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Principles of Gas Exchange 1: Animals Breathing in Water summarized lectures notes along with relevant pictures and notes from the assigned readings

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Biology 2601A/B
Brent Sinclair

Organismal Phys Lecture 15 Gas Exchange 1: Principles & Animals Breathing in Water  In general, it is important to note that metabolism and photosynthesis are dependent on O₂ and CO₂ exchange with the environment Gas Exchange in Respiration  Involves getting O₂ out of the external medium and into the cells o This is often done via the circulatory system  Also involves getting CO₂ out of the cells and into the external medium Gas Exchange in Photosynthesis  Getting CO₂ out of the external medium and into the cells o Usually via diffusion or stomata  Getting O₂ out of the cells and into the external medium o Or can also be used in respiration  Remember: water vapour is also a gas (important when considering water loss in plants)  Although this lecture discusses O₂, the principles also apply to CO₂ and water vapour Where is the Oxygen?  Most of the O₂ is in the air rather than in water o Cold sea water: Approximately 0.8% O₂ in solution o Air: Approximately 21% O₂ as a gas  The amount of O₂ that is able to dissolve in water is dependent upon temp o Warmer water temperature = less O₂ dissolved  Solutes also affect O₂ dissolving Partial Pressure  Each gas in a mix (either as a gas or in solution) exerts pressure  Partial pressure is the amount of pressure that the gas of interest exerts Partial Pressure of Gas X Partial Pressure  Air has o 20.95% O₂ o 78.09% N₂ o 0.93% Ar o 0.038% CO₂  Ex. PO₂ = 0.2095 x 101.3 kPa = 22.22 kPa  Note that when altitude changes (ex. when on Mt. Everest) the proportional composition of the air is still the same; the thing that differ is the air pressure  At higher altitudes = lower pressure o Remember that partial pressure will influence the rate of diffusion across the cell Gases Dissolve in Liquids  Not the same as having air bubbles  P(liquid) is proportional to P(air)  The amount of gas in a solution will depend on o 1. Temperature o 2. Salinity o 3. Gas  Gases that have reacted chemically do not contribute to partial pressure in solution  Under high partial pressures there will be more gases in liquid Henry’s Law  States that the amount of gas that will dissolve in a liquid is determined by the partial pressure of the gas and the solubility of the gas in the liquid All Gases are not Equal  Note that CO₂ is extremely soluble in comparison to the others  CO₂ > O₂ > Ar > N₂ Diffusion of Gases: The Fick Equation Diffusion Coefficient  Depends on gas, temperature, and medium  Also depend on the permeability of any barriers (ex. cell membrane, epidermis)  Diffusion is much faster in air than it is in water Putting it All Together  Note that D, S(gas) and √MW are all fixed for CO₂ and O₂; the organism can’t modify any of these things to alter its O₂/CO₂ diffusion  A , ∆P and X are all able to be modified by the structure and function of an organism Convection Can Break Dependence on Diffusion  Convection is a mechanism of transport resulting from the bulk motion fluids  Note that convection can also serve to ↑ diffusion Gas Transport in Animals is Often a Combination of Convection and Diffusion  Tidal convection ventilates the lungs  Unidirectional flow (convection) in the circulatory system  Diffusion from the capillaries into the tissues  Diffusion from the lungs into the bloodstream Breathing Water  Getting rid of CO₂ isn’t the problem since there is a high diffusivity and absorption; however getting O₂ to these organisms is (low absorption in H₂O = low PO₂)  The surface area of the lungs of water-breathing organisms is much higher than that of air- breathing organisms  Two Mechanisms o 1. Fast Ventilation – more water across the respiratory surface = more O₂ to absorb ; this is done through convection o 2. Efficient Absorption – a highly vascular
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