Respiratory System - GasExchange DiffusionistheKey Unlike...

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    Gas Exchange  Being an exercise in discovering  the limits of diffusion  
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    Diffusion is the Key Unlike dissolved solutes (inorganic ions, small proteins, sugars, etc.) the two principle respiratory gases cannot be transported across cell membranes. Thus, uptake and release of respiratory gases will always be along concentration gradients…animals and plants cannot transport gases against a concentration gradient! Multicellular organisms have evolved multiple morphological and physiological mechanisms to promote and enhance uptake/release of respiratory gases.
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    Respiratory Gases Cellular respiration = metabolism: C 6 H 12 O 6  + 6O 2    6CO 2  + 6H 2 O + ATP O 2  and CO 2  are the respiratory gases,  and physiological respiration = uptake  and transport of these two gases  through the organism
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    Solubility All metabolism occurs in aqueous environment,  hence these gases need to be in dissolved  state Oxygen has much lower solubility in water than  carbon dioxide, hence CO 2  moves from gas  to liquid (and vice versa) much faster (try  shaking a soda can, then open!).  Thus  respiratory surfaces are adapted for uptake of  oxygen more so than release of CO 2
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    Gas Movement in Plants
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    Stomata: Gateways for gas diffusion Stomata are pores on the undersurface of  leaves that allow carbon dioxide to diffuse into   the interior of the leaf, and oxygen to diffuse  out .  Stomata are necessary as leaf cuticles  are impermeable to gases (e.g. water vapor)
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    Photosynthesis is proportional  to diffusion of CO 2
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    Block the stomata with grease, diffusion of CO 2  ceases, as does photosynthesis!  Plants do need open airways, too
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    Roots? Root cells can only respire, thus need  supply of oxygen Typically, plant roots grow only to the  depth of the aerated layer of soil, that is,  the depth to which air can penetrate soil  until it reaches the water table
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    Some soils are oxygen poor Mangroves grow in soils that are fully  saturated with water, hence anoxic.   Oxygen is provided by  means of “living  snorkels,” called pneumatophores. These roots serve multiple functions: prop  up the tree, extract nutrients from the soil,  and act as conduits for the movement of  gases.
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    Rooted Aquatic Plants The roots of these water lilies are anchored in the bottom of the pond, and in  an environment that lacks oxygen.  Tubes in the stem allow oxygen to diffuse  down to the roots.
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    Flooding Vascular cylinder Air tubes Epidermis 100 μ m 100 μ m (a) Control root (aerated) (b) Experimental root (nonaerated) Many plants can respond to flooding by enzymatically destroying cells to create  pathways to supply oxygen to their roots. 
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