Renner text: Life 5th Edition - Purves..: Read Chapter 45 for this lecture

General Biology2

Spring 2000 end

Lecture Outline #5                                                                                            



I.  Gas Exchange in Animals  (Chapter 45)




Organic molecules must be broken down to yield energy.  The energy produced is captured and
stored in the form of ATP.  The production of ATP is an absolute requirement for all cells.  The
most direct source of energy (ATP) comes from the degradation of glucose.  Glucose degradation
to produce ATP can proceed in the presence or absence of oxygen.  Both pathways for ATP
production begin with glycolysis.




 A.  	Anaerobic respiration 



	Anaerobic respiration refers to the production of energy in the absence of oxygen.  The
process is used by some unicellular organisms that can survive in the absence of oxygen. 
Anaerobic respiration is also used by some organisms that can survive periods of oxygen
deprivation and by certain cell types, such as skeletal muscle.  There are two primary
products of anaerobic respiration.



	1.	glucose + 2Pi +2ADP             lactic acid +2ATP + 2H2O



	2.	glucose + 2Pi +2ATP              ethanol + 2ATP + 2CO2 + 2H2O




  B.  	 Aerobic Respiration




	Aerobic respiration refers to cellular respiration in the presence of oxygen.  Following
glycolysis, pyruvic acid is converted to acetyl CoA whcih is further oxidized in the Kreb
cycle.  




	glucose + 2NAD + 2Pi + 2ADP           2 pyruvic acid + 2NADH + 2ATP




	The pyruvic acid enters the Kreb cycle where molecules (which happen to be B complex
vitamins FAD, NAD) are reduced during enzymatic reactions.  The reduced molecules
enter the oxidative phosphorylation pathways to yield additional ATP.  Taking glycolysis
and the Kreb cycle together, the complete oxidation of 1 molecule of glucose can be
written:




	glucose + 6O2                         6CO2 + 6H2O + 36ATP




	Note that aerobic respiration provides a much more efficient way of producing energy.  In
order to utilize aerobic respiration, an organism must have a means of obtaining oxygen
and delivering it to the tissues.










II.  Comparison of Oxygen Properties in Air and Water Mediums		




  A.	At a given partial pressure of oxygen, much less oxygen will be present in water.  Oxygen
is soluble in water but solubility is limited.
                                                                                                                             
  B.	Gases diffuse faster in air than in water




  C.	Water is more viscous and dense than air.  An organism must expend greater energy to
move a given volume of water through the respiratory passages than air breathing
organisms.



  D.	In both water and air mediums, there is always a membrane separating the cell (in the case
of unicellular organisms) or tissues from the medium.  The movement of gases across the
membrane is by diffusion and depends on the concentration gradient.




III.  Gas Exchange in Protozoans and Small Multicellular Aquatic Organisms




In these organisms, gas exchange is by diffusion between the individual cells and the external
medium.  These organisms do not require specialized structures for gas exchange since the cell(s)
are either in contact with the external medium or are only a few cell layers away.




IV.  Gas Exchange in Larger Organisms




In larger organisms, diffusion of gases from air or water to individual cells would be ineffective.




  A.	With increased size, the increase in surface area available for gas exchange is much less
than the increase in volume.




  B.	Organisms with body volumes greater than a few cell layers thick would never receive
adequate amounts of oxygen from the external environment.  Similarly, removal of carbon
dioxide would present a problem.




  C.	Most organisms have evolved relatively impermeable external body surfaces (such as skin
or scales) further reducing the surface area available for gas exchange.




Larger animals have evolved specialized regions for gas exchange or true respiratory systems to
solve these problems.




V.  Gas Exchange in Aquatic Organisms 




The gill is the respiratory exchange organ in most aquatic organisms.  A gill is any type of
evaginated structure specialized for gas exchange.  




A.  Gills may be located on:						



	1.  body surface (echinoderms such as starfish)
	2.  thoracic appendages (crustaceans, marine annelids)
	3.  mantle cavity (mollusks such as clams or  squid)
	4.  oral cavity (fish)

B.  Gills have several features in common:



	1.	Most gills have finely divided surfaces to increase the surface area available for gas
exchange.  The divisions are thin plates of tissue called lamellae.




	2.	Gills often have a protective covering such as the operculum in fish.




	3.	Gills are highly vascularized to facilitate gas exchange between water, the gill
surface and blood vessels.




	4.	In general,  a mechanism is available to move water across the gills to increase the
rate of gas exchange.




C.  Fish as a Representative Gill Breather




Fish live in an environment that has low oxygen concentrations, limited oxygen diffusibility and
high viscosity when compared to an air environment.  Fish must expend considerable energy for
respiration.  The countercurrent exchange mechanism has evolved to improve the efficiency of 
oxygen transfer from the water to the blood.  In the countercurrent exchange system in the
lamellae of the fish gill, water and blood flow in opposite directions (figs. 38.18, 38.19, pg. 839).  




   1.	Water saturated with oxygen (100%) initially encounters blood which is nearly saturated
with oxygen (90%) when the water initially enters the gill apparatus.  Even though the
blood is partly saturated with oxygen the concentration gradient favors oxygen diffusion
into the blood. 




   2.	As the flow of water progresses across the gill, a concentration gradient is maintained that
always favors oxygen diffusion to the blood improving the efficiency of oxygen uptake.




   3.	Carbon dioxide is also removed from the blood through countercurrent exchange in most
fish.




   4.	A comparison of the counter current exchange mechanism to a concurrent (same direction
flow) system illustrates the advantage of countercurrent flow.  In a
concurrent system, the water and blood reach a point where the level of oxygen saturation
is essentially equal.  The diffusion gradient favoring oxygen diffusion to the blood is lost
and the blood is only partly saturated.




D.  Alternates to Gills in an Aquatic Environment					




Although most aquatic organisms use some form of gill respiration, there are a number of
exceptions.  For example, one group of echinoderms (the sea cucumbers) exchange gas through a
series of tubules called the respiratory tree.




IV.  Respiration in Terrestrial Organisms 




The transition from water to air presents different problems for an organism.  In both mediums a
wet or moist respiratory surface must be available and the respiratory surface must be thin enough
and have sufficient surface area for adequate exchange of gases.  In an aqueous environment,
where buoyancy and water viscosity serve as support, gills work well for oxygen extraction at a
high metabolic cost to the animal.  In air, gills do not function well:




   1.	Gills are not sufficiently rigid to stand up to atmospheric pressure in the absence of water.




    2.	Gills are subject to desiccation with exposure to water.




Air is an oxygen rich environment and the metabolic cost for gas exchange is much lower in
terrestrial organisms.  Most terrestrial animals have evolved invaginated respiratory structures. 
Invaginated respiratory structures have two major features:




    1.	The air inside the system is moist.  In vertebrates, the air is saturated with water at body
temperature.




    2.	The respiratory surface is not freely assessable to the outside air to minimize water loss
due to evaporation.

Respiration in Vertebrates with Lungs

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