Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Gas Exchange - Animals

I.  Importance of gas exchange
            recall the equation for respiration:   (CH₂O)_n + O₂ � CO₂ + H₂O + ATP (energy)

Take-Home-Lessons:

1. respiration provides energy source for aerobic organisms = critical importance

2. two of four major components are gases

3. requires uptake of O₂

4. requires removal of CO₂

5. understanding gas exchange is important

II.  Primer on molecular movement

A.  Diffusion vs. Bulk Flow

• Diffusion � net movement of individual molecules from one area to another; typically [Hi] � [Lo]

• Bulk flow � mass movement of molecules from one area to another; typically follows pressure gradient; from hi P � lo P; examples:  toilet, faucet

Take-Home-Lessons:

1. Gases, like oxygen and carbon dioxide, enter and exit organisms, respectively, by diffusion. Insert diagram

2. Oxygen typically gets to the animal by bulk flow or other active mechanism (i.e., breathing, fish sweep water across gills, frog swallowing.  More on this below)

3. Carbon dioxide is typically removed by bulk flow

B.  Fick's Law

• Mathematically expresses factors that effect the rate of diffusion

• J = D A (C1 � C2)/L    where:  J = flux density (diffusion rate; mol/m2/s); D = diffusion coefficient (function of molecule and medium); A = cross-sectional area; L = path length 

• insert diagram

Take-home-lessons:  This equation tells us that for a given molecule diffusion rate is:

1. directly proportional to area of absorptive surface (A).  The greater the area for diffusion, then the greater the rate.

2. directly proportional to the concentration gradient(C1 - C2).  The steeper the gradient, or in other words, the greater the difference in concentration between two areas, the greater the rate of diffusion

3. indirectly proportional to the distance of travel (L).  The longer the distance of travel

4. related to the medium of travel (D).  The more viscous and dense the medium, the slower the diffusion rate (which would you rather swim laps in � a pool of water or maple syrup?)
   

III.  Biological Implications of Fick's Law:  A Large Surface Area (A)  is Required
     There are various solutions to this problem.  The key feature is increase the total surface for gas exchange (oxygen uptake, carbon dioxide loss).  This is another good example of surface-to-volume ratios.  As we learned, to increase surface area for a particular volume, a filament or flattened is the best shape to be � so, how do organisms accomplish this:

• Vertebrates:  lungs, bronchi, alveoli

• Fish � gills

• Insect � trachea

• Plants - broad leaves, spongy mesophyll

Interesting Detour � based on your knowledge of gas exchange and s/v ratios, explain why giant insects are just figments of a film-maker's imagination.

IV.  Biological Implications of Fick's Law:  There must be a short diffusion distance (L)
    There must be a short diffusion distance between the environment and inside the organism. Fick's law tells us that diffusion rate is inversely related to distance � the greater the distance, the slower the rate of diffusion.  In fact, diffusion is painfully slow over long distances.  But, how much slower?  Let's calculate the rate for glucose:           

insert calculation for glucose here

Take Home Lessons:  No individual gas absorbing surface is more than a few cells thick (e.g.. gills, lungs, sea cucumbers, hydra  � tubes with a central cavity bathed in fluid, sponges � lots of chambers; leaves are flat, thin)

V.  Biological Implications of Fick's Law:   A Large Surface Area Provides a Large Area for Desiccation.
   A paradox � in order to exchange gases for metabolism, animals (and plants) need to have a large surface area means that the areas through which water is lost is also increased.

Solutions: 

1. put gas absorbing surface in side a humid chamber (i.e., humans � lungs)

2. live in water (i.e., gills)

3. plants (waxy cuticle with holes)

VI.  Biological Implications of Fick's Law:  There must be a way to get gases to the absorbing surface

A.  Positive Pressure breathing
    Frogs - push air down throat, lower throat - air enters - raise up to push down throat (= bulk flow)

B.  Negative Pressure Breathing

• humans/vertebrates

• pumping mechanism

• active, requires energy (ATP)

• lower pressure in organisms, gases enter by bulk flow

• function of diaphragm and movable ribs

• tidal volume - air inhaled per breath

• vital capacity - total possible (female - 3400, male - 4800)

• residual capacity - amount leftover

• inspirational volume - additional amount inhaled

• expirational volume - additional amount exhaled

C.  Concerns

• protect the surface from dust and other particles � mucus

• surface tension of water - surfactant 

D.  Gills

• move water across gills

• bulk flow

VII.  Biological Implications of Fick's Law:  Mechanism to maintain a large concentration (C) gradient

A.  Ventilation vs. perfusion (insert diagram)

B.  Partial Pressure 

• ultimately determines diffusion (exchange) of gases

• gas concentration expressed as mole fraction (moles gas / moles misture; oxygen in air = 21%; carbon dioxide = 0.036% or 336 ppm)

• gas affected by pressure (which is related to altitude)

• partial pressure = mole fraction x total pressure 

• at sea level = 760- mm Hg.  Therefore 760 mm Hg x 0.21 = 160 mm Hg = PO₂

• note:  the concentration of oxygen in air is not dependent on the altitude independent, but the availability (i.e., partial pressure) is dependent.  Thus,  mountain climbers at high elevation require bottled oxygen.

C.  Counter-current mechanisms

• gills

• especially critical because oxygen has low solubility in water (10 ml/L) as compared to air (200 ml/L)

VIII.  Hemoglobin 

A.  Oxygen low solubility in water.

• Only small amounts in blood plasma
• Evolution of oxygen carrying pigments (= hemoglobin, myoglobin)
• kept in cells (red blood cells) to avoid impacting the osmotic concentration of the plasma

B.  Structure

• quaternary protein
• 4 chains - 2 alpha, 2 beta
• each chain associated with heme unit (prophyrin ring system with iron)
• each heme unit binds an oxygen molecule (O₂); 4 total oxygen molecules carried

C.  Saturation curve - oxygen saturation (%) vs. PO₂ (mm Hg)

• binding shows cooperativity - first slow, more readily absorbs others (positive cooperativity)
• P  about 40 mm Hg entering heart = 75% saturation
• P  about 100 mm HG leaving heart = 100% saturation
• only 25% of the oxygen normally used, other 75% in reserve

C.  Factors that affect oxygen/hemoglobin binding

1. Hemoglobin composition - fetal hemoglobin 2 alpha, 2 gamma chains (higher affinity for oxygen)
   
2. pH (Bohr effect) - pH plasma typically about 7.6; metabolism, etc may lower it some; hemoglobin has a lower affinitiy for oxygen at lower pH, releases more oxygen
   
3. 2,3 diphosophoglyceric acid (DPG) - product of glycolysis, high levels under exercise and/or elevation; DPG binds to hemoglobin (allosteric regulator), changes its shape, lowers its affinity for oxygen, releases more oxygen

IX.  Carbon Dioxide Transport

A.  Form

B.  Carbon dioxide and water

CO₂ + H₂O  � CO₂ (aq)  � H₂CO₃ (aq)  � H⁺ + HCO₃^-

C.  Carbonic anhydrase - catalyzes formation of bicarbonate, fast enzyme, reversible; changes partial pressure of carbon dioxide to load/unload from blood stream