Introduction to Organismal Biology (BIOL221) - 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 in Plants & Animals

I.  The importance of gas exchange
            Recall the equation for photosynthesis & respiration:  

(CH₂O) _n + O₂ �� CO₂ + H₂O

 Take-Home-Lessons:

1. Energy production/liberation requires the exchange of gases with the environment
2. two of four (50%) major components of these processes are gases
3. photosynthesis requires uptake of CO₂ and releases O₂; respiration requires uptake of O₂ and release of CO₂
4. In order for plants to obtain carbon dioxide for photosynthesis, or both animals and plants to obtain oxygen for respiration, water loss will occur.  Thus, there must be a compromise between desiccation and gas uptake.

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 cells, respectively, by diffusion.  This requires a thin absorbing surface
2. Animals rely on bulk flow (or other active mechanism) to obtain oxygen and release carbon dioxide (hence breathing, fish sweep water across gills, frog swallowing)
3. Plants rely on diffusion to obtain carbon dioxide and release oxygen (hence leaves are thin)

B.  Fick's Law 

• Mathematically expresses factors that effect the rate of diffusion
• can be written in various forms
• J = k A (C₂ � C₁)/D    where: 
         J = flux density or diffusion rate in units of mol/m²/s)
         k = diffusion coefficient (function of molecule and medium)
         A = cross-sectional area available for gas exchange
         D = distance or path length of diffusion
         C₂ � C₁ (or P₂ � P₁ ) = concentration gradient or partial pressure gradient

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

III.  Biological Implications of Fick's Law:  A large surface area (A) for gas exchange is required
     There are various solutions to this problem.  The key feature is increase the total surface area 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 flat are the best shapes to be � so, how do organisms accomplish this:  

• Plants - broad leaves, spongy mesophyll
• Vertebrates:  lungs, bronchi, alveoli
• Fish, crayfish, nudibranchs � gills (external or internal)
• Insects � tracheae
• Note � small animals like insects and earthworms didn�t require the evolution of elaborate mechanisms for gas exchange; since they have a high SV ratio, they are able to absorb necessary gases through skin/body surface
• 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 (D)
    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?  As an example, the time it would take glucose to cross a typical membrane is 2.5 seconds.  However, the time it would take glucose to diffuse one meter is 32 years!

Take Home Lesson:  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 a large surface area, BUT, that means that the area through which water is lost is also increased.  This problem is exacerbated by the diffusion tendencies of water, carbon dioxide, and oxygen.  Diffusion is inversely related to molecular weight � this makes senses, football lineman can�t run as fast as a wide receiver.  Thus, the tendency of water [MW = 18] to evaporate from a plant is much greater than the tendency of carbon dioxide [MW = 44] to diffuse into the leaf for photosynthesis.

Solutions: 

• put gas absorbing surface in side a humid chamber (e.g., humans � lungs)
• live in water (e.g., gills)
• cover your body with a water impervious layer (e.g., plants - waxy cuticle with holes, insects)

VI.  Biological Implications of Fick's Law:  There must be a way to get gases to the absorbing surface.
    This can be a particular concern in species that stick the gas absorbing surfaces deep within the organism.  Remember that the two major ways of moving any molecules, including gases, are diffusion and bulk flow.  Ultimately organisms use one or a combination of these methods to get the gases to the absorbing surface.

A.  Diffusion Methods
    Diffusion is the primarily method of moving gas to the absorbing tissue in plants and some animals.  Similarly, oxygen reaches the lungs of slugs and spiders, and enters the tracheae of insects via diffusion.

    Bulk flow may play a role in some of these cases.  For example, studies on hawk moths have shown that when wing muscles beat that alternately contract the tracheae which will cause pressure changes resulting in the increased uptake of gas by bulk flow.  Gas moves through some plants via bulk flow, also.  Imagine a steady wind blowing across the upper surface of a leaf; this will essentially generate a slight negative pressure compare to the lower side of the leaf resulting in gas molecules being �sucked� through the leaf.

B.  Bulk Flow Methods
    Most organisms rely on bulk flow for moving gases to their respiratory surfaces.  These mechanisms are �active� and require cellular energy (ATP).  There are several methods:

1. Gills

• fish move water across gills; in mouth � out gill

• Some fish (tuna) that swim fast and have large energy requirements swim with mouth open to force water across gills.

2.  Positive Pressure breathing

• Frogs - push air down throat, lower throat - air enters - raise up to push down throat

3.  Negative Pressure Breathing

• humans/vertebrates

• pumping mechanism

• diaphragm expands � chest cavity volume increases � pressure drops � air enters by bulk flow due to pressure gradient (difference between inside and outside)

• controlled by pH � medullary respiratory center; acidic pH stimulates 

4. Additional Concerns:  Keeping the lung inflated (maintaining pressure)

• Lungs in chest cavity, surrounded by two linings (pleura) � one around lung, other lines chest cavity

• Negative pressure (-5 mm Hg) in pleural space; therefore normally little space between two layers

• Pressure in lung greater than chest cavity; therefore � lung inflated

Problems:

• Sucking chest wound � damage to outer layer, air enters, pressure increases � lung collapses

• "Growth spurt� � most common in young males, tall & thin � vesicle from lung blebs into cavity, punctures lining � pressure change � collapse lung

5.  Birds � require huge oxygen demands, especially during flight.  Respiratory system more efficient because:  (a) less dead space with shorter trachea; (b) gas exchange occurs during inhalation and exhalation (due to flow through system that is unidirectional and continuous.  Works because chest and air sac don�t compress at same time.  Similar to a bagpipe; (c) blood flows perpendicular to lung cells and is more efficient that capillary web around animals

6.  Additional concerns: 

• Protect the surface from dust and other particles � mucus

• What enables the lung to expand and contract during breathing(elastic)?   Answer:  the surface tension of water.  Pressure of breathing expands lung � surface tension contracts and returns to original position.  Mechanism is similar to formation of a raindrop.  But, there is a problem � the surface tension of the lung would be theoretically so large that the lung wouldn�t be able to expand.  Solution:  the lung excretes surfactants that help to cut the surface tension some � or else be too hard to breathe.  Problem with premature births � surfactants aren�t produced until about 32-34 weeks.  Lungs too rigid = respiratory distress syndrome.

VII.  Biological Implications of Fick's Law:  Mechanism to maintain a large concentration (ΔC) gradient
    Organisms have evolved to maximize the concentration gradient from inside to outside the absorbing surface that maximizes diffusion rates.

A.  General � Solubility

• oxygen has low solubility in water (10 ml/L) as compared to air (200 ml/L)
• solubility related to temperature (lower temperature = increased solubility)
• esp. important in aquatic systems
• running water higher oxygen levels � s/v issue

B.  Partial Pressure Primer

• Partial pressure ultimately determines diffusion (exchange) of gases
• Defined:  Gas concentration expressed as mole fraction (moles gas / moles mixture
• 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

• Mechanism to maintain large concentration gradient
• Flow of water opposite flow of blood

D. Ventilation � mechanism to bring fresh, high concentration air to absorbing surface

VIII.  Hemoglobin 

A.  Oxygen low solubility in water

• Only small amounts dissolved (1.5%) in blood plasma
• Required 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 α, 2 β
• each chain associated with a heme unit (porphyrin ring system with iron)
• each heme unit binds an oxygen molecule (O₂); 4 total oxygen molecules carrier

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

• Sigmoidal or S curve
• Binding shows cooperativity �� binding first slow, more readily absorbs others (positive cooperativity)
• Advantage � more oxygen released
• PO₂  about 40 mm Hg entering heart = 75% saturation
• PO₂  about 100 mm HG leaving heart = 100% saturation
• only 25% of the oxygen normally used, other 75% in reserve

D.  Factors that affect oxygen/hemoglobin binding

• pH (Bohr effect) - pH plasma typically about 7.6; metabolism, etc may lower it some; hemoglobin has a lower affinity for oxygen at lower pH, releases more oxygen
• Hemoglobin composition - fetal hemoglobin 2 alpha, 2 gamma chains (higher affinity for 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  

X.  Gas Exchange in Plants  

A.  General
    Gas exchange occurs primarily through stomata (pores in leaf) that are opened/closed by specialized epidermal cells called guard cells. Some gas exchange exchange occurs through other structures (e.g., lenticels in bark). 

B.  Guard cell structure 

1. Types of guard cells: (a.) elliptical or kidney-shaped. These are characteristic of dicots; and (b.) dumb-bell or dog-bone shaped - characteristic of grasses.  Click here for images of stomata from a variety of plants.  In addition, you will see many examples in lab.
    
2. Common features - (a.) thickened inner walls; (b.) bands of cellulose fibers that radiate out around the circumference of the pore; (c.) chloroplasts (in fact, guard cells are the only epidermal cells with chloroplasts); and (d) connected at the ends only.

B.  Guard cell Function
    Guard cells open the stoma because of the osmotic entry of entry of water into the GC.  In turn, this increases the turgidity (water pressure) in the GC and causes them to elongate. The radial orientation of cellulose microfibrils prevents increase in girth. Since GC are attached at the ends and because the inner wall is thicker, the guard cells belly out with the outer wall moving more and pulling open the guard cell. Guard cell closure essential involves reversing this process.  

    Water entry into the guard cells is controlled by increasing the solute concentration (osmotic concentration) in the guard cells. This occurs by: (a) transporting potassium (and chloride) ions into the guard cells from surrounding (subsidiary) cells. This process is mediated by a proton pump; and (b) by sugars produced during photosynthesis or from starch breakdown (recall that guard cells are the only epidermal cells with chloroplasts).  Thus, we can summarize the mechanics of GC action as follows: 

stoma closed (GC flaccid) → add solute → lower water potential → water uptake (osmosis) → increase pressure → stoma open (GC turgid) 

C.  Environmental Control of GC Action
    As a consequence of needing to keep open the stomata for photosynthesis, plants loose water (transpiration).  Thus, water loss is a �necessary evil� of photosynthesis.  However, plants tightly regulate the two processes � or in other words, they must compromise between the amount of photosynthesis and the amount of transpiration.  This is called the photosynthesis/transpiration compromise.  Guard cells control this compromise. 

    Guard cells are very responsive to their environment, especially any factors that impact the photosynthesis/transpiration compromise.  Thus, we expect any factor important in photosynthesis to exert regulatory control on GC. And, we also expect water, a major player in photosynthesis, to have the final word on control since if a plant dries out too much it's as good as dead!  Controls of GC action: 

1. Light - exerts strong control.   In general: light � open; dark � closed (reverse in CAM plants).   What kind of light is important? Red & blue light � these are important for photosynthesis which: (a) produces sugars (sucrose and glucose) for osmotic regulation; (b) produces ATP (via photophosphorylation) to power ion pumps; (c) reduces internal CO₂ levels which stimulates opening (see below). There is an additional effect of blue light on stomatal activity that is irrespective of its role in photosynthesis. What is blue light doing? Blue light: (a) activates a H+-ATPase in the membrane; and (b) stimulates starch breakdown.
    

2. Carbon dioxide - intracellular level is most critical. This is an important regulatory control.   lo CO₂ (i.e., during the day, used by photosynthesis) = open; hi CO₂ (i.e., at night, produced during respiration) = closed
    
3. Water - protects against excessive water loss. This is the prevailing and overriding control mechanism. There are two mechanisms by which water loss regulates stomatal closure, one is active and the other passive.
   
   Passive Control - simply put, as the plant looses water, the turgidity of the leaf cells, including guard cells, decreases and this results in stomatal closure. The plant is not "intentionally" closing the stoma, it is simply the consequence of drying out.
   
   Active Control - this mechanism is one in which the plant actually seems to monitor its water status. When the water potential drops below some critical level, it engages a cascade of events that close the stomata. Presumably the plant is measuring pressure (turgor) and then synthesizes or releases an anti-transpirant that is translocated (moved) to the GC to cause closure. The anti-transpirant is abscisic acid (ABA), one of the major plant growth regulators. It is active in very low concentration (10^-6 M) and appears very rapidly after water stress (within 7 minutes).
    
4. Temperature - increased temperatures usually increase stomatal action, presumably to open them for evaporative cooling. If the temperature becomes too high the stomata close due to water stress and increased CO₂ that result from respiration.
    
5. Wind - often causes closure because it: (a) brings CO₂ enriched air; and (b) increases the rate of transpiration that causes water stress which causes the stomata to close. In some cases, wind causes stomatal opening to increase transpiration for cooling.

D.  Mechanism of Guard Cell Action.  discussed in class