Autumn.wmf (12088 bytes)Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Gas Exchange in Plants & Animals

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

(CH2O) n + O2 �� CO2 + H2O


  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  


  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 

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:  

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.


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

2.  Positive Pressure breathing

3.  Negative Pressure Breathing

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


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: 

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

B.  Partial Pressure Primer

C.  Counter-current mechanisms � Gills

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

VIII.  Hemoglobin 

A.  Oxygen low solubility in water

B.  Structure

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

D.  Factors that affect oxygen/hemoglobin binding

IX.  Carbon Dioxide Transport

A.  Form

Table 1.  Forms in which carbon dioxide is transported



Dissolved in plasma (as CO2)

7 - 8

Bound to hemoglobin


Bicarbonate in plasma


B.  Carbon dioxide and water

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.
  1. 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
  2. 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).
  3. 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.
  4. 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

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Last updated: February 01, 2008        � Copyright by SG Saupe