Gas Exchange
 
 
  1. All organisms synthesize ATP
    1. ATP generated by oxidation of nutrients
    2. oxidation in aerobic organisms requires oxygen, produces CO2
  2. Gas exchange occurs through diffusion
    1. no active transport of gases
  3. in most animals, diffusion alone is insufficient
    1. surface area:volume ratio
    2. diffusion effective only over distances less than ca. 1mm
  4. Chemistry of oxygen in water and atmosphere
    1. partial pressure of gasses
      1. atmospheric pressure is actual physical pressure
        1. partial pressure is the pressure due to one component of atmosphere
        2. partial pressure of a component is a function of % composition
      2. at sea level, pressure of air is highest (highest density)
        1. at higher altitudes, total amount of air declines
        2. % of each component stays constant, pO2 declines
    2. in aqueous solution, concentration of O2 is much lower
      1. gases dissolve more slowly in water than in air
      2. oxygen is not highly soluble in water
      3. temperature, salinity, and surface area all alter concentration of gases in water
    3. in all animals except insects, gas exchange takes place in aqueous environment
  5. Comparative anatomy of gas exchange surfaces
    1. Insects: trachae (hemolymph only minor component of gas exchange)
    2. Fish: gills (countercurrent exchange)
    3. Birds: unidirectional lungs
    4. Other vertebrates: blind sac lungs
  6. Respiratory pigments
    1. may be dissoved in hemolymph (liquid portion of blood) or packaged in cells
    2. hemocyanin (copper) - many invertebrates, some insects
    3. hemoglobin (iron) - some invertebrates, all vertebrates
      1. vertebrate hemoglobin has 4 polypeptide chains (globins)
      2. 4 hemes (non-protein rings) that bind to one iron ion - pigmented
      3. each iron ion can bind one oxygen molecule
        1. hemoglobin dissociation curve due to interactions among 4 units
      4. Other factors affecting hemoglobin oxygen dissociation curve
        1. pH of local tissue (text fig 41.10)
        2. molecular composition of hemoglobin (text fig 45.13, pg 885)
  7. Getting rid of carbon dioxide
    1. CO2 forms bicarbonate through passive reaction + enzyme
      1. direction of reaction depends upon local relative concentrations
    2. HCO3 is highly soluble in water
      1. dissoved in blood plasma, returns to lungs
  8. in lungs and tissues, concentration gradients favor passive diffusion of oxygen, carbon dioxide
Study questions

1.  Why might natural selection have favored the evolution of unidirectional lungs in birds, and counter-current exchanges in fish?

2.  Why can the vertebrate lung (apart from birds) be considered highly inefficient?

3.  Why do many invertebrates have green "blood"?  Why is the blood of vertebrates red?

4.  Adult terrestrial insects have trachae, but many aquatic insect larvae have gills.  Would you expect there to be a pattern of presence / absence of respiratory pigments with these two broad categories of "habitat", and what pattern would you expect?

5.  Imagine that there is an invertebrate with hemoglobin dissoved in the blood, and this hemoglobin exists in the blood as single units (protein + heme + iron).  Would you expect the dissociation curve to have the same shape as in the 4-unit vertebrate hemoglobin?  Why or why not.

6.  Go to   interactive dissociation curves and move pH from normal to acid and basic.  How does the curve change, and what does this mean in terms of the number of oxygen molecules released to the tissues?  Why does it make "sense" that hemoglobin dissociation curve shifts with increased acidity?  Hint:  heavy exercise generates acid bi-products of cellular respiration in the muscles.

7.  Using the same interactive web page, explore what happens when body temperature changes (particularly, when there is a high fever).  Does this appear to be an "adaptation" or do these changes in dissociation simply reflect the chemistry of hemoglobin?s

8.  What is the oxygen reserve?  Why does it make evolutionary sense (i.e., why might natural selection favor) to have such a "reserve" of oxygen in the bloodstream?

9.   Look at figure 45.13 (pg 885).  Explain the relative dissociation curves of maturnal and fetal hemoglobin relative to the relationship between mother and fetus.

10.  There is molecular variation in hemoglobin among species found at different altitudes.  What shape would you expect the hemoglobin dissociation curve to take in a species always found at high altitudes compared to one found only at low altitudes?

See also Content review #1, 3, 4, 5;  Concept review #2, 3, 4;  Applying ideas #1, 3, 4, 6
 

Answers