The Living World 8th Edition by George Johnson

تمرين 20

How Does pH Affect a Protein's Function?
The red blood cells you see to the right carry oxygen to all parts of your body. These cells are red because they are chock full of a large iron-rich protein called hemoglobin. The iron atoms in each hemoglobin molecule provide a place for oxygen gas molecules to stick to the protein. When oxygen levels are highest (in the lungs), oxygen atoms bind to hemoglobin tightly, and a large percent of the hemoglobin molecules in a cell possess bound oxygen atoms. When oxygen levels are lower (in the tissues of the body), hemoglobin doesn't bind oxygen atoms as tightly, and as a consequence hemoglobin releases its oxygen to the tissues. What causes this difference between lungs and tissues in how hemoglobin loads and unloads oxygen? Oxygen concentration is not the only factor that might be responsible. Blood pH, for example, also differs between lungs and body tissues ( pH is a measure of how many H+ ions a solution contains). Tissues are slightly more acid (that is, they have more H+ ions and a lower pH) because their metabolic activities release CO2 into the blood, which you will recall from chapter 3, quickly becomes converted to carbonic acid.
The graph to the right displays so-called "oxygen loading curves" that reveal the effectiveness with which hemoglobin binds oxygen. The more effective the binding, the less oxygen required before hemoglobin becomes fully loaded, and the further to the left a loading curve is shifted. To assess the impact of pH on this process, O2 loading curves were carried out at three different blood pH values. In the graph, oxygen levels in the blood are presented on the x axis (the x axis runs along the bottom of the graph), and for each data point the corresponding % hemoglobin saturation (a %, or percent, is the numerator [top part] of a fraction whose denominator [bottom part] is 100-in this case, a measure of the fraction of the hemoglobin that is bound to oxygen) is presented on the y axis (the y axis runs up the left side of the graph). The oxygen-loading curve was repeated at pH values of 7.6, 7.4, and 7.2, corresponding to the blood pH that might be expected in resting, exercising, and very active muscle tissue.
Interpreting Data
a. What is the percent hemoglobin bound to O2 for each of the three pH concentrations at saturation? at an oxygen level of 20 mm Hg? at 40 mm Hg? at 60 mm Hg?
b. What general statement can be made regarding the effect of the oxygen levels in the blood (as measured by partial pressure of oxygen, measured in mm Hg) on the binding of oxygen to hemoglobin?
c. Are there any significant differences in the hemoglobin saturation values for the three pHs at high oxygen levels?