dissociation curve

(noun)

The oxygen–hemoglobin dissociation curve plots the proportion of hemoglobin saturated with oxygen on the vertical axis against the partial pressure of oxygen on the horizontal axis. 

Related Terms

  • hemoglobin
  • binding capacity
  • acidity

Examples of dissociation curve in the following topics:

  • Oxygen Transport

    • The lower areas of the curve show saturation when oxygen is unloaded into the tissues.
    • The oxyhemoglobin dissociation curve can shift in response to a variety of factors.
    • A change in the P50 of the curve is a sign that the dissociation curve as a whole has shifted.
    • The oxygen–hemoglobin dissociation curve plots the percent hemoglobin saturation (y-axis) against the partial pressure of oxygen in the blood (PO2).
    • The blue curve is standard curve, while the red and green curves are right and leftward shifts respectively.
  • Transport of Oxygen in the Blood

    • The resulting graph, an oxygen dissociation curve, is sigmoidal, or S-shaped .
    • The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right.
    • A similar shift in the curve also results from an increase in body temperature.
    • The oxygen dissociation curve demonstrates that as the partial pressure of oxygen increases, more oxygen binds hemoglobin.
  • RBC Physiology

    • As a result, the oxygen-binding curve of hemoglobin (also called the oxygen saturation or dissociation curve) is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding.
    • This curve shows the saturation of oxygen bound to hemoglobin compared to the partial pressure of oxygen (concentration) in blood.
    • That's because most carbon dioxide travels through the blood as a bicarbonate ion, which is the dissociated form of carbonic acid in solution.
    • This dissociates in solution into bicarbonate and hydrogen ions, the driving force of pH in the blood.
    • A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the Haldane effect.
  • Diprotic and Polyprotic Acids

    • Diprotic and polyprotic acids show unique profiles in titration experiments, where a pH versus titrant volume curve clearly shows two equivalence points for the acid; this is because the two ionizing hydrogens do not dissociate from the acid at the same time.
    • Dissociation does not happen all at once; each dissociation step has its own Ka value, designated Ka1 and Ka2:
    • This first dissociation step of sulfuric acid will occur completely, which is why sulfuric acid is considered a strong acid; the second dissociation step is only weakly dissociating, however.
    • A triprotic acid (H3A) can undergo three dissociations and will therefore have three dissociation constants: Ka1 > Ka2 > Ka3.
    • The titration curve of a polyprotic acid has multiple equivalence points, one for each proton.
  • Polyprotic Acid Titrations

    • Monoprotic acids are acids able to donate one proton per molecule during the process of dissociation (sometimes called ionization) as shown below (symbolized by HA):
    • If the pH of this titration were recorded and plotted against the volume of NaOH added, a very clear picture of the stepwise neutralization emerges, with very distinct equivalence points on the titration curves.
    • Oxalic acid is an example of an acid able to enter into a reaction with two available protons, having different Ka values for the dissociation (ionization) of each proton.
    • This image shows how Oxalic Acid will lose two protons in successive dissociations.
  • Dissociation

    • The major characteristic of all dissociative phenomena involves a detachment from reality.
    • Although some dissociative experiences involve memory loss, others do not.
    • At the pathological end of the dissociation spectrum are the dissociative disorders.
    • Psychoactive drugs can often induce a state of temporary dissociation.
    • Pathological dissociation involves the dissociative disorders, including dissociative fugue and depersonalization disorder.
  • Calculating Percent Dissociation

    • To determine percent dissociation, we first need to solve for the concentration of H+.
    • However, because the acid dissociates only to a very slight extent, we can assume x is small.
    • As we would expect for a weak acid, the percent dissociation is quite small.
    • However, for some weak acids, the percent dissociation can be higher—upwards of 10% or more.
    • Calculate percent dissociation for weak acids from their Ka values and a given concentration.
  • Acid Dissociation Constant (Ka)

    • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
    • Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution.
    • The larger the value of pKa, the smaller the extent of dissociation.
    • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .
    • The acetic acid partially and reversibly dissociates into acetate and hydrogen ions.
  • Dissociative Disorders

    • More pathological dissociation involves dissociative disorders.
    • These are both examples of dissociation.
    • Dissociation of this sort is fairly normal from time to time; however, there are five types of dissociative disorders which are considered psychopathological: dissociative identity disorder, disociative amnesia, depersonalization/derealization disorder, other specified dissociative disorder, and unspecified dissociative disorder.
    • Dissociative fugue, while it used to be its own diagnosis in the previous DSM-IV-TR, is now subsumed under dissociative amnesia as a specifier (i.e., dissociative amnesia with or without dissociative fugue).
    • The old category of dissociative disorder not otherwise specified is now split into two according to the DSM-5 (2013): other specified dissociative disorder and unspecified dissociative disorder.
  • Bond Energy

    • These energy values (493 and 424 kJ/mol) required to break successive O-H bonds in the water molecule are called 'bond dissociation energies,' and they are different from the bond energy.
    • The bond energy is the average of the bond dissociation energies in a molecule.
    • A Morse curve shows how the energy of a two atom system changes as a function of internuclear distance.
    • The attractive and repulsive forces are balanced at the minimum point in the plot of a Morse curve.
    • A Morse curve will have different energy minima and distance dependence for bonds formed between different pairs of atoms.
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