Question

Hemoglobin plays a part in a series of equilibria involving protonation- deprotonation and oxygenation-deoxygenation. The overall reaction is approximately as follows: $${\mathrm{HbH}}^{+}(aq)+{\mathrm{O}}_2(aq)\rightleftharpoons {\mathrm{HbO}}_2(aq)+{\mathrm{H}}^{+}(aq)$$ where $\mathrm{Hb}$ stands for hemoglobin and ${\mathrm{HbO}}_2$ for oxyhemoglobin. (a) The concentration of ${\mathrm{O}}_2$ is higher in the lungs and lower in the tissues. What effect does high $\left[{\mathrm{O}}_2\right]$ have on the position of this equilibrium? (b) The normal $\mathrm{pH}$ of blood is 7.4. Is the blood acidic, basic, or neutral? (c) If the blood $\mathrm{pH}$ is lowered by the presence of large amounts of acidic metabolism products, a condition known as acidosis results. What effect does lowering blood pH have on the ability of hemoglobin to transport ${\mathrm{O}}_2$ ?

Short answer

(a) A higher $[{\mathrm{O}}_2]$ in the lungs shifts the equilibrium to the right, favoring the formation of oxyhemoglobin (${\mathrm{HbO}}_2$). (b) The normal blood pH of 7.4 indicates that blood is slightly basic. (c) Lowering blood pH due to acidosis increases $[{\mathrm{H}}^{+}$] concentration, shifting the equilibrium to the left, reducing the formation of ${\mathrm{HbO}}_2$, and decreasing hemoglobin's ability to transport oxygen to the tissues.

Step-by-step solution

(a) Effect of Oxygen Concentration on Equilibrium.

The given reaction is: $${\mathrm{HbH}}^{+}(aq)+{\mathrm{O}}_2(aq)\rightleftharpoons {\mathrm{HbO}}_2(aq)+{\mathrm{H}}^{+}(aq)$$ In the lungs, the concentration of oxygen $[{\mathrm{O}}_2]$ is higher. According to Le Chatelier's Principle, if the concentration of a reactant is increased, the equilibrium will shift towards the product side to minimize the change. Therefore, a higher $[{\mathrm{O}}_2]$ would shift the equilibrium to the right, favoring the formation of oxyhemoglobin (${\mathrm{HbO}}_2$).

(b) Normal pH of Blood

Blood pH is a measure of the acidity or basicity of blood. A pH value of 7 is considered neutral, while values less than 7 are acidic, and values greater than 7 are basic. Since the normal pH of blood is given as 7.4, the blood is slightly basic.

(c) Effect of Lower Blood pH on Oxygen Transport

Acidosis is a condition in which the blood pH is lowered due to the presence of large amounts of acidic metabolic products. Lowering the blood pH means an increase in $[{\mathrm{H}}^{+}$] concentration. Referring to the given reaction: $${\mathrm{HbH}}^{+}(aq)+{\mathrm{O}}_2(aq)\rightleftharpoons {\mathrm{HbO}}_2(aq)+{\mathrm{H}}^{+}(aq)$$ According to Le Chatelier's Principle, when the concentration of a product (${\mathrm{H}}^{+}$ in this case) is increased, the equilibrium will shift towards the reactant side to minimize the change. Therefore, lowering the blood pH, the equilibrium would shift to the left, reducing the formation of oxyhemoglobin (${\mathrm{HbO}}_2$) and decreasing the ability of hemoglobin to transport oxygen fo the tissues.

Key concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that explains how equilibrium systems respond to changes in reactant or product concentrations. It states that if a change is imposed on a system at equilibrium, the system will adjust itself in such a way as to counteract that change and restore a new equilibrium state.
This principle is particularly relevant in biological systems like the hemoglobin reaction. Hemoglobin ($\mathrm{Hb}$) binds with oxygen to form oxyhemoglobin (${\mathrm{HbO}}_2$), as described by the equation:

• $${\mathrm{HbH}}^{+}(aq)+{\mathrm{O}}_2(aq)\rightleftharpoons {\mathrm{HbO}}_2(aq)+{\mathrm{H}}^{+}(aq)$$

In areas where oxygen concentration is high, such as in the lungs, Le Chatelier's Principle predicts that the equilibrium will shift to the right, favoring the production of more oxyhemoglobin. Conversely, in tissues where oxygen concentration is lower, the equilibrium tends to shift to the left, releasing oxygen from hemoglobin, which is crucial for oxygen delivery to tissues.

blood pH

Blood pH is an important indicator of the blood's acidity or alkalinity. A pH of 7 is considered neutral. If the pH is below 7, the blood is acidic; above 7, it is basic. Human blood typically has a pH of about 7.4, which classifies it as slightly basic.
Maintaining this pH is vital because even small changes can affect biological function. Various systems in the body work together to keep blood pH within a narrow range. This includes buffering systems, respiratory mechanisms, and renal function.
Disruptions in these systems can lead to conditions like acidosis, where the blood becomes more acidic. This can occur due to increased $[{\mathrm{H}}^{+}]$ from metabolic by-products, which affects hemoglobin's efficiency in transporting oxygen. Understanding blood pH is critical when studying oxygen transport because hemoglobin's ability to bind and release oxygen is pH dependent.

oxygen transport

Oxygen transport in the human body is primarily facilitated by hemoglobin in the red blood cells. Hemoglobin's ability to bind or release oxygen changes depending on the surrounding environment, particularly pH and oxygen concentration.
In the lungs, where oxygen concentration is high, hemoglobin binds oxygen readily, forming oxyhemoglobin. This forms as a result of an equilibrium shift towards the right side of the reaction equation:${\mathrm{HbH}}^{+}(aq)+{\mathrm{O}}_2(aq)\rightleftharpoons {\mathrm{HbO}}_2(aq)+{\mathrm{H}}^{+}(aq)$.
However, in tissues requiring oxygen, the concentration of oxygen is lower, and factors like lower pH due to carbon dioxide production promote the release of oxygen. This process is aided by the previously discussed blood pH and ensures that tissues receive adequate oxygen for metabolic needs. Efficient oxygen transport is crucial for supporting cellular respiration and energy production throughout the body.