A Level Biology Practice Exam

What effect does increased carbon dioxide have on the hemoglobin's ability to bind oxygen?

1. It increases the affinity for oxygen

2. It has no effect on affinity

3. It decreases the affinity for oxygen

4. It completely inhibits binding

The correct answer is: It decreases the affinity for oxygen

Increased carbon dioxide levels lead to a phenomenon known as the Bohr effect, where hemoglobin's affinity for oxygen decreases as carbon dioxide concentration rises. When carbon dioxide enters the blood, it reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. This increase in hydrogen ion concentration lowers the pH of the blood. The decrease in pH enhances the release of oxygen from hemoglobin. This occurs because the structural changes in hemoglobin resulting from the binding of hydrogen ions promote the release of oxygen, thereby facilitating oxygen delivery to actively respiring tissues, which generally produce more carbon dioxide due to enhanced metabolic activity. Therefore, as carbon dioxide increases, hemoglobin's ability to bind oxygen decreases, ensuring that more oxygen is released where it is most needed.