Hemoglobin and oxygen transport: evaluate the statement that each hemoglobin molecule can transport two molecules of oxygen when fully saturated.

Introduction / Context:
Hemoglobin is the oxygen carrying protein in red blood cells and is central to respiratory physiology. Each hemoglobin molecule contains several heme groups that bind oxygen reversibly. Many exam questions test whether learners know how many oxygen molecules a single hemoglobin molecule can bind when it is fully saturated. This question asks you to evaluate a specific statement about that number and choose the most accurate interpretation.

Given Data / Assumptions:

• The statement claims that each hemoglobin molecule can transport two oxygen molecules.
• Hemoglobin structure includes polypeptide chains and heme groups.
• Each heme group has an iron atom that binds oxygen.
• The question asks whether the statement is correct or incorrect and by how much.

Concept / Approach:
Adult human hemoglobin, often called hemoglobin A, is a tetramer made of four polypeptide chains, typically two alpha and two beta chains. Each chain contains one heme group. Each heme group has one iron ion that can bind one oxygen molecule. Therefore, a single hemoglobin molecule can bind a maximum of four oxygen molecules when fully saturated, one per heme group. The statement that each hemoglobin molecule can carry only two oxygen molecules is therefore incorrect. The best option will explicitly correct the statement and give the correct maximum number of oxygen molecules.

Step-by-Step Solution:
Step 1: Recall the basic structure of adult hemoglobin: four globin chains and four heme groups. Step 2: Note that each heme group has one iron ion that binds one molecule of oxygen reversibly. Step 3: Multiply the number of heme groups by the number of oxygen molecules each can bind. Four heme groups times one oxygen molecule each equals four oxygen molecules per hemoglobin molecule at full saturation. Step 4: Compare this with the statement in the question, which claims that each hemoglobin molecule can transport two oxygen molecules. Step 5: Recognize that this number is only half of the correct maximum and therefore the statement is inaccurate. Step 6: Select the option that clearly labels the statement as incorrect and provides the correct maximum capacity of four oxygen molecules.

Verification / Alternative check:
Standard physiology and biochemistry references describe the oxygen dissociation curve of hemoglobin, showing cooperative binding of up to four oxygen molecules. Laboratory measurements of oxygen binding capacity are based on this four molecule limit. Genetic variants such as fetal hemoglobin still maintain four heme groups and four potential binding sites, even though they differ in oxygen affinity. There is no normal situation in which adult hemoglobin can bind only two oxygen molecules at full saturation. This confirms that the statement is incorrect and that four is the correct maximum number of oxygen molecules bound per hemoglobin molecule.

Why Other Options Are Wrong:
Option A: This option accepts the incorrect statement and repeats the wrong number of two oxygen molecules, so it is not correct.

Option C: Fetal hemoglobin differs from adult hemoglobin in affinity and chain composition, but it still has four heme groups and can bind four oxygen molecules. The statement is not magically correct only in fetal blood.

Option D: Venous blood has hemoglobin that is often partially unsaturated with oxygen, but the maximum capacity of each molecule remains four oxygen molecules. The statement does not become correct only in venous blood.

Common Pitfalls:
Students may confuse the number of heme groups with partial saturation states and think that a typical hemoglobin molecule carries two oxygen molecules on average under resting conditions. Another pitfall is to mix up oxygen molecules and oxygen atoms, or to forget that saturation refers to maximum binding capacity, not to typical physiological saturation in blood. Always recall the structural basis of binding capacity: four heme groups mean four potential oxygen molecules at full saturation.

Final Answer:
The statement is incorrect; one hemoglobin molecule can carry a maximum of four oxygen molecules when fully saturated.