How many molecules of `O_2` can associate with a molecule of haemoglobin in man?

Explanation

To determine how many molecules of oxygen can associate with a molecule of hemoglobin in humans, we can follow these steps:

1. **Understand the Structure of Hemoglobin**: Hemoglobin is a protein found in red blood cells (RBCs) and is composed of four subunits. These subunits are made up of two alpha (α) and two beta (β) chains.

2. **Identify the Heme Groups**: Each subunit of hemoglobin contains a central heme group, which is crucial for oxygen binding. The heme group contains iron (Fe), which is the site where oxygen molecules attach.

3. **Calculate the Binding Capacity**: Since there are four subunits in a hemoglobin molecule, and each heme group can bind one molecule of oxygen, the total number of oxygen molecules that can bind to one hemoglobin molecule is equal to the number of subunits.

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