Haemoglobin (Hb) transports oxygen from lungs to tissue. The partial pressure of oxygen in lungs is different from that in tissues. Each Hb can bind to up to four oxygen molecules. Suppose we have an equal number of Hb and oxygen molecules and all the oxygen molecules are in bound form. Then, which of the following is TRUE?

Options

A. Almost all the Hb molecules have one bound oxygen molecule.
B. Nearly half of all the Hb molecules are each bound to two oxygen molecules.
C. Nearly one-fourth of all the Hb molecules are bound to four oxygen molecules each.
D. Most of the Hb molecules have one bound oxygen molecule each, the rest either have no bound oxygen or have two or more bound oxygen molecules.

Answer

To solve the problem, we need to analyze the relationship between hemoglobin (Hb) and oxygen (O2) binding. Here’s the step-by-step solution:

### Step 1: Understand the Binding Capacity of Hemoglobin
Each hemoglobin molecule can bind to a maximum of four oxygen molecules. This means that for every hemoglobin molecule, there can be up to four oxygen molecules attached.

### Step 2: Set Up the Scenario
We are given that there are an equal number of hemoglobin and oxygen molecules. Let's assume there are 100 hemoglobin molecules and 100 oxygen molecules.

### Step 3: Calculate the Number of Hemoglobin Required
Since each hemoglobin can bind to four oxygen molecules, we can calculate how many hemoglobin molecules are needed to bind all 100 oxygen molecules:
- If 1 hemoglobin binds 4 oxygen, then to bind 100 oxygen molecules, we need:
\[ \text{Number of hemoglobin needed} = \frac{\text{Total oxygen}}{\text{Oxygen per hemoglobin}} = \frac{100}{4} = 25 \]

### Step 4: Determine the Binding Status of Hemoglobin
Since we have 100 hemoglobin molecules available and only 25 hemoglobin molecules are needed to bind all 100 oxygen molecules, it implies that:
- Only 25 hemoglobin molecules will be fully saturated with 4 oxygen molecules each.
- The remaining 75 hemoglobin molecules will not be bound to any oxygen.

### Step 5: Conclusion
From this analysis, we can conclude that:
- Only a fraction of the hemoglobin molecules (specifically, 25 out of 100) are fully saturated with oxygen.
- Therefore, the correct statement is that nearly 1/4 of the hemoglobin molecules are bound to 4 oxygen molecules each.

### Final Answer
The correct option is that nearly 1/4 of the hemoglobin molecules are bound to 4 oxygen molecules each.

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