Question

Assertion: Oxygen dissociation curve is sigmoid. Reason: Affinity of oxygen for \(\mathrm{Hb}\) increases with addition of each molecule of oxygen one after another.

Short answer

The assertion is correct and the reason provided is also correct. The affinity of oxygen for Hb does increase with the binding of each molecule of oxygen, resulting in a sigmoid, or S-shaped, oxygen dissociation curve.

Step-by-step solution

Understand the Terms

We need two important terms clarified: 'Oxygen dissociation curve' and 'sigmoid curve'. An oxygen dissociation curve is a way to represent how much oxygen is carried by hemoglobin at different partial pressures of oxygen, and a sigmoid curve is an S-shaped curve.

Determine Relation

The assertion mention the curve is sigmoid in shape. The reason provided is 'affinity of oxygen for Hb increases with addition of each molecule of oxygen...' So, we need to determine if increased affinity leading to more binding of oxygen with Hb can generate a sigmoid (S-shaped) curve.

Verify Reasoning

When the partial pressure of oxygen is low, Hemoglobin binds with oxygen but less readily. As the pressure increases, more oxygen molecules can bind, hence the affinity increases. When hemoglobin binds with one molecule of Oxygen, it alters its shape favorably to allow another Oxygen molecule to bind more easily, and this continues as more Oxygen is available. The adaptation of hemoglobin after each oxygen binding explains why affinity increase after each oxygen molecule addition and eventually it forms a sigmoid curve as more and more binding sites are occupied by Oxygen.

Key concepts

Sigmoid Curve

The sigmoid curve is an important concept in biology, especially in understanding how hemoglobin interacts with oxygen. Imagine the curve looking like an 'S' lying on its side. This S-shape is not just a random pattern but a result of a specific process. When plotted on a graph, this curve represents how saturated hemoglobin is with oxygen at different oxygen concentrations. At first, the curve rises slowly, then steeply, and finally levels off. This pattern shows the changing ease with which oxygen binds to hemoglobin at different pressures.

Initially, oxygen binds slowly because hemoglobin's affinity for oxygen is low. As more oxygen molecules attach to hemoglobin, it becomes easier for additional molecules to bind, thanks to changes in hemoglobin's structure. This accelerates the increase until saturation is almost reached, then the curve flattens as all the binding sites are occupied. Understanding this curve helps in comprehending how oxygen is loaded in the lungs and released in tissues.

Hemoglobin Affinity

Hemoglobin's affinity for oxygen is a crucial factor in how efficiently our blood can transport oxygen. Affinity refers to the tendency of hemoglobin to bind to oxygen molecules.

When hemoglobin's affinity is high, it means it binds to oxygen more readily. Conversely, when its affinity decreases, hemoglobin will release oxygen more easily into the tissues. The way hemoglobin's affinity changes is peculiar: it increases with each oxygen molecule that binds.

• Initially, with low affinity, hemoglobin binds oxygen slowly.
• After the first oxygen molecule binds, hemoglobin's shape alters, increasing its affinity.
• This conformational change makes it easier for subsequent oxygen molecules to bind.

This process is known as cooperative binding, which is key to the oxygen dissociation curve's sigmoid shape. Hence, understanding affinity changes is vital to grasping oxygen transport dynamics in the body.

Partial Pressure of Oxygen

Partial pressure of oxygen, often abbreviated as \( P_{O_2} \), is a measure of oxygen concentration in blood or lungs. It's fundamental to understanding how oxygen binds to hemoglobin.

In the atmosphere, oxygen exerts a certain pressure, and this is described as partial pressure. The same holds true within our bodies. When \( P_{O_2} \) is low, oxygen binds less readily to hemoglobin. As \( P_{O_2} \) increases, so does the likelihood of hemoglobin taking up oxygen, increasing its saturation. This relationship is beautifully depicted in the sigmoid oxygen dissociation curve.

• Low \( P_{O_2} \): Low hemoglobin saturation.
• Moderate \( P_{O_2} \): Rapid increase in saturation.
• High \( P_{O_2} \): Saturation levels off as hemoglobin is nearly full.

This concept is pivotal for comprehending how efficient oxygen transport can adjust with varying oxygen levels in our environment or body.

Oxygen Binding

Oxygen binding defines how oxygen molecules attach to hemoglobin. It's more than just a simple attachment; it's a dynamic process ensuring that oxygen is efficiently transported throughout the body.

The initial binding of oxygen to hemoglobin is less favorable at low oxygen pressures. However, once the first oxygen molecule binds, it induces a structural change in hemoglobin, increasing its affinity for oxygen.

• This enhanced binding ensures quick saturation at certain pressures, crucial for survival in varying conditions.
• It guarantees oxygen uptake in the lungs and release in body tissues, where it's needed.

The binding process is influenced by several factors, such as \( P_{O_2} \), pH levels, and the presence of carbon dioxide. All these elements ensure that hemoglobin releases oxygen when required, while easily picking it up under oxygen-rich conditions. Understanding this binding helps us appreciate the intricacy of cellular respiration and transport mechanisms.