Question

Oxygen dissociation curve is (a) Parabola (b) Sigmoid (c) Hyperbola (d) Straight line

Short answer

(b) Sigmoid

Step-by-step solution

Understanding the Oxygen Dissociation Curve

The first step would be to understand what the oxygen dissociation curve is. It is a graph that illustrates the relationship between the partial pressure of oxygen (x-axis) and the oxygen saturation of hemoglobin (y-axis). It's crucial to understand what kind of shape this curve could create on a graph.

Identifying the Shape

By studying the oxygen dissociation curve or looking at a plotted graph, it is seen that as the partial pressure of oxygen increases, hemoglobin's oxygen saturation also generally increases. But this relationship isn't linear or hyperbolic. The curve starts slowly, then steepens before leveling out. This creates an 'S' shaped curve, also known as a sigmoid curve.

Key concepts

Sigmoid Curve

The oxygen dissociation curve is an example of a sigmoid curve, which is an 'S' shaped graph often seen in biological systems. The presence of a sigmoid curve in the relationship between two variables indicates a complex, non-linear relationship. For the oxygen dissociation curve, this is due to the cooperative binding of oxygen to hemoglobin.

In the initial phase, as oxygen begins to bind to hemoglobin, the curve rises slowly. This is because the first few oxygen molecules find it relatively difficult to bind, due to the need to change the structure of the hemoglobin to favor binding. As more oxygen molecules bind, it becomes easier for subsequent molecules to attach, leading to the steep middle section of the curve. This phenomenon is known as cooperative binding, and it contributes to the S-shape of the graph.

Eventually, as hemoglobin becomes saturated with oxygen, the curve starts to level off, forming the top part of the 'S'. At this point, most of the hemoglobin binding sites are occupied, so additional increases in oxygen partial pressure result in only small increases in oxygen binding. This plateauing effect is crucial as it allows hemoglobin to deliver oxygen effectively across a wide range of oxygen levels.

Hemoglobin

Hemoglobin is a protein found in red blood cells that plays a key role in oxygen transport from the lungs to the tissues. Aided by its four subunits, hemoglobin can bind up to four molecules of oxygen. The binding and release of these oxygen molecules are vital to its function in the body.

One of the remarkable features of hemoglobin is its ability to undergo conformational changes during oxygen binding. Initially, hemoglobin is in a T (tense) state, which has a lower affinity for oxygen. As oxygen binds, it triggers a transition to the R (relaxed) state, which has a higher affinity for oxygen. This conformational change is central to hemoglobin's cooperative binding characteristic, enhancing its efficiency in oxygen uptake and release.

The design of hemoglobin enables it to perform with high responsiveness to changes in oxygen needs within different tissues. This adaptability is what allows it to effectively transport and offload oxygen where it is most needed, depending on the varying partial pressures encountered throughout the circulatory system.

Partial Pressure of Oxygen

The partial pressure of oxygen (\( pO_2 \)) is a measure of oxygen's pressure in a mixture of gases, and is a crucial factor in the oxygen dissociation curve. It essentially reflects the concentration of oxygen, and is a direct driver of how much oxygen will bind to hemoglobin at any given moment.

Within the lungs, where \( pO_2 \) is highest, hemoglobin binds oxygen efficiently, capturing oxygen molecules into the bloodstream. As hemoglobin travels to the body's tissues experiencing lower \( pO_2 \), it releases oxygen to fulfill metabolic demands. This ability to adapt based on \( pO_2 \) levels demonstrates the curve's dynamic function in matching oxygen delivery with local tissue needs.

The shape of the oxygen dissociation curve reflects the variations in hemoglobin affinity across different \( pO_2 \) levels. It indicates how readily hemoglobin can release oxygen when \( pO_2 \) drops, ensuring tissues with active metabolism receive adequate oxygen supplies, adjusted in real-time according to their requirements.