Introduction to Haemoglobin and Gas Transport
Welcome! In this chapter, we are going to look at one of the most hardworking molecules in your body: haemoglobin. Think of haemoglobin as a specialized delivery truck. Its only job is to pick up oxygen from your lungs and drop it off exactly where it's needed—your hardworking cells. We will explore how its structure allows it to do this, how it reacts to different environments, and why a baby in the womb needs a slightly different version than an adult.
Note: This chapter connects closely to "Mass Transport and the Circulatory System," which covers how the blood moves around the body.
The Structure of Haemoglobin
Haemoglobin is a globular protein. In Biology, "globular" just means it is folded into a compact, ball-like shape that is soluble in water. This is vital because it needs to be carried in the watery environment of your red blood cells.
Here are the key features of its structure:
1. Quaternary Structure: It is made of four polypeptide chains (two alpha chains and two beta chains) joined together.
2. Haem Groups: Each of the four chains contains a special "non-protein" part called a haem group.
3. Iron Ions: At the center of each haem group is an iron ion (\(Fe^{2+}\)). It is this iron that actually binds to the oxygen.
Quick Fact: Because there are four haem groups, one single haemoglobin molecule can carry up to four oxygen molecules (\(4O_2\)) at once!
Oxygen Transport and Affinity
The main role of haemoglobin is to transport oxygen. When haemoglobin binds with oxygen, it becomes oxyhaemoglobin. This is a reversible reaction:
\(Haemoglobin + Oxygen \rightleftharpoons Oxyhaemoglobin\)
Whether haemoglobin "grabs" or "lets go" of oxygen depends on the partial pressure of oxygen (\(pO_2\)). This is just a fancy way of saying the concentration of oxygen.
- High \(pO_2\): (e.g., in the lungs) Haemoglobin has a high affinity for oxygen, meaning it binds easily to it.
- Low \(pO_2\): (e.g., in active muscles) Haemoglobin has a low affinity for oxygen, meaning it releases it so the cells can use it for respiration.
The Oxyhaemoglobin Dissociation Curve
If we plot a graph of how "saturated" haemoglobin is with oxygen at different partial pressures, we get an S-shaped curve, also known as a sigmoid curve.
Why is it S-shaped?
Don't worry if this seems tricky! It is all due to something called cooperative binding:
1. When the first oxygen molecule binds to the first haem group, it is actually quite difficult.
2. However, once that first molecule binds, it changes the shape (conformational change) of the entire haemoglobin molecule.
3. This change makes it much easier for the second and third oxygen molecules to bind.
4. The fourth molecule is harder to bind simply because most of the "seats" on the truck are already full!
Key Takeaway: The S-shape ensures that haemoglobin is very efficient. It loads up fully in the lungs and can dump its oxygen quickly when oxygen levels drop even a little bit in the tissues.
The Bohr Effect
When you exercise, your cells produce carbon dioxide (\(CO_2\)). Haemoglobin is "smart" enough to realize that if there is a lot of \(CO_2\) around, those cells must be working hard and need more oxygen!
The Bohr Effect describes what happens when \(CO_2\) levels are high:
1. High \(CO_2\) levels make the conditions slightly more acidic.
2. This changes the shape of the haemoglobin slightly, reducing its affinity for oxygen.
3. On a graph, the curve shifts to the right.
4. This means oxygen is released (unloaded) more easily to the tissues that need it most.
Memory Aid: Bohr shift = Both Right and Release. High \(CO_2\) moves the curve to the Right and causes the Release of oxygen.
Fetal vs. Adult Haemoglobin
A fetus (unborn baby) gets its oxygen from its mother's blood across the placenta. By the time the mother's blood reaches the placenta, it has already lost some oxygen.
To survive, fetal haemoglobin must have a higher affinity for oxygen than adult haemoglobin.
- On a graph, the fetal haemoglobin curve is shifted to the left of the adult curve.
- This allows the fetus to "pick up" oxygen from the mother's blood even when the oxygen concentration is relatively low.
Quick Review Box:
- Shift Right (Bohr): Lower affinity, happens during exercise/high \(CO_2\), helps unload oxygen.
- Shift Left (Fetal): Higher affinity, happens in babies, helps grab oxygen from the mother.
Carbon Dioxide Transport
While most of this chapter focuses on oxygen, remember that haemoglobin also helps transport carbon dioxide. Carbon dioxide is transported in three main ways:
1. Dissolved directly in the plasma (about 5%).
2. Bound directly to haemoglobin (forming carbaminohaemoglobin).
3. As hydrogencarbonate ions in the plasma (this is the most common way).
Summary of Key Terms
Affinity: The degree to which a substance tends to combine with another (how "sticky" haemoglobin is for oxygen).
Dissociation: The process of releasing oxygen from haemoglobin.
Partial Pressure (\(pO_2\)): A measure of the concentration of a gas in a mixture.
Globular Protein: A spherical, water-soluble protein like haemoglobin.
Common Exam Pitfall:
Avoid saying "the baby's blood mixes with the mother's." It doesn't! The oxygen just moves from the mother's haemoglobin to the baby's haemoglobin because the baby's has a higher affinity (the shift to the left).