Haemoglobin and the Bohr effect

In a nutshell

Red blood cells contain a protein called haemoglobin which is responsible for carrying oxygen around the body. This summary details the properties and functions of the different haemoglobin proteins that different organisms encode.

Haemoglobin

Plasma is largely made up of water and oxygen isn't very soluble in water, therefore it needs to find an alternative way to be transported in blood. Haemoglobin (Hb) is a protein found in red blood cells (erythrocytes) that allows oxygen to be transported around the body efficiently. It is made up of four polypeptide chains each with a haem group. The haem group contains an iron ion ( $Fe{^2}{^+}$ ) which gives haemoglobin and therefore blood, its red colour. The structure can be seen below.

Biology; Exchange and transport; KS5 Year 12; Haemoglobin and the Bohr effect — The quaternary structure of haemoglobin. 1. Alpha-globin subunit, 2. Beta-globin subunit, 3. Haem group, 4. Oxygen molecule

Each haemoglobin protein can bind to four oxygen molecules. This is shown in the equations below.

$haemoglobin\ +\ oxygen \rightleftharpoons oxyhaemoglobin$

Hb\ + 4O_2\ \rightleftharpoons HbO_8

The forward reaction occurs in the lungs. In this reaction, oxygen will bind to the haem group in haemoglobin with high affinity to form oxyhaemoglobin. When the oxyhaemoglobin reaches the body cells, the reverse reaction occurs.

Loading and unloading of oxygen

The partial pressure of oxygen ( $pO_2$ ) is the concentration of dissolved oxygen in cells.

Partial pressure of oxygen	Consequence	Example
High	Oxygen loads onto haemoglobin.	The alveoli have a high $pO_2$ . Therefore, oxygen is loaded onto haemoglobin in the alveoli.
Low	Oxyhaemoglobin unloads its oxygen.	Respiring cells will have a lower $pO_2$ as they are using oxygen. Therefore, oxygen is unloaded in these cells.

Dissociation curves

Dissociation curves show how the saturation of haemoglobin with oxygen changes with the partial pressure of oxygen.

Biology; Exchange and transport; KS5 Year 12; Haemoglobin and the Bohr effect

1.	The shape of the haemoglobin molecule hinders the ability of oxygen to bind to one of the four binding sides on the polypeptide subunits. Therefore, at low oxygen concentrations, not much oxyhaemoglobin will form. This is shown as a low gradient.
2.	The binding of oxygen to haemoglobin causes a conformational change in the haemoglobin protein which makes it easier for other oxygen molecules to bind. This is called positive cooperativity as the binding of the first oxygen molecule increases the ability of other oxygen molecules to bind. As a result, the gradient of the line increases.
3.	However, after binding the third oxygen molecule, it is harder to bind the fourth oxygen molecule. This is because there is a lower probability that the oxygen molecule will find a free binding site. Therefore, the gradient of the curve lowers and the graph starts to plateau.

The Bohr effect

The Bohr effect involves the partial pressure of carbon dioxide ( $pCO_2$ ). When the $pCO_2$ is greater, the more oxygen is unloaded from haemoglobin. As a result, haemoglobin acts differently in different parts of the body. This is explained below.

Location	Explanation
Gas exchange surfaces	The $pCO_2$ is low which means haemoglobin will have a higher affinity for oxygen. As a result, more oxygen will be loaded onto haemoglobin.
Respiring tissues	The $pCO_2$ is high which means that the haemoglobin will have a reduced affinity for oxygen and that the oxygen will be unloaded from haemoglobin.

Biology; Exchange and transport; KS5 Year 12; Haemoglobin and the Bohr effect — Red line: Low carbon dioxide concentration, Purple line: Medium carbon dioxide concentration, Blue line: High carbon dioxide concentration

The reason for this change is that the dissolved carbon dioxide is acidic which causes a decrease in pH. The change in pH triggers a change in the shape of the haemoglobin molecule and the release of oxygen.

Types of haemoglobin

Different organisms have different dissociation curves based on their adaptations to their environments. A species that lives in an environment with a low $pO_2$ will have haemoglobin that has a higher affinity for oxygen than a species that lives in an environment with a high $pO_2$ .

Example

The lugworm is a species of marine worm that lives in a burrow under the water. Oxygen from the water will diffuse into the lugworm's blood. However, during low tide there is little oxygen available. Its haemoglobin has therefore adapted to have a high affinity for oxygen so that even in low oxygen environments, they can still survive.

Example

Llamas often live in high altitude areas with low $pO_2$ . This means that it has also evolved haemoglobin with a higher affinity for oxygen than human haemoglobin.