Exchange between Organisms (Mass Transport) - Haemoglobin Flashcards
What are haemoglobin molecules?
The haemoglobins are a group of chemically similar molecules found in a wide variety of organisms.
Haemoglobins are water soluble protein molecules (respiratory pigments) with a quaternary structure that has evolved to make them efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions. They combine reversibly with oxygen and greatly increase the capacity of blood to transport oxygen and deliver it to the tissues.
What is the structure of a haemoglobin molecule?
Primary Structure
Sequence of amino acids in the four polypeptide chains.
Secondary Structure
In which each of these polypeptide chains is coiled into a helix.
Tertiary Structure
In which each polypeptide chain is folded into a precise shape - an important factor in its ability to carry oxygen.
Quaternary Structure
In which all four polypeptides (two identical beta and two identical alpha chains) are linked together as a functional unit by bonds to form an almost spherical molecule. Each polypeptide complex is associated with a haem group - which contains a ferrous (Fe2+) ion. Each Fe2+ ion can combine with a single oxygen molecule (O2), making a total of four O2 molecules that can be carried by a single haemoglobin molecule in humans.
How is oxygen loaded and unloaded?
The process by which haemoglobin binds with oxygen is called loading, or associating. In humans this takes place in the lungs.
The process by which haemoglobin releases its oxygen is called unloading, or dissociating. In humans this takes place in the tissues.
What do haemoglobins with a high and low affinity for oxygen do?
Haemoglobins with a high affinity for oxygen take up oxygen more easily, but release it less easily. Haemoglobins with a low affinity for oxygen take up oxygen less easily, but release it more easily.
What is the role of haemoglobin?
The role of haemoglobin is to transport oxygen. To be efficient at transporting oxygen, haemoglobin must:
- readily associate with oxygen at the surface where gas exchange takes place
- readily dissociate from oxygen at those tissues requiring it
How are the requirements for haemoglobin achieved?
Haemoglobin changes its affinity (chemical attraction) for oxygen under different conditions. It achieves this because its shape changes in the presence of certain substances. such as carbon dioxide. In the presence of carbon dioxide, the new shape of the haemoglobin molecule binds more loosely to oxygen. As a result, haemoglobin releases its oxygen.
How does an environmental change affect a proteins structure?
A change in the environment of any protein changes its tertiary structure and therefore affects the way it functions. This explains why haemoglobin binds with oxygen in the lungs and releases it in the tissues.
What happens at the gas exchange surface?
- oxygen concentration: high
- carbon dioxide concentration: low
- affinity of haemoglobin for oxygen: high
- result: oxygen is associated
What happens at the respiring tissues?
- oxygen concentration: low
- carbon dioxide concentration: high
- affinity of haemoglobin for oxygen: low
- result: oxygen is dissociated
Why are there different haemoglobins?
Each species produces a haemoglobin with a slightly different amino acid sequence. The haemoglobin of each species therefore has a slightly different tertiary and quaternary structure and hence different oxygen binding properties. Depending on its structure, haemoglobin molecules range from those that have a high affinity for oxygen to those that have a low affinity for oxygen.
How many oxygen molecules can each molecule of haemoglobin carry?
Haemoglobin is a water soluble globular protein which consists of two beta polypeptide chains and two alpha helices. Each molecules forms a complex containing a haem group. It carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group. Each molecule can carry four oxygen molecules.
What does the affinity of oxygen for haemoglobin depend on?
- The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen which is a measure of oxygen concentration.
- The greater the concentration of dissolved oxygen in cells, the greater the partial pressure.
- Therefore, as partial pressure increases, the affinity of haemoglobin for oxygen increases, that is, oxygen binds to haemoglobin tightly.
- This occurs in the lungs in the process known as loading (associating).
- During respiration, oxygen is used up and therefore the partial pressure decreases, thus decreasing the affinity of oxygen for haemoglobin.
- As a result of that, oxygen is released in respiring tissues where it is needed.
- After the unloading process (disassociating), the haemoglobin returns to the lungs where it binds to oxygen again.
What is affinity?
an attraction of one thing to another
What is the Bohr effect?
states that haemoglobin’s oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide
What is dissociation?
the separation of one molecule from another
What is association?
the joining of one molecule to another
What is haemoglobin?
a globular protein in blood that readily combines with oxygen (reversibly) to transport it around the body - it comprises four polypeptide chains around an iron-containing haem group (prosthetic group)
What is partial pressure?
a measure of the concentration of a substance in a particular area
How does oxygen bind to each polypeptide chain in haemoglobin?
- first is hard to find
- exposes second and third, making them easier to find
- fourth is even harder to find
- results in a sigmoid shape
Why do red blood cells contain haemoglobin?
Red blood cells contain haemoglobin, enabling them to carry far more oxygen than if it was only dissolved in their cytoplasm.
What is the equation for when oxygen binds to haemoglobin?
Four oxygen molecules can bind to each haemoglobin molecule as haemoglobin is a protein consisting of four polypeptide chains, each containing a haem group that includes an iron ion.
4O2 + Hb (deoxyhaemoglobin) -> Hb(O2)4