3.4a Haemoglobin

Why does CO2 change the affinity of haemoglobin?

CO2 is mildly acidic so it lowers the pH of the blood when it is present in high concentrations. This pH change causes the protein structure of the haemoglobin to change, therefore the shape to change, affecting the affinity.

What is the haem group like?

Contains an iron ion (Fe2+) which can bind with an oxygen molecule.

Why might different species have different haemoglobins?

Different species have different DNA codes. the different bases code for a different amino acid sequence for haemoglobin. the different primary structure affects the entire shape of the haemoglobin including which affects the oxygen affinity. therefore different species have different haemoglobins with different affinities based on what is required in their environment.

How much oxygen can it carry?

Each haemoglobin molecule can carry up to four oxygen molecules because it has four haem groups. This much is not always carried.

Describe the structure of haemoglobin.

Four globin polypeptide chains which each contain a prosthetic group called a haem group. These are twisted into an almost perfectly spherical molecules with the four haem groups contained within.

How is the concentration of a gas measured?

Gases are normally in a mixture of other gases which exert a combined pressure. The concentration of a certain gas depends on the proportion of the full combined pressure it exerts.

What type of protein structure does it have?

Globular quaternary structure

What kind of molecule is haemoglobin?

Haemoglobin is a protein molecule found in the red blood cells within blood.

Where does haemoglobin need a high affinity? Where doe sit need a low one?

Haemoglobin needs to pick up oxygen easily in the lungs so it needs a high affinity in the lungs. It needs to dissociate (release) oxygen easily in the tissues so it needs a low affinity in the tissues.

What saturation does haemoglobin usually reach?

Haemoglobin rarely reaches 100% saturation because it is so hard for the final oxygen molecule to be loaded. normally round 97% saturation with oxygen is reached.

describe how oxygen affinity is adjusted in the lungs.

In the lungs, CO2 concentration is low because it is constantly being exhaled. Therefore the pH is raised, the shape is changed and the affinity is raised. The high affinity means lots of oxygen is loaded in the lungs.

Describe how and where oxygen loses and gains haemoglobin.

In the lungs, haemoglobin LOADS or ASSOCIATES with oxygen to pick it up. In the respiring tissue, haemoglobin UNLOADS or DISSOCIATES with oxygen to release it.

Describe how oxygen affinity is adjusted in the tissues.

In the tissues, the CO2 concentration is high because it is constantly being produced by the respiring cells. Therefore pH is lowered, shape is changed and the affinity is lowered. The low affinity means oxygen is unloaded into the cells to use in respiration.

Why is the curve shallow at first? What real life situation does this represent?

It is difficult for haemoglobin to load oxygen when the oxygen pressure is low. This represents the way it is difficult for the first oxygen molecule to be loaded because the haemoglobin is very tightly held together.

How easily does the fourth molecule bind? Why?

It is very difficult for the final oxygen to bind simply due to probability. There is only one binding site available and this reduces the likelihood of an oxygen molecule binding to it. This is why 100% saturation is not often reached.

How do you know it has a quaternary structure?

More than one polypeptide chain, and the prosthetic groups.

What molecule is responsible for being a reserve supply of oxygen?

Myoglobin - a molecule with a very very low affinity for oxygen which carries only one oxygen molecule. it is used to supply emergency oxygen to cells as a reserve.

What happens to increase the gradient of the curve?

Once the first oxygen molecule has been loaded, this changes the haemoglobin shape to make it easier for the next to load. This changes the shape to make it easier for the third to load, etc. This is positive co-operativity.

Describe how different environments mean different haemoglobins are needed?

Organisms which live in an environment where oxygen is scarce (eg high up, in water etc) need to have a high affinity for oxygen because they need to load it easily so they can collect as much as possible.

What does "oxygen affinity" mean?

Oxygen affinity is basically how easily oxygen is loaded and unloaded. If it has a high affinity, oxygen is loaded with ease and unloaded with difficulty. If it has a low affinity, oxygen is unloaded with ease and loaded with difficulty.

What is the shape of the curve?

Sigmoid. This means it starts out quite shallow, then gets very steep, then flattens out again before 100% saturation.

What is the Bohr Effect?

The Bohr effect is the way the concentration of carbon dioxide affects the affinity of haemoglobin for oxygen therefore allowing it to work efficiently at different parts of the body.

What is foetal haemoglobin like and why?

The foetus lives in an environment with restricted oxygen as it must all come through the mothers blood stream. It therefore has a higher affinity so it can load lots of oxygen to supply its needs.

If the curve shifts to the right, what does this mean?

The haemoglobin has a lower affinity for oxygen.

If the curve shifts to the left, what does this mean?

The haemoglobin loads oxygen at a lower pressure - i.e, the haemoglobin ahs a higher affinity for oxygen.

How does the Bohr effect change the oxygen dissociation curve?

The higher the carbon dioxide concentration, the lower the affinity, the further to the right. The lower the carbon dioxide concentration, the higher the affinity, the further to the left.

How does the Bohr effect ensure the most active tissues get the most oxygen?

The most active tissues respire the most therefore produce the most carbon dioxide. therefore the carbon dioxide concentration is highest so the pH is lowest (most acidic) this means the haemoglobin will have a very low affinity for oxygen so probably around 3 molecules will be released into the tissue - lots of oxygen so the respiration rate can be maintained.

What change in the haemoglobin leads to a change in the oxygen affinity?

The oxygen affinity is affected by the shape of the haemoglobin so when the shape changes, so does the affinity.

What relationship is modelled in an oxygen dissociation curve?

The relationship between the partial pressure (concentration) of oxygen and the saturation of the haemoglobin molecule with oxygen. Generally, the higher the concentration the more saturated the molecule will be.

Why is it useful that haemoglobin only unloads one oxygen molecule into a resting tissue?

The resting issue is doing little respiration therefore only needs a small amount of oxygen. this means that the rest of the oxygen in the haemoglobin molecule can be used as a reserve when the tissue becomes more active.

What is positive co-operativity?

The way each oxygen molecule makes it easier for the next to be loaded,

What are the polypeptide chains like?

There are four polypeptide chains, two alpha and two beta. They are called globins.

What is the main role of haemoglobin?

To transport oxygen from the lungs to the respiring tissues through the blood by combining to form oxyhaemoglobin.

What units does partial pressure of oxygen take?

Units of pressure - kiloPascals kPa

When CO2 is present, what happens to the oxygen affinity and why?

When CO2 is present, the pH is lowered slightly. This changes the tertiary structure of the haemoglobin and affects the shape. The changed shape means it has a lower affinity for oxygen so it unloads oxygen.

When CO2 is not present, what happens to the oxygen affinity?

When there is no CO2, the pH is raised slightly. This changes the tertiary structure of the haemoglobin which changes the shape. this changed shape means it has a higher affinity for oxygen so it loads oxygen easily.

What kind of haemoglobin might a very active animal need?

a very active animal needs lots of oxygen supplied to their cells. they therefore need a haemoglobin with a low oxygen affinity so it unloads oxygen easily into the cells.