7.2 Transport of oxygen by haemoglobin

Question 1

What is an oxygen dissociation curve

Answer 1

. When haemoglobin is exposed to different partial pressures of oxygen, it does not bind to the oxygen evenly

The graph of relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen is known as the oxygen dissociation curve

Question 2

How does the first oxygen molecule bind to haemoglobin

How does this affect the shape of the curve

Answer 2

. Shape of haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of its 4 polypeptide subunits because they are closely united

. Therefore at low oxygen concentrations, little oxygen binds to haemoglobin so the gradient of the curve is shallow initially

Question 3

How do the second and third oxygens bind to haemoglobin now that the first is bonded

Positive cooperativity

How does this affect the shape of the curve

Answer 3

. When the first oxygen molecule binds, it changes the quaternary structure of the haemoglobin molecule
. This causes it to change shape
. This makes it easier for other subunits to bind to an oxygen molecule

As a result, it takes a smaller increase in the partial pressure of oxygen to bind the second oxygen molecule to haemoglobin than it did for the first.

This is known as positive cooperativity as the binding of the first molecule makes the binding of second and third easier
So the curve steepens

Question 3

How does the fourth oxygen bind to a haemoglobin

Answer 3

. After the binding of the third, you would expect the fourth to bind even easier.

However, due to the probability with most of the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind to

As a result, the curve reduces and graph flattens off

Question 3

Why is there a large number of different dissociation curves

Answer 3

There are many different types of haemoglobin molecules in different species.
Also, the shape of a haemoglobin molecule can change under different conditions

They all have a rough similar shape but differ in their position on the axis

Question 4

Where would the graph move if there is a higher affinity of haemoglobin for oxygen

Answer 4

. It would move to the left

This is because the haemoglobin can take up oxygen more easily, so can get saturated with oxygen at a lower partial pressure of oxygen

Question 5

Where would the graph move if there is a lower affinity of haemoglobin for oxygen

Answer 5

It would move to the right

If the haemoglobin isn’t able to take up the oxygen as easily, it will require moore partial pressure of oxygen for it to get saturated with it

Question 6

How does carbon dioxide concentration affect the affinity of haemoglobin for oxygen

THE BOHR EFFECT

Answer 6

In the presence of carbon dioxide, haemoglobin has reduced affinity for oxygen

So the greater the concentration of carbon dioxide, the more readily the haemoglobin releases its oxygen

So this explains why the behavior of haemoglobin changes in different regions of the body

Question 7

How does the concentration of carbon dioxide affect affinity of haemoglobin for oxygen

IN THE LUNGS / GAS EXCHANGE SURFACE

What does this do to the curve

Answer 7

. The concentration of CO2 is low here because it diffuses across exchange surface and is excreted out of the organism
. The affinity of haemoglobin for oxygen has increased, and as there is a high concentration of oxygen in the lungs, the oxygen is loaded by the haemoglobin.

This reduced carbon dioxide has shifted the curve to the left

Question 8

How does the concentration of carbon dioxide affect affinity of haemoglobin for oxygen

IN THE RESPIRING TISSUES EG MUSCLES

Answer 8

. The concentration of CO2 here is high here do to respiration

. So the affinity of haemoglobin for oxygen is reduced, and there is also a low concentration of oxygen here

. As a result, the oxygen is readily unloaded/dissociated from haemoglobin in the muscle cells

As a result the curve shifts to the right

Question 9

Why does carbon dioxide make the haemoglobin dissociate oxygen more easily

Answer 9

. Dissolved carbon dioxide is acidic and has a low PH which can change the tertiary structure of the haemoglobin causing it to change shape

Question 10

What happens to oxygen at exchange surfaces eg lungs and what does this do to the haemoglobin

Answer 10

. The carbon dioxide is always being removed here, so the PH is slightly raised so is more alkaline due to there being a low concentration of CO2 which changes the tertiary structure of haemoglobin so it can load oxygen readily

. So the haemoglobin has a higher affinity for oxygen at these places

Question 11

What happens to oxygen in the tissues

Answer 11

. There are high levels of CO2 here due to respiring cells so the blood has a low PH as CO2 is acidic
. This changes shape of haemoglobin so it has a lower affinity for oxygen, so the oxygen is dropped off at the cells and is unloaded to the respiring cells

Question 12

The more active a tissue, the more oxygen is unloaded here.
Why is this

Answer 12

. Active tissues have a higher rate of respiration so more CO2 is produced there
. So there is a lower PH in the blood which changes structure of haemoglobin even more making it have a lower affinity for oxygen
. SO more oxygen is unloaded here

This means more oxygen is available for respiration

Question 13

Why is the overall saturation of haemoglobin at atmospheric pressure around 97 degrees

Answer 13

Not all haemoglobin molecules are fully saturated with their maximum 4 oxygen molecules.

So when this haemoglobin reaches a tissue with a low respiratory rate only one of these molecules is released so the blood returning to the lungs will still be 75% saturated with oxygen

However tissues with a high respiratory rate eg muscle cells will result in haemoglobin offloading 3 oxygen molecules

Question 14

Describe the haemoglobin of a lugworm that lives in a burrow in sand next to see which gets flooded in all the time and is not very active

Answer 14

. When water washes in over the lugworm, the oxygen can diffuse into the lugworms blood, and it uses haemoglobin to transport the oxygen to its tissues

. When the tide goes out the lugworm can’t circulate a fresh supply of oxygenated water in its burrow so there is less oxygen available here.

As a result the haemoglobin of the lugworm will have a high affinity for oxygen to be able to extract it from the water, so a low partial pressure of oxygen is needed for it to bind to haemoglobin

Question 14

Why do different species have different types of haemoglobin

eg Llama

Answer 14

Mammals living in different climates and conditions mean their haemoglobin is adapted differently.

Llamas living in a very high altitude environment with a low partial pressure of oxygen will have adapted haemoglobin that have a higher affinity for oxygen than human haemoglobin.