Haemoglobin

Question 1

What are haemoglobins

Answer 1

Protein molecules with a quarternary structure that has evolved to make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions.

Question 2

Describe the structure of a haemoglobin molecule

Answer 2

Primary structure: the sequence of amino acids in the four polypeptide chains.
Secondary structure: Each of the four polypeptide chains is coiled into a helix.
Tertiary structure: Each polypeptide chain is folded into a specific shape- this is key to its ability to carry oxygen.
Quaternary structure: the four polypeptides are all linked together to form an almost spherical molecule. Each polypeptide 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 oxygen molecules that can be carried by a single haemoglobin molecule in humans.

Question 3

What is loading/associating and where does it take place in humans

Answer 3

The process by which haemoglobin binds with oxygen. It takes place in the lungs.

Question 4

What is unloading/dissociating and where does it take place in humans

Answer 4

The process by which haemoglobin releases oxygen. It takes place in the tissues.

Question 5

What does a haemoglobin having a high affinity for oxygen mean

Answer 5

It can take up oxygen more easily, but release it less easily.

Question 6

What does a haemoglobin having a low affinity for oxygen mean

Answer 6

It can take up oxygen less easily, but release it more easily.

Question 7

What two things must haemoglobin be able to do to be efficient at transporting oxygen

Answer 7

• readily associate with oxygen at the surface where gas exchange takes place
• readily dissociate from oxygen at the tissues requiring it

Question 8

What key thing does haemoglobin do to make it efficient at transporting oxygen

Answer 8

Haemoglobin changes its affinity (chemical attraction) for oxygen under different conditions. It achieves this by changing its shape in the presence of certain substances such as carbon dioxide.

Question 9

Describe the affinity of haemoglobin when found at the gas exchange surface of the body (lungs) and what this means.

Answer 9

The affinity of haemoglobin is high so oxygen is associated.
This is because there is a high oxygen concentration and low carbon dioxide concentration.

Question 10

Describe the affinity of haemoglobin when found in respiring tissues and what this means

Answer 10

The affinity of the haemoglobin is low which means oxygen is dissociated.
This is because at respiring tissues the oxygen concentration is low and the carbon dioxide concentration is high.

Question 11

How do different haemoglobin molecules have different affinities for oxygen

Answer 11

Each species produces a haemoglobin molecules 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.

Question 12

What is an oxygen dissociation curve

Answer 12

The graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen.

Question 13

Explain why the gradient of the oxygen dissociation curve is shallow initially.

Answer 13

• The shape of the haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of the sites on its four polypeptide subunits because they are closely united.
• Therefore at low oxygen concentrations, little oxygen binds to haemoglobin and so the gradient of the curve is shallow initially.

Question 14

Explain why the gradient of the oxygen dissociation curve steepens (in the middle)

Answer 14

• The binding of the first oxygen molecule changes the quaternary structure of the haemoglobin molecule, causing it to change shape.
• This change makes it easier for the other subunits to bind to an oxygen molecule-the binding of the first oxygen molecule induces the other subunits to bind to an oxygen molecule.
• It therefore takes a smaller increase in the partial pressure of oxygen to bind the second oxygen molecule than it did to bind the first one.
• This is known as positive cooperativity because binding the first molecule makes binding of the second easier and so on- this causes the gradient to steepen.

Question 15

Explain why the gradient of the oxygen dissociation curve reduces and the graph flattens off (towards the end)

Answer 15

• After the third oxygen molecule binds to the haemoglobin, in theory it should be easier for the fourth oxygen molecule to bind.
• However, in practice it is harder and this is due to probability. With the majority of the binding sites occupied, it is less likely that a single oxygen molecule will find an empty sister to bind to.
• This causes the gradient of the curve to decrease- the grpah flattens off.

Question 16

Which two facts allow us to easily compare different oxygen dissociation curves

Answer 16

• The further to the left the curve is, the greater affinity for oxygen the haemoglobin has (so it loads oxygen readily but unloads it less easily)
• The further to the right the curve is, the affinity for oxygen that the haemoglobin has is lower (so it loads oxygen less readily but unloads it more easily)

Question 17

What affect does carbon dioxide concentration have on haemoglobin

Answer 17

Haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide. The greater the concentration of carbon dioxide, the more readily the haemoglobin releases its oxygen.

Question 18

Explain how the behaviour of haemoglobin changes when at the gas exchange surface in the body and the affect this has on the oxygen dissociation curve.

Answer 18

• At the gas exchange surface (lungs), the concentration of carbon dioxide is low because it diffuses across the exchange surface and is excreted from the organism.
• The affinity of haemoglobin for oxygen is increased, which, coupled with the high concentration of oxygen in the lungs, means that oxygen is readily loaded by haemoglobin.
• The reduced carbon dioxide concentration has shifted the oxygen dissociation curve to the left.

Question 19

Explain how the behaviour of haemoglobin changes when at respiring tissues in the body and how this affects the oxygen dissociation curve

Answer 19

• In rapidly respiring tissues, the concentration of carbon dioxide is high.
• The affinity of haemoglobin for oxygen is reduced, which, coupled with the low concentration of oxygen in the muscles, means that oxygen is readily unloaded from the haemoglobin into the muscle cells.
• The increased carbon dioxide concentration has shifted the oxygen dissociation curve to the right.

Question 20

Why does haemoglobin more readily release its oxygen (have a low affinity to) when there is a high concentration of carbon dioxide present

Answer 20

Dissolved carbon dioxide is acidic and the low PH causes the haemoglobin to change shape and have a lower affinity for oxygen

Question 21

Explain how haemoglobin changes shape under different conditions to alter its affinity for oxygen.

Answer 21

• At the gas exchange surface carbon dioxide is being constantly removed.
• The PH is raised slightly due to the low concentration of carbon dioxide.
• The higher PH changes the shape of the haemoglobin into one that enables it to load oxygen readily.
• This shape also increases the affinity of haemoglobin for oxygen so it is not released whilst being transported in the blood to the tissues.
• In the tissues, carbon dioxide is produced by respiring cells.
• Carbon dioxide is acidic in solution, so the PH of the blood within the tissues is lowered.
• The lower PH changes the shape of haemoglobin into one with a lower affinity for oxygen.
• haemoglobin releases its oxygen into respiring tissues.

Question 22

At around what percentage is the overall saturation of haemoglobin at atmospheric pressure

Answer 22

97%

Question 23

What happens when the haemoglobin in humans reaches a tissue with a low respiratory rate

Answer 23

Only one of the oxygen molecules is normally released. The blood returning to the lungs therefore contains haemoglobin that is still 75% saturated with oxygen.

Question 24

What happens when the haemoglobin in humans reaches a tissue with a high respiratory rate (for example, and exercising muscle)

Answer 24

Three oxygen molecules are unloaded from each haemoglobin molecule.

Question 25

Describe how the structure of haemoglobin varies when adapting to the environment

Answer 25

Species of animals that live in an environment with a low partial pressure of oxygen have evolved haemoglobin that has a higher affinity for oxygen than the haemoglobin of animals that live where the partial pressure of oxygen is higher.