Exchange between Organisms (Mass Transport) - Haemoglobin

Question 1

What are haemoglobin molecules?

Answer 1

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.

Question 2

What is the structure of a haemoglobin molecule?

Answer 2

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.

Question 3

How is oxygen loaded and unloaded?

Answer 3

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.

Question 4

What do haemoglobins with a high and low affinity for oxygen do?

Answer 4

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.

Question 5

What is the role of haemoglobin?

Answer 5

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

Question 6

How are the requirements for haemoglobin achieved?

Answer 6

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.

Question 7

How does an environmental change affect a proteins structure?

Answer 7

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.

Question 8

What happens at the gas exchange surface?

Answer 8

• oxygen concentration: high
• carbon dioxide concentration: low
• affinity of haemoglobin for oxygen: high
• result: oxygen is associated

Question 9

What happens at the respiring tissues?

Answer 9

• oxygen concentration: low
• carbon dioxide concentration: high
• affinity of haemoglobin for oxygen: low
• result: oxygen is dissociated

Question 10

Why are there different haemoglobins?

Answer 10

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.

Question 11

How many oxygen molecules can each molecule of haemoglobin carry?

Answer 11

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.

Question 12

What does the affinity of oxygen for haemoglobin depend on?

Answer 12

• 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.

Question 13

What is affinity?

Answer 13

an attraction of one thing to another

Question 14

What is the Bohr effect?

Answer 14

states that haemoglobin’s oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide

Question 15

What is dissociation?

Answer 15

the separation of one molecule from another

Question 16

What is association?

Answer 16

the joining of one molecule to another

Question 17

What is haemoglobin?

Answer 17

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)

Question 18

What is partial pressure?

Answer 18

a measure of the concentration of a substance in a particular area

Question 19

How does oxygen bind to each polypeptide chain in haemoglobin?

Answer 19

• 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

Question 20

Why do red blood cells contain haemoglobin?

Answer 20

Red blood cells contain haemoglobin, enabling them to carry far more oxygen than if it was only dissolved in their cytoplasm.

Question 21

What is the equation for when oxygen binds to haemoglobin?

Answer 21

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

Question 22

Where does oxygen load and unload?

Answer 22

O2 loads onto haemoglobin at a high partial pressure of O2. Haemoglobin becomes fully saturated with oxygen in the lungs.

O2 unloads at the cells where partial pressure of O2 is low.

Question 23

What happens to haemoglobin when oxygen molecules associate with it?

Answer 23

• The shape of haemoglobin changes as oxygen molecules associate with it.
• This changes the ability of haemoglobin to associate with further oxygen molecules.

Question 24

What are oxygen dissociation curves?

Answer 24

When haemoglobin is exposed to different partial pressures of oxygen, it does not bind to the oxygen evenly. The graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen is known as the oxygen dissociation curve.

Question 25

Why is the gradient of the curve shallow initially?

Answer 25

• 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.
• The gradient of the curve is shallow initially.

Question 26

Why does the gradient of the curve steepen (cooperative binding)?

Answer 26

• The binding of this first oxygen molecules 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 (induction).
• 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 of the first molecule makes binding of the second easier and so on.
• The gradient of the curve steepens.

Question 27

Why does the gradient of the curve reduce and flatten out?

Answer 27

• The situation changes after the binding of the third molecule.
• While in theory it is easier for haemoglobin to bind the fourth oxygen molecule, in practice it is harder.
• This is simply due to probability.
• With the majority of the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind on.
• The gradient of the curve reduces and the graph flattens out.
• The curve plateaus just below 100%.

Question 28

Why is there a large number of different oxygen dissociation curves?

Answer 28

• there are different types of haemoglobin molecules in different species, each with a different shape and hence a different affinity for oxygen
• the shape of any one type of haemoglobin molecule can change under different conditions
• they all have a roughly similar shape but differ in their positions on the axes

Question 29

What does it mean if the curve is to the left?

Answer 29

the further to the left of the curve, the greater is the affinity of haemoglobin for oxygen (so it loads oxygen readily but unloads it less readily)

Question 30

What does it mean if the curve is to the right?

Answer 30

the further to the right the curve, the lower is the affinity of haemoglobin for oxygen (so it loads oxygen less readily but unloads it more easily)

Question 31

How do you measure gas concentration?

Answer 31

the amount of a gas that is present in a mixture of gases is measured by the pressure it contributes to the total pressure of the gas mixture - this is known as the partial pressure of the gas

Question 32

What is the effect of carbon dioxide on haemoglobin?

Answer 32

Haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide because of the lowered pH. The greater the concentration of carbon dioxide, the more readily the haemoglobin releases its oxygen (the Bohr effect).

Question 33

How does the Bohr effect explain why the behaviour of haemoglobin changes at the gas-exchange surface?

Answer 33

• At the gas-exchange surface (e.g. 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 34

How does the Bohr effect explain why the behaviour of haemoglobin changes in rapidly respiring tissues?

Answer 34

• In rapidly respiring tissues (e.g. muscles), 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 35

Why does a greater concentration of carbon dioxide mean haemoglobin releases its oxygen more readily?

Answer 35

because carbon dioxide dissolves in water to form carbonic acid and the low pH causes haemoglobin to change shape so that it has a lower affinity for oxygen

Question 36

How does a change in pH affect the affinity of haemoglobin for oxygen at the gas-exchange surface?

Answer 36

• At the gas-exchange surface, carbon dioxide is constantly being removed.
• The pH is slightly raised due to the low concentration of carbon dioxide.
• The higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily (more oxygen binding).
• This shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues.

Question 37

How does a change in pH affect the affinity of haemoglobin for oxygen in respiring tissues?

Answer 37

• 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 the respiring tissues (more oxygen dissociation).

Question 38

The more active a tissue, the more oxygen is unloaded to ensure there is sufficient oxygen for respiring tissues. How does this work?

Answer 38

• > the higher the rate of respiration
• > the more carbon dioxide the tissue produces
• > the more the carbon dioxide dissolves in water to form carbonic acid
• > the lower the pH
• > the greater the haemoglobin shape change
• > the more readily oxygen is unloaded
• > the more oxygen is available for respiration

Question 39

What happens when haemoglobin reaches a tissue with a low and high respiratory rate?

Answer 39

• When haemoglobin reaches a tissue with a low respiratory rate, only one of the four oxygen molecules will normally be released. The blood returning to the lungs will therefore contain haemoglobin that is still 75% saturated with oxygen.
• If a tissue is very active, then three oxygen molecules will usually be unloaded from each haemoglobin molecule.

Question 40

How have different types of haemoglobin evolved within species?

Answer 40

Different species have different types of haemoglobin, each with its own different oxygen dissociation curve. These different types have evolved within species as adaptations to different environments and conditions.

For example, species of animals that live in an environment with a lower 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.

Question 41

What does the oxygen dissociation curve of a lugworm look like?

Answer 41

The lugworm is not very active, spending almost all of its life in a u-shaped burrow. Most of the time, the lugworm is covered by sea water, which it circulates through its burrow. Oxygen diffuses into the lugworm’s blood from the water and it uses haemoglobin to transport oxygen to its tissues.

When the tide goes out, the lugworm can no longer circulate a fresh supply of oxygenated water through its burrow. As a result, the water in the burrow contains progressively less oxygen as the lugworm uses it up (oxygen-depleted environment). The lugworm has to extract as much oxygen as possible from the water in the burrow if it is to survive until the tide covers it again.

The dissociation curve is shifted far to the left. This means that the haemoglobin of the lugworm is fully loaded with oxygen even when there is little available in its environment.

Question 42

What does the oxygen dissociation curve of a llama look like?

Answer 42

A llama is an animal that lives at high altitudes. At these altitudes, the atmospheric pressure is lower and so the partial pressure of oxygen is also lower. It is therefore difficult to load haemoglobin with oxygen. Llamas have a type of haemoglobin that has a higher affinity for oxygen, so the curve is shifted to the left.

Question 43

Explain how oxygen is loaded, transported and unloaded in the blood.

Answer 43

• Haemoglobin carries oxygen / has a high affinity for oxygen;
• Found in red blood cells;
• Association (loading) of oxygen takes place in the lungs;
• At high pO2;
• Dissociation (unloading) of oxygen to respiring cells or tissues;
• At low pO2;

Question 44

What does the oxygen dissociation curve of smaller animals look like?

Answer 44

• smaller animals (shrews/mice) and birds
• have high metabolism
• curve shifts to the right
• need oxygen to be released more readily

Question 45

Why does carbon monoxide cause a left shift?

Answer 45

Binding of one CO molecule to haemoglobin increases the affinity of the other binding spots for oxygen, leading to a left shift in the dissociation curve. This shift prevents oxygen unloading in peripheral tissue and therefore the oxygen concentration of the tissue is much lower than normal.

Question 46

What feature of a respiratory pigment determines its oxygen-carrying capacity?

Answer 46

• the haem (prosthetic) group

- the level of oxygen in the blood

Question 47

How is the oxygen-carrying capacity of haemoglobin related to metabolic activity across different animal taxa?

Answer 47

the higher the metabolic activity, the higher the oxygen-carrying capacity

Question 48

What is myoglobin?

Answer 48

In the muscles, oxygen from haemoglobin is transferred to and retained by myoglobin, a molecule that is chemically similar to haemoglobin except that it consists of only one haem-globin unit.

Myoglobin has a greater affinity for oxygen than haemoglobin and acts as an oxygen store within muscles, releasing the oxygen during periods of prolonged or extreme muscular activity.

Question 49

How do respiratory pigments increase the carrying capacity of the blood?

Answer 49

by combining with oxygen in a reversible reaction

Question 50

What is the significance of the very high affinity of myoglobin for oxygen?

Answer 50

Allows myoglobin to pick up oxygen from haemoglobin and therefore serves as an oxygen store when oxygen tensions begin to fall.

Question 51

What is cooperative binding?

Answer 51

The more oxygen that is bound to haemoglobin, the easier it is for more oygen to bind.

Question 52

What is the need for a transport system?

Answer 52

Required to take materials from cells to exchange surfaces and from exchange surfaces to cells. They must have:

• a suitable medium to carry materials
• a form of mass transport in which the transport medium is moved around in bulk over large distance
• a closed system of tubular vessels
• a mechanism for moving the transport medium within vessels.

Question 53

Why must fetal haemoglobin have a higher affinity than adult haemoglobin?

Answer 53

• fetal haemoglobin has a different affinity for oxygen compared to adult haemoglobin
• as it needs to be better at absorbing oxygen because by the time oxygen reaches the placenta, the oxygen saturation of the blood has decreased
• therefore, fetal haemoglobin must have a higher affinity for oxygen in order for the foetus to survive at low partial pressure