3.1.2i/j Transport of Oxygen

Question 1

What contains Haemoglobin

Answer 1

Red blood cells

Question 2

What is Haemoglobin

Answer 2

• A globular protein with a quaternary structure
• Contains 4 subunits (2 alpha & 2 beta subunits), each consisting of a polypeptide (protein) chain & in the centre of each globin unit is a a haem (non-protein) group, where oxygen binding takes place. Therefore Hb molecule can bind with 4 oxygen molecules (one on each haem group)
• The haem group in each chain contains iron atom (Fe2+) (giving hb its red colour)
• Haem group has a high affinity for oxygen - each molecule can carry 4 oxygen molecules

Question 3

Equation of Haemoglobin

Answer 3

Haemoglobin + Oxygen <–> Oxyhaemoglobin

Hb + 4O2 <–> Hb(O2)4

This is a reversible reaction - when oxygen leaves oxyhemoglobin (dissociates from it) near the body cells, it turns back to haemoglobin

Question 4

What happens to the solution when oxygen binds to Haemoglobin

Answer 4

As oxygen binds to Hb, it is taken out of solution.
This maintains a steep concentration gradient

Question 5

What does Haemoglobin saturation depend on

Answer 5

Haemoglobin saturation depends on the partial pressure of oxygen

Question 6

What is the partial pressure of oxygen

Answer 6

• The partial pressure of oxygen (pO2) is a measure of oxygen concentration.
• The greater the concentration of dissolved oxygen in cells, the higher the partial pressure.

Question 7

How does Haemoglobin’s affinity for oxygen relate to partial pressure

Answer 7

Haemoglobin’s affinity for oxygen varies depending on the partial pressure of oxygen:
- Oxygen loads onto haemoglobin to form oxyhaemoglobin where theres a high pO2.
- Oxyhaemoglobin unloads its oxygen where theres a lower pO2.

Question 8

Haemoglobin’s loading/unloading of oxygen - lungs

Answer 8

1. Oxygen enters blood capillaries at the alveoli in the lungs. Alveoli have a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin
2. When cells respire, they use up oxygen - this **lowers the pO2). Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen
3. The haemoglobin then returns to the lungs to pick up more oxygen

Question 9

What does an oxygen dissociation curve show

Answer 9

An oxygen dissociation curve shows how saturated the haemoglobin is with oxygen at any give partial pressure.
The cooperative binding process of Hb is highlighted in the oxygen dissociation curve
(have graph on pg86 up)

Question 10

Dissociation curve: what does 100% saturation mean

Answer 10

100% saturation means every haemoglobin molecule is carrying the maximum of 4 molecules of oxygen

Question 11

Dissociation curve: what does 0% saturation mean

Answer 11

0% saturation means none of the haemoglobin molecules are carrying any oxygen

Question 12

Dissociation curve: what does it mean where pO2 is high

Answer 12

Where pO2 is high (eg. in the lungs), haemoglobin has a high affinity for oxygen (ie. it will readily combine with oxygen), so it has a high saturation of oxygen

Question 13

Dissociation curve: what does it mean where pO2 is low

Answer 13

Where pO2 is low (eg. in respiring tissues), haemoglobin has a low affinity for oxygen, which means it releases oxygen rather than combines with it. That’s why it has a low saturation of oxygen

Question 14

Dissociation curve: why is the graph sigmoidal (‘S-shaped’)

Answer 14

1. The graph is sigmoidal (‘S-shaped’) because when Haemoglobin combines with the first O2 molecule, its shape alters in a way that makes it easier for other molecules to join too.
2. But as Hb starts to become saturated it gets harder for more oxygen molecules to join.
3. As a result, the curve has a steep bit in the middle where its really easy for oxygen molecules to join, & shallow bits at each end where its harder. When the curve is steep, a small change in pO2 causes a big change in the amount of oxygen carried by the Hb.

Question 15

Difference between adult haemoglobin & fetal haemoglobin

Answer 15

Adult haemoglobin & fetal haemoglobin have different affinities for oxygen.
- Fetal haemoglobin has a higher affinity for oxygen (the fetus’s blood is better at absorbing oxygen than its mothers blood) at the same partial pressure of oxygen at the placenta

Question 16

Why does fetal haemoglobin have a higher affinity for oxygen than adult haemoglobin

Answer 16

1. The fetus gets oxygen from its mother’s blood across the placenta
2. By the time the mother’s blood reaches the placenta, its oxygen saturation has decreased (bc some has been used up by the mother’s body)
3. For the fetus to get enough oxygen to survive its haemoglobin has to have a higher affinity for oxygen (so it takes up enough)
4. If its haemoglobin had the same affinity for oxygen as adult haemoglobin, its blood wouldnt be saturated enough

Question 17

Difference between dissociation curve for fetal & adult haemoglobin

Answer 17

See pg87 for graph

Fetal is above adult

Question 18

How does haemoglobin get more oxygen to cells during activity

Answer 18

• Haemoglobin gives up its oxygen more readilyat higher partial pressures of CO2 (pCO2). This is a way of getting more oxygen to cells during activity.
• When cells respire, they produce CO2, which raises the pCO2, increasing the rate of oxygen unloading. The reason for this is linked to how CO2 affects blood pH

Question 19

How are RBCs adapted to carry more haemoglobin

Answer 19

RBCs do not have a nucleus, increasing the amount of space for Hb.
There are approximately 300 million Hb molecules in each RBC

Question 20

Where does oxygen bind to Haemoglobin

Answer 20

• Oxygen binds to Hb in the lungs, where the concentration of oxygen is high. Binding of oxygen is a cooperative process.
• This means that when an oxygen molecule binds to one of the haem groups, it causes a conformational change in the protein which makes it easier for oxygen to bind to the other haem group

Question 21

Dissociation curve in terms of joining of the 4 oxygen molecules

Answer 21

see on goodnotes images slide

Question 22

Transpiration of oxygen using Dissociation curve

Answer 22

• Once oxygen has loaded onto Hb, it is transported by the RBCs to tissues around the body, & unloaded into cells which need oxygen for respiration. (where pO2 is high - high end of curve)
• CO2 in the tissues creates an acidic environment, affecting the structure of Hb & lowering its affinity for oxygen. A small decrease in the pH results in a large decrease in the % saturation of oxygen (Hb unloads, lower pCO2 near start of curve)

Question 23

Unloading of oxygen from Haemoglobin

Answer 23

Oxygen unloads from Hb one molecule at a time & Hb returns to its deoxyhaemoglobin structure

Question 24

Structure of fetal haemoglobin

Answer 24

Fetal Hb has 2 alpha & 2 gamma globin chains - this structure enables it to bind oxygen w a greater affinity. This means that the fetal Hb will always load oxygen from the mother’s Hb

Question 25

Where is Haemoglobin found

Answer 25

Hb is found in the **RBCs** of most **vertebrates** & **some invertebrates**. Different organisms have different types of Hb & their dissociation curve reflects this (eg. an animal living at a high altitude (llama), where oxygen levels are reduced, needs to be able to pick up oxygen even at low levels so the curve shifts to the left)

Question 26

EXAM Q: Describe the role of haemoglobin in transporting oxygen around the body (3)

Answer 26

- Oxygen binds to the haem group, firing oxyhaemoglobin - In certain conditions, like increased CO2 concentrations, the affinity of Hb for O2 decreases. - As a result, oxygen dissociates from the Hb

Question 27

EXAM Q: Explain why the blood off-loads more oxygen to actively respiring tissues than to resting tissues (2)

Answer 27

- In actively respiring tissues, CO2 concentrations will be high (high pCO2) - High pCO2 reduces the affinity of Hb for oxygen - (use point of graph if given)

Question 28

EXAM Q: Explain why the difference between the two curves is essential for the survival of the fetus (4)

Answer 28

- Fetal blood & maternal blood run **close** to eachother in **placenta**. Therefore, there is diffusion of O2 from maternal Hb to fetal Hb - Fetal Hb has a **higher affinity** for O2. So O2 dissociates from maternal Hb & binds to the fetal Hb - Fetal Hb releases O2 for respiring cells in regions of **low pO2**.

Question 29

What is the PO2 in placenta

Answer 29

2-4kPa

Question 30

EXAM Q: Why is it necessary for the adult form of Hb to gradually replace the fetal form of Hb after birth (2)

Answer 30

- Oxygen will not be released readily enough for actively respiring cells - If the fetus becomes a mother, there will need to be a difference in the affinities of their Hb