transport of O2 and CO2

What are erythrocytes

Red blood cells

What is the role of erythrocytes

Transport oxygen around the body

What is haemoglobin

• a pigment in red blood cells

• Globular protein made of four polypeptide chains (2 α-alpha chains and 2 β- chains)

• Each chain has a haem prosthetic group contain iron

• Oxygen binds to haemoglobin to form oxyhemoglobin in a reversible reaction

What are the three ways which carbon dioxide is transported in the blood

• 5% dissolved in blood plasma

• 10-20% combines with haemoglobin to form carbaminohaemoglobin

• 75-85% converted into hydrogen carbonate ions (HCO3-) in the cytoplasm of erythrocytes

Which enzyme catalyses the production of hydrogen carbonate ions

Carbonic anhydrase

Describe how hydrogen carbonate ions are produced in erythrocytes

• carbon dioxide diffuses into erythrocytes

• Reacts with water, catalysed by carbonic anhydrase

• Forms carbonic acid (H2CO3)

• Carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions

Give the equation for the conversion of CO2 to HCO3-

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

What happens to hydrogen carbonate ions after they have been produced

• diffuse down concentration gradient

• Out of red blood cells and into plasma

What is the chloride shift

The movement of Cl- ions into erythrocytes to balance the electrical change due to the negative hydrogen carbonate ions leaving.

How does haemoglobin act as a buffer

Prevents changes in pH by accepting 3 H+ ions to form haemoglonobic acid

Define partial pressure

The contributing pressure of a single gas to the total pressure of a mixture of gases

What happens to hydrogen carbonate ions at the lungs

• low partial pressure of carbon dioxide (pCO2) at lungs

• HCO3- diffuses back to erythrocytes

• Reacts with H+ to form H2CO3

• Carbonic anhydrase catalyses conversion of H2CO3 back into water and CO2

• CO2 diffuses out of blood into lungs

• Cl- ions diffuse out of erythrocytes into plasma

Describe the movement of oxygen in the lungs

• high partial pressure of oxygen (pO2) in alveoli

• Lower partial pressure of oxygen in blood in capillaries

• Oxygen diffuses from alveoli to blood

Describe the movement of oxygen at respiring tissues

• high partial pressure of oxygen in blood

• Lower partial pressure of oxygen in respiring tissues

• Oxygen diffuses from blood to respiring cells

Describe the role of haemoglobin in transporting oxygen around the body

• Haemoglobin has a high affinity for oxygen so it binds to haemoglobin in lungs (high pO2) forming oxyhemoglobin.

• Oxygen is released in respiring tissues (low pO2)

By what process does oxygen bind to haemoglobin to produce the sigmoidal shape of the oxygen dissociation curve

Cooperative binding

Explain how the binding of one oxygen molecule to a haem group affects haemoglobin

• binding of first oxygen molecule causes conformational change altering tertiary structure of haemoglobin (slightly)

• Affinity for oxygen increases so further loading of oxygen molecules is easier

Explain how removal of the first oxygen at respiring tissues affects haemoglobin

• removal of first molecule causes conformational change

• Affinity for oxygen decreases (next molecule can leave more easily)

Explain the significance of the oxygen dissociation curve

• High pO2 in lungs so haemoglobin rapidly loaded with oxygen

• Relative small drop in pO2 at respiring tissues leads to rapid dissociation of oxygen so it is free to diffuse into cells.

Explain the oxygen dissociation curve for human haemoglobin

• Haemoglobin becomes saturated at very high pO2

• As pO2

Describe difference in oxygen affinity between fetal haemoglobin and adult haemoglobin

Fetal haemoglobin has a higher affinity for oxygen

Explain why fetal haemoglobin curve is to the left of the adult haemoglobin curve

• placenta has low pO2

• Adult haemoglobin will release O2 at the placenta

• Foetal haemoglobin has a higher affinity for oxygen at low pO2

• It is able to take up oxygen in the placenta

What effect does carbon dioxide have on haemoglobin

at higher pCO2, haemoglobin gives up oxygen more easily

What effect does higher pCO2 have on the oxygen dissociation curve

• shifts to the right

• Bohr shift

Why is the bohr effect important

• Actively respiring tissues have high pCO2, haemoglobin gives up oxygen more easily

• Lungs have lower pCO2, haemoglobin binds to oxygen more easily

Explain how an increase in CO2 in the blood leads to the release of more O2 from red blood cells

• more carbon dioxide diffuses in to red blood cells

• More carbonic acid is formed by carbonic anhydrase

• More hydrogen ions formed

• Haemoglobin has a high affinity for hydrogen ions

• Haemoglobin binds to hydrogen ions to form haemoglobinic acid (HHb)

• Formation of HHb decreases the affinity of haemoglobin for oxygen

• Carbon dioxide binds to -NH2 of polypeptides to form carbaminohaemoglobin

• Causes change in tertiary structure, releasing more oxygen

Outline the benefits of the bohr shift to actively respiring tissue

• Actively respiring tissue requires more oxygen for aerobic respiration

• Actively respiring tissue produces more CO2

• Haemoglobin involved in transport of CO2 so there’s less haemoglobin available to combine with O2

• Bohr shift causes more oxygen to be released

What effect does pH have on Hb’s affinity for oxygen?

• Affinity decreases as pH decreases (more H+ ions)