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8.2.38 · Transport in Mammals

Plasma and Carbon Dioxide — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 13 of 21 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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8.2.3 Plasma and Carbon Dioxide — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the three ways in which carbon dioxide is transported in the blood and the part played by plasma.Origin — aerobic respiration in mitochondria produces CO2, mainly in the link re…

  • Need to remove it
  • Homeostasis
  • Reaction in plasma
  • Carbamino compounds
  • Haldane effect
  • Plasma proteins

Part 1 of 8Source of carbon dioxide

Origin
  • Origin aerobic respiration in mitochondria produces CO2, mainly in the link reaction and the Krebs cycle.
Movement
  • Movement it diffuses down a concentration gradient out of cells into tissue fluid and then into capillary blood.
Need to remove it
  • Need to remove it if CO2 accumulated it would lower blood pH and could denature enzymes.
Homeostasis
  • Homeostasis breathing rate adjusts to the CO2 concentration of the blood.

Part 2 of 8Three forms of transport

FormApproximate percentageWhere
Hydrogencarbonate ions, HCO3-About 85Formed in red cells then carried in plasma
CarbaminohaemoglobinAbout 10In red cells
Dissolved CO2About 5In plasma

Part 3 of 8Hydrogencarbonate in plasma

Formation
  • CO2 diffuses into red cells, where carbonic anhydrase converts it to carbonic acid, which dissociates into H+ and HCO3-.
Carriage
  • HCO3- leaves the red cell and is carried in plasma, which gives plasma a large capacity for CO2.
Reaction in plasma
  • Reaction in plasma a small amount of CO2 reacts directly with water in plasma, but far more slowly because plasma has little carbonic anhydrase.

Part 4 of 8Carbaminohaemoglobin and dissolved CO_{2}

Carbamino compounds
  • CO2 binds to the amino groups at the ends of haemoglobin polypeptide chains, not to the haem group.
Haldane effect
  • Haldane effect deoxyhaemoglobin binds CO2 more readily than oxyhaemoglobin.
  • More CO2 is carried in the tissues, where oxygen has been released.
  • It is released in the lungs as haemoglobin takes up oxygen.
Plasma proteins
  • Plasma proteins they can bind a small amount of CO2 in the same way.
Dissolved CO2
  • CO2 is more soluble than oxygen, but this is only a small fraction.
  • Dissolved CO2 diffuses across membranes and sets the partial pressure.
  • This partial pressure controls the direction of the other reactions.
  • For comparison, only 1.5 per cent of oxygen is dissolved and 98.5 per cent is combined with haemoglobin.

Part 5 of 8Gas exchange of CO_{2} in tissues and lungs

Tissues
  • Tissues pCO2 in respiring tissue is about 6 kPa, compared with about 5.3 kPa in arriving blood, so CO2 diffuses into the blood.
Lungs
  • Lungs alveolar air is about 5.3 kPa and blood reaching the lungs about 6 kPa, so CO2 diffuses out.
Reversal
  • Reversal the fall in plasma CO2 draws more CO2 from the red cells and reverses the earlier reactions.

Part 6 of 8Control of breathing

Chemoreceptors
  • Chemoreceptors they are in the medulla oblongata, carotid bodies and aortic bodies.
Stimulus
  • Stimulus a rise in CO2 lowers blood pH, which these receptors detect.
Response
  • Response impulses increase the rate and depth of breathing, so more CO2 is exhaled and pH returns to normal.
Negative feedback
  • Negative feedback this is a negative feedback loop.
Breath holding
  • Breath holding the urge to breathe is triggered by rising CO2, not falling oxygen.

Part 7 of 8More from the specification

1Carbamino-haemoglobin terminal binding sites
  • Carbamino-haemoglobin terminal binding sites carbon dioxide combines directly with terminal amino groups, regulating affinity at oxygen binding sites.
2Oxygen Dissociation Curve and Cooperative Binding
  • The oxygen dissociation curve of haemoglobin exhibits a sigmoidal shape on a saturation graph due to cooperative binding: the binding of the first oxygen molecule alters quaternary shape,
  • increasing affinity for subsequent oxygen molecules.
3The Bohr Shift and Oxygen Release
  • Elevated partial pressures of carbon dioxide in actively respiring tissues shift the oxygen dissociation curve to the right,
  • which means haemoglobin has a lower affinity for oxygen and unloads O2 more readily at respiring tissues.
4Carbon Dioxide Transport in Blood Plasma and Erythrocytes
  • Carbonic anhydrase within red blood cells converts dissolved CO2 into carbonic acid,
  • which dissociates into hydrogen ions (H+) and hydrogencarbonate ions (HCO3−);
    • HCO3− ions then diffuse out into blood plasma.
5The Chloride Shift in Red Blood Cells
  • To maintain electrical neutrality across the erythrocyte membrane when negatively charged hydrogencarbonate ions leave the cell, chloride ions (Cl−) diffuse in from plasma, a mechanism known as the chloride shift.
6Oxygen Dissociation Curve and Cooperative Binding
  • The oxygen dissociation curve of haemoglobin exhibits a sigmoidal shape on a saturation graph due to cooperative binding: the binding of the first oxygen molecule alters quaternary shape,
  • increasing affinity for subsequent oxygen molecules.
7The Bohr Shift and Oxygen Release
  • Elevated partial pressures of carbon dioxide in actively respiring tissues shift the oxygen dissociation curve to the right,
  • which means haemoglobin has a lower affinity for oxygen and unloads O2 more readily at respiring tissues.
8Carbon Dioxide Transport in Blood Plasma and Erythrocytes
  • Carbonic anhydrase within red blood cells converts dissolved CO2 into carbonic acid,
  • which dissociates into hydrogen ions (H+) and hydrogencarbonate ions (HCO3−);
    • HCO3− ions then diffuse out into blood plasma.
9The Chloride Shift in Red Blood Cells
  • To maintain electrical neutrality across the erythrocyte membrane when negatively charged hydrogencarbonate ions leave the cell, chloride ions (Cl−) diffuse in from plasma, a mechanism known as the chloride shift.
10Oxygen Dissociation Curve and Cooperative Binding
  • The oxygen dissociation curve of haemoglobin exhibits a sigmoidal shape on a saturation graph due to cooperative binding: the binding of the first oxygen molecule alters quaternary shape,
  • increasing affinity for subsequent oxygen molecules.
11The Bohr Shift and Oxygen Release
  • Elevated partial pressures of carbon dioxide in actively respiring tissues shift the oxygen dissociation curve to the right,
  • which means haemoglobin has a lower affinity for oxygen and unloads O2 more readily at respiring tissues.
12Carbon Dioxide Transport in Blood Plasma and Erythrocytes
  • Carbonic anhydrase within red blood cells converts dissolved CO2 into carbonic acid,
  • which dissociates into hydrogen ions (H+) and hydrogencarbonate ions (HCO3−);
    • HCO3− ions then diffuse out into blood plasma.
13The Chloride Shift in Red Blood Cells
  • To maintain electrical neutrality across the erythrocyte membrane when negatively charged hydrogencarbonate ions leave the cell, chloride ions (Cl−) diffuse in from plasma, a mechanism known as the chloride shift.
14Chloride shift mechanism
  • Chloride shift mechanism chloride ions diffuse into erythrocytes from plasma to counterbalance hydrogencarbonate ions exiting the cell.
15Carbaminohaemoglobin formation
  • Carbaminohaemoglobin formation carbon dioxide binds directly to terminal amino groups of haemoglobin molecules for transport in blood.
16Bohr shift to the right
  • Bohr shift to the right acidic conditions shift the oxygen dissociation graph right, promoting oxygen unloading to active muscle tissue.
17Saturation
  • The point where all transport proteins are in use.
18Affinity
  • How readily haemoglobin binds oxygen.
19Dissociation curve
  • Graph of percentage saturation of haemoglobin against pO2.
20Chloride shift
  • Exchange of bicarbonate for chloride across the red cell membrane.
21Binding site
  • The region of a protein with a shape complementary to a ligand.
22Percentage saturation
  • The proportion of haem groups combined with oxygen.

Part 8 of 8More on this topic

  • Method
    • read across from the percentage saturation to the curve and down to pO2, quoting kPa.

Key terms

TermMeaning
Hydrogencarbonate ionHCO_{3}^{-}, the main form in which CO_{2} is carried in plasma.
CarbaminohaemoglobinHaemoglobin combined with CO_{2} at its amino groups.
Haldane effectDeoxyhaemoglobin binds CO_{2} more readily than oxyhaemoglobin.
Partial pressure of CO_{2}The pressure exerted by CO_{2} in a mixture, which drives diffusion.
ChemoreceptorA receptor that detects a chemical change such as a fall in blood pH.
Carbonic anhydraseEnzyme in red cells that catalyses CO_{2} + H_{2}O ⇌ H_{2}CO_{3}.

Avoid these mistakes

1
Saying most CO_{2} is carried dissolved in plasma — only about 5 per cent is.
2
Saying CO_{2} binds to the haem group — it binds to amino groups of the globin chains.
3
Saying carbonic anhydrase is in plasma — it is in red cells.
4
Saying breathing is driven mainly by low oxygen — it is driven mainly by rising CO_{2}.

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