L6 - oxygen dissociation

• Erythrocytes:

  -red blood cells

  -biconcave shape

  -no nucleus

  -numerous
• Biconcave shape:

  -increases surface area for oxygen absorption

  -enables red blood cells to flex and squeeze through narrow capillaries
• No nucleus:

  -maximises space for oxygen transport

  -limits life span of red blood cells to approx 120 days as they cannot divide or repair
• Numerous:

  -outnumber white blood cells 1000:1

  -continuously produced in bone marrow (approx 2 million per second) to replace old cells
• Haemoglobin (Hb) structure and function:

  -globular protein made of four polypeptide subunits

  -each subunit contains a haem group with an iron (Fe2+) ion

  -each haem group binds one oxygen molecule (a single haemoglobin molecule carries four oxygen molecules)
• Haemoglobin (Hb):

  -there are about 300 million haemoglobin molecules in each red blood cell

  -in theory they can carry 1200 million oxygen molecules in each cell
• Association and dissociation:

  -in the lungs, where oxygen levels are high, haemoglobin binds to oxygen to form oxyhaemoglobin

  -in body tissues, where oxygen levels are low, haemoglobin dissociates from (releases) the oxygen
• 98% of all the bloods oxygen is carried as oxyhaemoglobin
• Partial pressure of oxygen($pO_2$) is a measure of the concentration of oxygen in a mixture of gases
• Partial pressure of oxygen:

  -the greater the concentration of dissolved oxygen in cells, the higher the partial pressure

  -it can also be referred to as oxygen tension

  -$pO_2$is measured in kPa
• Alveoli have a higher $pO_2$ than red blood cells, causing oxygen to associate with haemoglobin, forming oxyhaemoglobin
• Saturation: the extent to which something is filled or occupied to its maximum capacity
• If all four haem groups are occupied by oxygen the molecule is said to be saturated
• Affinity: an attractive force between substances or particles
• Affinity:

  -the haem group has an affinity for oxygen

  -haemoglobin’s affinity for oxygen varies depending on the partial pressure of oxygen
• Association curve: the graph is “s-shaped” or “sigmoid” because of the way in which the haemoglobin molecules bind and release their 4 oxygen molecules

  
• At low partial pressure, the first oxygen molecule binds with difficulty because the haem groups are positioned within the haemoglobin structure, making them less accessible. This results in a shallow gradient at the start of the dissociation curve

  
• Once the first oxygen molecule binds, a conformational change occurs, altering the quaternary structure of haemoglobin.This change exposes the remaining haem groups, making it easier for additional oxygen molecules to bind.The curve steepens, reflecting the increased rate of oxygen uptake.

  
• As more oxygen binds, haemoglobin approaches full saturation. With most haem groups already occupied, there are fewer available binding sites slowing the rate of further oxygen association. This causes the curve to level off

  
• Fetal haemoglobin has a higher oxygen affinity than the adult haemoglobin found in the mother‘s blood. This is vital as it allows a foetus to obtain oxygen from its mother‘s blood at the placenta:

  fetal haemoglobin can bind to oxygen at a low partial pressure

  -at this low partial pressure, the mother’s haemoglobin is dissociating with oxygen
• Adaptations:

  -species that live where oxygen is scarce (such as high altitudes, deep water, or burrows) need haemoglobin that can bind more easily even at low partial pressures

  -their oxygen dissociation curve shifts to the left, meaning haemoglobin becomes more saturated with oxygen at lower oxygen levels
• Adaptations:

  -active species in oxygen rich environments need haemoglobin that quickly releases oxygen to their tissues for respiration

  -their oxygen dissociation curve shifts to the right, meaning haemoglobin releases oxygen more readily at higher partial pressures

  -examples include hawks, cheetahs, seals, and other animals that require rapid oxygen delivery for high energy activities
• Oxygen dissociation curve summary:

  -a left shift ensures oxygen binds easily in low oxygen environments

  -a right shift ensures oxygen is readily released for energy demanding activities
• Carbon dioxide is released from respiring tissues. It must be removed from these tissues and transported by the blood to the lungs
• Carbons dioxide is transported in three ways:

  -about 5% is dissolved directly in the plasma

  -about 10% combines directly with haemoglobin to form carbaminohaemoglobin

  -about 85% is transported in the form of hydrogencarbonate ions ($HCO_3^-$)
• Hydrogen carbonate ion formation:

  -carbon dioxide in the blood plasma diffuses into red blood cells, where is combines with water to form carbonic acid ($H_2CO_3$)

  -this reaction is catalysed by the enzyme carbonic anhydrase

  -carbonic acid then dissociates, releasing hydrogen ions (H+) and hydrogencarbonate ions (HCO3−​)
• Chloride shift:

  -the hydrogencarbonate ions diffuse out of the red blood cell into the plasma

  -to maintain charge balance, chloride ions ($Cl^-$) move from the plasma into the red blood cell
• pH stability:

  -as hydrogen ions accumulate, they could make the red blood cell contents too acidic

  -to prevent this, haemoglobin acts as a buffer, binding with hydrogen ions to form haemoglobonic acid (HHb), helping to maintain pH stability
• Effect of increasing $CO_2$ concentration:

  -the partial pressure of oxygen is lower in the lungs. As a result, oxyhaemoglobin dissociates, releasing oxygen to the tissues

  -this also makes haemoglobin available to bind hydrogen ions, forming haemoglobonic acid. In highly active tissues, where more carbon dioxide is released, this process becomes even more pronounced, affecting haemoglobin’s function
• The Bohr effect: describes how increasing carbon dioxide concentration affects haemoglobin’s ability to bind oxygen
• The Bohr effect:

  -when carbon dioxide partial pressure is high in the blood, haemoglobin’s affinity for oxygen is reduced

  -this is the case in respiring tissues, where cells produce carbon dioxide as a waste product
• The Bohr effect:

  -carbon dioxide combines with water to form carbonic acid

  -carbonic acid dissociates into hydrogen carbonate ions and hydrogen ions

  -hydrogen ions bind to haemoglobin, causing the release of oxygen