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Biology 3.3: Mass transport - Coggle Diagram
Biology 3.3: Mass transport
3.3.4.1: Haemoglobin
Haemoglobin is a protein that carries oxygen around the body.
Different species have different versions of it depending on where each species lives.
Oxygen is carried around the body by haemoglobin:
1)
Red blood cells
/ Erythrocytes contain
haemoglobin
(Hb)
2) Haemoglobin is a large
protein
with a
quaternary
structure - it's made up of
more than one
polypeptide chain (
4
of them in fact)
3) Each chain has a
haem group
, which contains an
iron ion
and gives haemoglobin its
red
colour.
4) Haemoglobin has a
high affinity for oxygen
- each molecule can carry
4
oxygen molecules.
5) In the lungs, oxygen
associates / binds
to haemoglobin in red blood cells to form
oxyhaemoglobin
6) This is a
reversible reaction
- when oxygen
dissociates
from oxyhaemoglobin near the body cells, it turns back to haemoglobin
Hb + 4O2 ⇌ HbO8
Haemoglobin + oxygen ⇌ oxyhaemoglobin
Haemoglobin saturation depends on the partial pressure of oxygen:
1)The partial pressure of oxygen ( p02) is a measure of oxygen concentration. The greater the concentration of dissolved oxygen in cells, the higher the partial pressure.
2) Similarly, the partial pressure of carbon dioxide ( pC02) is a measure of the concentration of CO , in a cell.
3) Haemoglobin's affinity for oxygen varies depending on the partial pressure of oxygen. Oxygen loads onto haemoglobin to form oxyhaemoglobin where there's a high pO2. Oxyhaemoglobin unloads its oxygen where there's a lower pO2.
4) Oxygen enters the blood capillaries at the alveoli in the lungs. Alveoli have a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin.
5) When cells respire, they use up oxygen - this lowers the pO2. Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads oxygen.
6) The haemoglobin then returns to the lungs to pick up more oxygen.
Dissociation Curves Show How Affinity for Oxygen Varies:
A dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure.
Where p02 is high (e.g. in the lungs) haemoglobin has a high affinity for oxygen (i.e . it will readily combine with oxygen), so it has a high saturation of oxygen.
Where p 0 2 is low (e.g. 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.
The graph is 'S-shaped' because when haemoglobin (Hb) combines with the first O2 molecule, its shape alters in a way that makes it easier for other molecules to join too.
But as the Hb starts to become saturated, it gets harder for more oxygen molecules to join.
As a result, the curve has a steep bit in the middle where it's really easy for oxygen molecules to join, and sh allow bits at each end where it's harder.
When the curve is steep, a small change in p 0 2 causes a big change in the amount of oxygen carried by the Hb.
Carbon Dioxide Concentration Affects Oxygen Unloading:
To complicate matters, haemoglobin gives up its oxygen more readily at higher partial pressures of carbon dioxide ( pCO2).
1) When cells respire they produce carbon dioxide,
which raises the pCO2.
2) This increases the rate of oxygen unloading (i.e. the rate at which oxyhaemoglobin dissociates to form haemoglobin and oxygen) — so the dissociation curve 'shifts' right. The saturation of blood with oxygen is lower for a given p02, meaning that more oxygen is being released.
3) This is called the Bohr effect.
Haemoglobin is Different in Different Organisms:
Different organisms have different types of haemoglobin with different oxygen transporting capacities.
Having a particular type of haemoglobin is an adaptation that helps the organism to survive in a particular environment.
1) Organisms that live in environments with a low concentration of oxygen have haemoglobin with a higher affinity for oxygen than human haemoglobin — the dissociation curve is to the left of ours.
2) Organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen than human haemoglobin — the curve is to the right of the human one.
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