Unit 3 - Organisms exchange substances with their environment
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Summary Notes on Mass Transport in Animals - AQA A Level Biology
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Unit 3 - Organisms exchange substances with their environment
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Summary Notes on Mass Transport in Animals - AQA A Level Biology
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Unit 3 - Organisms exchange substances with their environment
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Mass transport in animals – AQA A Level Biology Summary Notes
Haemoglobins are a group of chemically similar molecules found in many different organisms
Hameoglobin is a protein (globular and soluble) with a quaternary structure
Structure of haemoglobin:
- 4 polypeptide chains (2 alpha chains, 2 beta chains)
- Each chain with a haem group (Fe2+)
- Held together by disulphide bonds
- Each haem group can bind to one O2 molecule, so Hb can carry a total of 4 oxygens
Variants of haemoglobin
- Associated with diseases such as sickle-cell anemia (HbS) – a special high affinity variant in
the foetus (HbF) – 1 base substitution (mutation) so change in tertiary structure
Haemoglobin is found in RED BLOOD CELLS and has an important role in transporting oxygen from
the lungs to respiring tissues. To do this:
1. Haemoglobin must bind/associate with oxygen to form oxyhaemoglobin at the lungs
2. Oxyhaemoglobin must unbind/dissociate to unload oxygen at respiring tissues
(Haemoglobin is able to do both, as the conditions are different in the two areas)
Oxygen dissociation curves
How much oxygen binds to a population of Hb molecules varies depending on the partial pressure of
oxygen (p(02) in the surrounding environment
The amount of oxygen bound to Hb is measured as percentage saturation
- 100% saturation means that EVERY Hb molecule in the population has all 4 binding sites fully
occupied
- 0% saturation means NO Hb molecule has any bound bounded
Increasing the partial pressure of oxygen, causes an increase in Hb saturation
- This is shown by an oxygen dissociation curve
, Plateau at top = takes longer for 4th oxygen to bind as
there are less available binding sites
Slower at start = due to cooperative binding
The gradient of the oxygen dissociation curve represents
the affinity Hb has for oxygen
Steeper gradient – small increase in p02 causes a large
increase in Hb saturation with oxygen
In lungs, p02 is very high, and therefore oxygen binds to Hb
In respiring tissues, p02 is lower so less oxygen is bound to Hb
- This difference in percentage saturation of Hb with oxygen between the tissues and lungs
represents the oxygen that dissociates when oxyhaemoglobin reaches the tissues – this
dissociated oxygen is then used for aerobic respiration
(Positive) cooperative binding
1. Binding of first oxygen changes tertiary structure of haemoglobin
2. Uncovers another/second oxygen binding site
3. Allows more 02 to bind more easily/greater saturation
4. Cooperative nature of oxygen binding
The effect of carbon dioxide on the dissociation of oxyhaemoglobin (the Bohr effect/shift)
Describe the Bohr effect
- Haemoglobin has a reduced affinity for oxygen in the presence of BECAUSE
- When cells are respiring more quickly, they produce more CO2
- CO2 dissolves in blood forming carbonic acid - lowers the pH of the plasma
- Decreased pH causes change in tertiary structure of Hb, altering shape of oxygen binding
sites
- Binding sites are slightly less complementary to oxygen
- Hb’s affinity for oxygen is reduced, and at same p02, more oxygen is unloaded from
oxyhaemoglobin
This is represented on an oxygen dissociation curve as a shift to the RIGHT and is called the Bohr
effect
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