3.3.4.1 Mass Transport in animals Flashcards
The haemoglobins
A group of chemically similar molecules found in a variety of organisms
What type of protein are haemoglobins?
Quaternary structure proteins - evolved to make it efficient at loading oxygen under one set of conditions and unloading oxygen under a different set of conditions
Haemoglobin: primary structure
Sequence of amino acids in the 4 polypeptide chains
Haemoglobin: secondary structure
Each polypeptide chain is coiled into a helix
Haemoglobin: Tertiary structure
Each polypeptide is folded into a precise shape (important for its ability to carry oxygen)
Haemoglobin: Quaternary structure
- All 4 polypeptides are linked together to form an almost spherical molecule.
- Each polypeptide is associated with a haem group
Structure of a haem group and how it works
Contains a ferrous ion (Fe2+) which can combine with a single oxygen molecule (O2)
So together it makes 4 x )2 molecules that can be carried by a single haemoglobin molecule in humans
Loading or associating oxygen
The process by which haemoglobin binds with oxygen
Unloading or dissociating oxygen
The process by which haemoglobin releases its oxygen
Where does loading/associating oxygen occur in humans?
Lungs
Where does unloading/dissociating oxygen occur in humans?
Tissues
Affinity of haemoglobin
Haemoglobin with high affinity for oxygen takes up O2 more easily, and releases it less easily
Role of haemoglobin
- Readily associate with O2 at the surface where gas exchange takes place
- Readily dissociate from O2 at those tissues requiring it
These 2 requirements contradict eachother!!
How are the contradicting requirements of haemoglobin achieved?
Haemoglobin can change its affinity for O2 under different conditions (its shape changes under the presence of CO2 or O2)
Affinity
Chemical attraction
Why do different species have different haemoglobins?
Each species has a different shape haemoglobin (due to slightly different amino acid sequence -> leads to slightly different tertiary and quaternary structure -> different O2 binding properties)
Oxygen dissociation curves
Graph that shows the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen
Shape of oxygen dissociation curve: 1
- The haemoglobin molecule’s shape makes it difficult for the 1st O2 molecule to bind to one of the sites on its 4 polypeptide subunits -> due to them being closely united.
- Therefore at low O2 concentrations, little O2 binds to haemoglobin -> shallow gradient
Shape of oxygen dissociation curve: 2
- The binding of the 1st O2 molecule changes the quaternary structure of the haemoglobin molecule -> causing it to change shape
- The change in shape makes it easier for the other subunits to bind to the O2 -> **the binding of the 1st O2 molecule induces the other subunits to bind to an O2 molecule
Shape of oxygen dissociation curve: 3
- It only takes smaller increase in the partial pressure of O2 to bind the 2nd O2 molecule than it did to bind to the 1st one = positive cooperativity -> gradient steepens
Positive cooperativity
Binding of the 1st molecule makes binding of the 2nd easier and so on.
Shape of oxygen dissociation curve: 4
After the binding of the 3rd molecule, although theoretically it should be easer for the 4th molecule to bind, it is not practically due to probability -> gradient of the curve reduces and the graph flattens off
Movement of the oxygen dissociation curve
- Shift to the left
- Shift to the right
Shift to the left of the oxygen dissociation curve
The further to the left the curve, the greater the affinity of haemoglobin for oxygen (so it loads oxygen readily but unloads it less easily)
