mass transport Flashcards
what is haemglobin
haemoglobin are a group of chemically similar molecules found in a wide variety of organisms
haemoglobin are protein molecules with a quaternary structure that has evolved to make them efficient at loading oxygen under one set of conditions and unloads oxygen under a different set of conditions
what is the structure of haemoglobin
like most proteins, haemoglobins have:
- primary structure
- secondary structure
- tertiary structure
- quaternary structure
what is the primary structure of haemoglobin
sequence of amino acids in the four polypeptide chains
what is the secondary structure of haemoglobin
in which each of these polypeptide chains coiled into a helix
what is the tertiary structure of haemoglobin
in which each of these polypeptide chains is folded into a precise shape - an important factor in its ability to carry oxygen
what is the quaternary structure of haemoglobin
in which ll four polypeptides are linked together to form an almost spherical molecule
Each polypeptide is associated with a haem group - which contains Fe2+ ion
Each a total of four o2 molecules that can be carried by a single haemoglobin molecule in humans
what is loading
the process by which haemoglobin binds with oxygen is called loading/ associating
In humans, this takes place in the lungs
what is unloading
the process by which haemoglobin releases its oxygen is called unloading or dissociating
what happens when haemoglobin has a high affinity for O2
takes up O2 easily and release it less easily
what happens when O2 has a low affinity for O2
haemoglobin with a low affinity for oxygen take oxygen less easily, but release it more easily
what is the role of haemoglobin
the role of haemoglobin is to transport oxygen
what is the most efficient way that haemoglobin can transport oxygen
to be efficient at transporting oxygen, haemoglobin must:
- readily associate with oxygen at the surface where gas exchange takes place
- readily dissociate from oxygen at the tissues requiring it
how does haemoglobin associate and dissociate at the same time
it changes its affinity (chemical attraction) for oxygen under different conditions
It achieves this because its shape changes in presence of certain substances, such as CO2
In the presence of CO2, the new shape of hemoglobin binds more loosely to oxygen
- as a result the hemoglobin releases its oxygen
different organisms have different haemoglobin - what does this mean
it means that they take up oxygen differently
why do different haemoglobins have different affinities fr oxygen
each species produces a haemoglobin with a slightly different amino acid sequence
the haemoglobin of each species therefore has a slightly different tertiary binding properties
depending on its structure haemoglobin molecules range from those that have a high affinity for to those that have a low affinity for oxygen
under what conditions causes haemoglobin to bind unevenly with oxygen
when haemoglobin is exposed to different partial pressures of oxygen it does not bind the oxygen evenly
what is the graph showing the relationship between the saturation of haemoglobin with oxygen ad the partial pressure called
the oxygen dissociation curve
explain the shape of the oxygen dissociation curve i
1) the shape of the haemoglobin molecule makes it difficult for the 1st oxygen molecule to bind to one of the sites on its four polypeptide subunits because they are closely united
Therefore at low oxygen concentrations, little oxygen binds to haemoglobin
-the gradient of the curve is shallow initially
2) however, the binding of this first oxygen molecule changes the quaternary structure of the haemoglobin molecule causing it to change
This change makes it easier for the other subunits to bind to an oxygen molecule
3) It, therefore takes a smaller increase in the partial pressure of oxygen to bind the second oxygen than it did to bind the first one
- know n as positive cooperativity because binding of the first molecule makes binding of the first molecule make the binding of the 2nd easier and so on - the gradient STEEPENS
4) Even though in theory, binding to the fourth oxygen molecule should be easier, in practice it is harder
This is simply due to probability with the majority the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind to
- the gradient of the curve reduces and the graph FLATTENS off
each species has a different haemoglobin shape what does this mean
each has different shapes and therefore different affinity for oxygen
when does the shape of haemoglobin change
the shape of any haemoglobin molecule can change under different conditions
this means that there are a large number of different oxygen dissociation curves
- they all have a roughly similar shape but differ in their position on the axes
what must be kept in mind when we are reading a dissociation curve
- the further to the LEFT the curve, the greater is the affinity of haemoglobin for oxygen (so it loads readily but unloads it less easily)
- the further to the RIGHT the curve, the lower is the affinity of haemoglobin for oxygen (so it loads less readily but unloads it more easily)
if the graph is to the left then…
increases in pH in tissue (low CO2)
Large animals with low metabolic rate e.g. elephant
myoglobin (pigment in muscles)
foetal HB
if the graph is to the right then…
decreases in CO2 (high CO2)
what does haemoglobin do in the presence of CO2
haemoglobin has a reduced affinity for O2 in the presence of CO2
the greater the concentration of CO2, the more readily the haemoglobin releases its oxygen - this is known as the Bohr effect and explains why the behaviour of haemoglobin changes in different regions of the body