The Variety of Life
Living organisms have many similarities and differences of the molecules of which they are composed of; this is to do with the structure of proteins which different species of organisms possess.
The haemoglobins are a group of chimically similar molecules found in a wide variety of organisms.
The structure of a haemoglobin is made up as follows:
- Primary structure - consisting of four polypeptide chains.
- Secondary structure - each of the four polypeptide chains is coiled into a helix.
- Tertiary structure - the polypeptide chain is folded into a precise shape - an important factor in its ability to carry oxygen.
- Quaternary structure - all four polypeptide chains are linked together to form an almost spherical molecule. Each polypeptide chain is associated with a haem group, which contains a ferrous ion. Each ferrous ion can combine with a single oxygen molecule, allowing a single haemoglobin molecule to carry four oxygen molecules in total in humans.
The role of the haemoglobin:
- The role of the haemoglobin is to transport oxygen.
- To be efficient at transporting oxygen, the haemoglobin molecule must:
- Readily associate with oxygen at the surface where gas exchange takes place.
- Readily disassociated from oxygen at those tissues requiring it.
- The haemoglobin changes its affinity for oxygen under certain conditions to reach these requirements.
- It achieves this, because it changes shape in the presense of certain substances, such as carbon dioxide; this means in the presense of carbon dioxide, the haemoglobins new shape allows oxygen to bind more loosely and as a result the haemoglobin releases its oxygen.
- At the gas exchange surface there is a high concentration of oxygen but a low concentration of carbon dioxide, because of this the haemoglobin has a high affinity for oxygen and so oxygen is readily associated.
- At respiring tissues the concentration of oxygen is low but there is a high concentration of carbon dioxide; this causes the haemoglobin to change shape and its affinity for oxygen to decreased and so oxygen disassociates with the haemoglobin.
Why do different species have different haemoglobins?
- Scientists found out that haemoglobins can vary from different species due to these factors:
- Haemoglobins with a high affinity for oxygen - these take up oxygen more easily but release it less readily.
- Heamoglobins with a low affinity for oxygen - these take up oxygen less easily but release it more readily.
- Scientists found a correlation between the type of haemoglobin in an organism and factors such as the environment which it lived in or its metabolic rate.
- The explanations of these correlations are as follows:
- An organism living in an environment with little oxygen, requires a haemoglobin that readily combines with oxygen if it is to absorb a sufficient amount of oxygen in need to survive; therefore this type of haemoglobin takes up oxygen more easily but releases it less readily. Provided that the organism's metabolic rate is not very high, the fact that this form of haemoglobin does not release its oxygen readily…