Oxygen is required by many organisms for survival, luckily it is plentiful in the air, but how does it get into all the tiny cells all over the body?
First, Oxygen is a highly electronegative atom. This means that it attracts electrons very well and can pull them away from other molecules. Only one other atom is more electronegative and that’s the most reactive element in the periodic table, Fluorine. Electronegativity becomes useful biologically because electrons are capable to storing energy that can be passed along from one molecule to the next. But , to do this, each molecule must be more electronegative than the last. Therefore, it is not surprising that Oxygen is used as the final electron acceptor in the electron transport chain of cellular respiration. This reaction is required by many organisms, and can be highly beneficial even to some organisms that can live without Oxygen. An electron transport chain is a process by which molecules in a membrane pass en electron down the line using its energy in a controlled way to extract even more energy from sugar.
But how does Oxygen get to the cells that need it? Two molecules account for much of this action, myoglobin and hemoglobin, both illustrated below:
Hb- hemoglobin, Mb-myoglobin.
What this image also elegantly portrays is the amazing similarity between the molecules that belies their evolutionary relationship.
Each of these molecules is capable of binding Oxygen, but each occurs in a different tissue. Hemoglobin is present in Red Blood Cells, making them capable of transporting oxygen from the lungs to other tissues. Myoglobin occurs in these ‘other’ tissues, particularly muscle cells.
Despite the fact that they both bind oxygen, they do not bind it equally well under all conditions. While hemoglobin is just as good at binding Oxygen in high concentration environments (like the lung after inhalation), it is not as good at retaining oxygen when found in less Oxygen-rich environments (such as tissues like muscle). Under these conditions, myoglobin is much better at binding Oxygen and can pull the molecules away from hemoglobin.
Recently, several groups have published new data about how subtle differences amongst proteins involved in Oxygen transport through blood and muscle result in different binding properties. In turn, the variability in these properties underlie the amazing diversity of lifestyles found in nature, from humans to birds to giant whales capable of holding their breath for up to an hour.
That brings us back to the electron transport chain and Oxygen’s electronegativity, where O2 is used as a ‘magnet’ for the electron traveling down the pathway from one molecule to the next. As it goes it loses some of its energy, which is converted into a new form that the cell can use. Once the electron, now at a lower energy state, gets to O2, the Oxygen splits and takes up a Hydrogen ion to form water.
So, electronegativity and binding affinity are the forces that both transport Oxygen through the body and pulls electrons from one molecule to another. Together, the movement of electrons, like that of water through a mill, powers processes that lead to the synthesis of ATP, the energy currency of the cell (see below).
Given what we’ve discussed here, how do you think a baby ever gets to pull the Oxygen away from its mother’s blood / hemoglobin?