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Molecular Oxygen (O2)

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Humans and Oxygen

Pioneer human plaque

Respiration is the mechanism in which sugars and fats are "burned" releasing CO2 and energy. The liberated energy sustains life and enables biosynthetic processes that result in growth, anabolic and catabolic reactions.

The central role of O2 in respiration in most animals including humans is accomplished via a circulatory system (blood) that enables dissolved oxygen to be carried to the tissues where the respiratory proteins are found. The circulation is a means of "oxygen cooling" the heart and preventing the equivalent of a reactor meltdown.

The figure left was carried on Pioneer's 10 & 11 into outer space

"We are made of stars"

Oxygen Biology

The key biochemical process of oxygenic respiration is summarised below energetically as a scheme for the respiratory electron transfer chain. A simplification is that the preceding steps of glycolysis and the TCA cycle are not shown. These preceding steps are critical for the generation of the reducing equivalents such as NADH that feed the reaction below and involve the breakdown of glucose. The NADH generated is then used in the pathway below and electrons flow from NADH all the way to O2 via the respiratory complexes (termed complexes I to complex IV). The important functionality about this chain is the redox difference between NADH and O2 represents about 1000 mV (~17 kJ/mol) in free energy. It is this difference in free energy that results in reactions being driven forward against equilibria and this large difference in free energy is the reason for aerobic respiration generating the consummate amounts of energy for biology to survive upon.

redox respiration figure

 

However, O2 as it turns out is is not indispensable for biology, and 2.5 billion years ago - as is generally accepted - there was no O2 on earth. Back in this era there was life and and things lived then, as they can do so today, under anaerobic conditions. The difference, however, is that these anaerobic organisms are small, very small, living as single celled bacteria. No large organism can survive long without oxygen as the density of tissue and rates of metabolism require energy requirement that can only be met by oxygen. In the absence of oxygen the sugars are oxidised but only partially, and other biochemical reactions take place in the absence of oxygen. One example is fermentation which, in the absence of oxygen, liberates about ~25% of the total energy and release ethanol as the end product.

So if you are a big organism and want to use all your energy available energy you will need to use oxygen for respiration. Then too you need a solution for moving oxygen about to the respiratory tissues that generate energy to sustain live. That means a circulatory system that uses haemoglobin (or equivalent protein) that acts as oxygen carrier to transport oxygen to tissues where it can then diffuse into a cell and react in the mitochondria.

Oxygen Transport
The human body has approximately 5 Litres of blood that travels a vascular systems made up of veins, capillaries and arteries. Oxygen is carried about at high concentrations bound to a storage protein called haemoglobin. This protein is functionally a tetrameter and contains a heme group that binds O2. The bound oxygen molecule can then be carried from the lungs to the tissues such as the muscle tissues and heart where it is needed for respiration.
heamoglobin structure

(Left) The protein structure of Haemoglobin. Close up of the heme oxygen binding pocket.

The different colours are the different chains of the haemoglobin tetrameter and the blue spheres are the diatomic oxygen molecule binding to the heme group. Oxygen binding causes a small distortion of the heme ring plane and this is propagated into the protein and results in a conformational change effecting the overall structure of the protein. This in turn results in changes in the binding affinity of oxygen.

 
 
 

Reactive Oxygen (Radicals) and Antioxidants

Living with oxygen has consequences. Molecular oxygen is reactive and can be involed in deleterious chemical reactions that lead to cell dammage. One of the incurable diseases or conditions of humanity is growing old. It is an inalienable fact - birth, aging and then death. Much of this aging process is attributable to reactive oxygen intermediates that cause cellular damage. The appearance of oxygen on earth those millions of years back brought with it the necessity for Biology to solve oxygen damage to cells. The solutions were a range of antioxidants and detoxification processes invovling enzymes to scavange the reactivey oxygen intermediates and to repair cellular damage. The reactive oxygen species (ROS) in question were derivatives of O2 that were partially reduced. They include superoxide, peroxide, hydroxide radical that are intermediates between O2 and water. Another reactive form is singlet oxygen a form of energised O2 that results from flipping an electron. Normal oxygen is tripplet and is relativly unreactive. Singlet O2 is the most potent form of o2.

 
 

High altitude and Diving conditions

 
 
 
 

 

Capsule StoryHuman Blood and what can go in or come out <interesting link>

Capsule Story Work In Progress . Linc . .

Capsule Story Work In Progress . Linc .

Capsule Story Work In Progress . Linc .

Capsule Story Work In Progress . Linc .

Capsule Story Work In Progress . Linc .

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