How do fish separate oxygen from H20 & consume it? Do they break the water molecule and absorb the oxygen only?
The answer to this, I reckon, is that they don't.
They use molecular oxygen (O2) dissolved in the water for respiration, where it acts as a terminal electron acceptor, just as we use molecular oxygen in the air for respiration. We can speak of the water as being oxygenated.
Water is split in photosynthesis, where reducing equivalents from water are used to reduce NADP+ (giving NADPH).
One of the great discoveries of biology, IMO, is that the oxygen formed in green-plant photosynthesis comes from water, not CO2.
Tricarboxylic Acid Cycle (Krebs Cycle) Rant
Despite claims to the contrary, most infamously by Racker (1976, pp 28 - 29) and Wieser (1980), but also by Madeira (1988) and Mego (1986) for example, water is not split in the tricarboxylic acid cycle (Krebs Cycle). Banfalvi (1991) also sails pretty close to the wind on this one.
That is, reducing equivalents from water are not passed down the respiratory chain, or in any way used to make ATP, or are in any way a 'source' of free energy. Such claims, IMO, are nonsense.
The definitive answers to the Wieser (1980) paper are given by Atkinson (1981) and Herreros & Garcia-Sancho (1981). Both of these articles are models of clarity, and categorically refute the claims of Wieser (1980). Nevertheless, as shown by the references above, the controversy surfaces periodically.
The only sources of reducing equivalents in the TCA cycle are carbon compounds, and the only electrons passed down the respiratory chain are those 'held' in C-H and C-C bonds (Herreros & Garcia-Sancho, 1981). An ionization is neither an oxidation nor a reduction (see Atkinson, 1981) and neither is a hydration. Adding water to (say) a double bond does not make the compound any more oxidized or reduced. As far as oxygen and electrons are concerned, and to generalize from a biological point of view, what is has it holds - except in photosynthesis.
As you may have guessed, the splitting of the water in the TCA cycle is a pet rant of mine. Thanks for the opportunity of airing my views!
Found this one when searching Google (Brière et al, 2006). In an invited review for the American Journal of Physiology (Cell Physiology) at that!
So now the TCA cycle is producing oxygen from water. Wonders will never cease!
In air-saturated buffer at 25oC the concentration of oxygen (O2 molecules) is about 0.24 mM (0.24 μmoles/ml, or about 0.474 μg-atoms of oxygen per ml). [Chappell (1964)]. This figure decreases with increasing temperature.
Great question, BTW.
(Apologies for the incomplete Atkinson and Herreros & Garcia-Sancho references. I have a photocopy of these papers but have been unable to trace the full source. They are both in the 'Letters to the Editor' section of the February 1981 edition of Trends in Biochemical Sciences. They do not appear to be in Pubmed, or anywhere else on-line. Has anyone ever seen these references quoted, or can provide me with a full source?. I'll update if I find anything)