MadSci Network: Biochemistry

Re: I am intersted in how the heavy isotope of oxygen affects photosynthesis

Date: Thu Nov 18 20:03:29 1999
Posted By: Bettye Smith, Staff, Biophysics, Univeristy of California
Area of science: Biochemistry
ID: 942693325.Bc



Very good question. I'm not sure many studies have been done on the effects of heavy oxygen and photosynthesis.

As you are probably already aware, photosynthesis is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems.   All photosynthetic organisms that produce oxygen have two types of reaction centers, named photosystem I and photosystem II (PS I and PS II), both of which are located in specialized membranes called thylakoids. PS II is the "light reaction" where water splitting and oxygen evolution occurs.  All of the oxygen in the atmosphere is thought to occur from photsynthesis.  Most speculate that the major source of photosynthesis occurs in open oceans.  Without photsyntesis, the oxygen in the atmosphere would eventually be depleted and all respiring organisms would cease to exist!

So, how would heavy oxygen affect photosynthesis?

We know from Professor Hengee's response regarding heavy water (containing deuterium or tritium), that adverse reactions were observed when mice or animals were treated with heavy water.  This is because the heavy isotope slows down the chemical reaction rate which in turn slows down related biological processes. This is known as the kinetic isotope effect, and is contigent upon the difference in the mass of the isotopes. In the case of heavy hydrogen, the difference is a factor of two, which is double the normal mass of the hydrogen. In the case of oxygen, if the oxygen (16) was replaced with oxygen (18), the mass of the subsequent water molecule would be about the same.& nbsp; But, and this is the important factor:  the isotope effect from oxygen- 16 would be much smaller with respect to oxygen-18 because the difference in the mass of the oxygen atom from 16 to 18 is much smaller than the mass of the hydrogen atoms from 1 to 2 (which is double). Oxygen-18 isotope generally only slows down the chemical reaction by about 7 % compared with Oxygen-16.  So, one would expect negligible effects on biological homeostasis of plants or animals.

From the perspective of the effect of O-18 on photosynthesis, then, would be negligible. However, scientists use O-18 uptake to track water in environmental and pollution studies, for example to study snowmelt, runoff, and melting glaciers.  It is also possible that O-18 would not develop into free radicals, culprits known for aging and cancer.



Check out the question/ answer from MadSci Network by Alan Hengge

Arizona State University (basics of photosynthesis)

Using O-18 in environmental studies

O-18 / O-16 ratios

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