chlorophyll fluorescence

PELMONT Jean jp10 at calvanet.calvacom.fr
Thu Jan 2 01:30:58 EST 1997


>From: David Walker <David at alegba.demon.co.uk>
>Subject: chlorophyll fluorescence
>
>White light  (sunlight, normal artificial room lighting) is a
>mixture of all of the colours of the rainbow including blue, red and
>green. In white light, chlorophyll looks green because it absorbs
>the blue and the red wavelengths and reflects and transmits the
>green. However, when illuminated, chlorophyll molecules also get
>excited. In this process light energy gets converted into electrical
>energy just as it does in the photocell of a camera or solar cells
>in a satellite. Some of this electrical energy is then converted
>back into light (fluorescence) just as it is when you switch on a
>fluorescent light. Chlorophyll fluorescence is a deep red colour.
>You can see some of  this very easily if you illuminate a solution
>of chlorophyll and view it from the side. If you view it from the
>front, so much green light comes through the solution that the
>retina in your eye is saturated and you can only see green.
>Similarly, on a paper tissue, you can not easily see it from the
>side and the green predominates.
>        The red excited state from which chlorophyll fluorescence emanates
>is the starting point, the driving force, of the chemistry of
>photosynthesis. So ultimately, via the food we eat, it is what
>drives you.
>...................
 This is only a comment and a question, in addition to the very clear
explanations of Dr. David Walker. I am not an expert in photosynthesis as a
whole, but I wonder if this problem of chlorophyll fluorescence is not
actually misleading. Most of the energy captured within chloroplasts by
chlorophyll is channelled to an other chlorophyll molecule, or to a
reaction center where it is converted to a low-potential redox potential,
i.e. to a chemical potential. This in turn is used to create reductants
that shall be used for the reductive assimilation of CO2 or other stuffs.

Some losses of energy during these transfers appear under the form of
emitted light (fluorescence) or heat.

My question to the specialists is the following : what is the average
proportion of energy re-emitted as fluorescence by chlorophyll in vivo in
the course of photosynthesis ?

It is generally assumed, according to precise measurements, that
photosynthesis is a very efficient process, the so-called quantum yield
being high. Therefore fluorescence appears to be largely quenched during
energy transfers. It is explained that it is so because the reactions
involved are extremely fast (in the picosecond range) due to the specific
protein environment and structure in antenna complexes and reaction
centers.

When chlorophyll is put into solution, no such conversion from light to
chemical energy can take place, there is no more quenching. A fraction of
energy is dissipated as heat, an other part as light with higher wavelength
compared to the incident beam. Therefore fluorescence appears in this case
to be an artefact. A similar situation is met in the case of flavins.
Although they are strongly fluorescent when in free state, they are barely
so when located in the various enzymes such as dehydrogenases or
monooxygenases.

For teaching purposes, one on my colleagues in Grenoble (Roland DOUCE)
devised years ago an experiment to demonstrate energy conversion from light
by chloroplasts. It was done in the presence of the students while
lecturing. As I can remember, freshly isolated thylakoids vesicles where
used as a suspension in a weakly buffered isotonic medium, and the pH was
monitored with a sensitive apparatus.I guess that the tylakoids were from
spinach leaves. When switching the light on, a slight shift in the pH of
the medium could be demonstrated after a few seconds. I do not remember if
he used inside-out vesicles or not. In the first case, a change of the pH
should be acidification, showing that protons are expelled to the medium.
Of course this had nothing to do with fluorescence, but although the
conversion of light energy to ion translocation is a complicated process
needing detailed explanations, this was a vivid illustration of the primary
events of photosynthesis.

Cheers to everyone.








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