Photosynthesis: Some Fundamental Points
1. This energy-transduction process, which results in the annual formation of massive quantities of
organic matter around the Earth, is central to most organisms (autotrophic and heterotrophic) and the
functioning of most ecosystems.
2. It is accomplished by plants (except for a small, but interesting, number of achlorophyllous,
holoparasitic plants), some protists (e.g., various “algae” and “protozoans”), and some bacteria (esp.
3. The part of the light spectrum responsible for most photosynthesis is approx. in the range of 400-
700 nanometers (approx. the same wavelengths as seen by us, i.e., “visible light”), and roughly half of
the incoming solar radiation is in that part of the spectrum.
O are the reactants, but a number of other inorganic nutrients, e.g., nitrogen,
phosphorus, calcium, iron, magnesium, …, are essential for functioning of photosynthetic organisms.
is an important byproduct formed by the oxidation of H
Our currently O
atmosphere is a result of past photosynthesis with much of that “fossil” organic matter being buried by
geologic processes rather than being decomposed (oxidized).
6. Photosynthesis by terrestrial plants requires significant amounts of water.
Only a small fraction of
that is actually used as a reactant.
Most is expended as transpiration, i.e., water vapor exiting the
plant at the same time that air, including CO
, is diffusing in photosynthetic tissues.
7. Plants accomplish photosynthesis as a multi-step set of reactions occurring within chloroplasts of
photosynthetic portions of plants.
These are commonly categorized as the “light reactions” and
subsequent “carbon fixation and reduction reactions.”
a. Light energy is absorbed by chlorophyll molecules (Chl
) and accessory
photosynthetic pigments, such as carotenoids.
b. That energy is used to accomplish photolysis of water and then initiate an electron flow,