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5. Photosynthesis

Photosynthesis (assimilation) and respiration (dissimilation) are complementary processes.
Photosynthesis results in reductive carbon fixation and production of oxygen, energy driven by
light energy, hν.
hν
CO 2 + 2H 2O* → {CH 2O} + O 2* + H 2O

-
Carbon dioxide is reduced, in a 4e reduction, to {CH 2O} (symbolising a carbohydrate such as
1/6 of glucose). Reducing agent is water, which is oxidised to O 2. Instead of resorting to light
as an energy source, chemical energy (energy liberated in the course of a chemical reaction)
can be employed, and sources for carbon other than CO 2, e.g. CO or acetate, can be used.
Depending on the energy and the carbon source, one distinguishes the following categories:
Light energy: phototrophic
Chemical energy: chemotrophic
CO 2 as C-source: autotrophic
Other C-sources: heterotrophic
Green plants, cyanobacteria and other photosynthetically active bacteria, and protozoa
containing chlorophyll produce bio-mass photo-autotrophically. A 100 year old beech tree
produces about 1000 l of O 2 and 12 kg of carbohydrates per day (this corresponds to 100 ml of
2
O 2 and 1.2 g of glucose per 1 m of foliage). The major part of bio-mass is, however, produced
by chemotrophic microorganisms. Examples for chemical processes supplying energy are:
3+
2+
-
Fe → Fe + e
+
-
H 2 → 2H + 2e
-
2-
-
+
HS + 4H 2O → SO 4 + 9H + 8e
2+
+
-
Mn + 3H 2O → MnO(OH) 2 +4H + 2e

In the photosynthetic process carried out by plants, one distinguishes between the light reaction
and the light-independent (or dark) reaction on the one hand and, within the light reaction,
between photosystems I and II (PSI and PSII, also referred to as light harvesting complexes
LHC) on the other hand:

Light reaction:
-
+
2+
PSII: P 680 + hν → [P 680] + e (via phaeophytin, a ”chlorophyll“ depleted of Mg :
+
+
2[P 680] + H 2O → 2P 680 + ½O 2 + 2H (catalysed by water oxidase)

-
e transfer chain from PSII to PSI (Fig. 14)

-
+
PSI: P 700 + hν → [P 700] + e
+ -
[P 700] + e → P 700
-
+
+
+
NADP + 2e + 2H → NADPH + H (catalysed by [2Fe,2S])

+
+
Dark reaction: 2(NADPH + H ) + CO 2 → {CH 2O} + 2 NADP + H 2O (energy driven by ATP)

The photosystems are collectives of pigment molecules (ca. 200), mainly chlorophyll-a and -b,
carotinoids, anthocyanes and xantophylls. These pigments act as collectors over the complete
spectrum of the (visible) sun light. The energy thus collected is transferred to the reaction
centres, which represent specific molecules of chlorophyll-a, termed P 680 in PSII, and P 700 in
PSI; see Fig. 15.