Page 137 - Color Atlas of Biochemistry

128 Metabolism
Photosynthesis: light reactions
Sunlight is the most important source of en-
ergy for nearly all living organisms. With the
help of photosynthesis, light energy is used to located in the thylakoid membrane, whereas
produce organic substances from CO 2 and
water. This property of phototrophic organ-
isms (plants, algae, and some bacteria) is ex-
ploited by heterotrophic organisms (e. g., ani- one redox system to the next in an electron
mals), whichare dependent ona supply of
organic substances in their diet (see p. 112).
The atmospheric oxygen that is vital to higher
organisms is also derived from photosynthe-
sis.
A. Photosynthesis: overview
The chemical balance of photosynthesis is
simple. Six molecules of CO 2 are used to
form one hexose molecule (right). The hydro-
gen required for this reduction process is
taken from water, and molecular oxygen is
formed as a by-product (left). Light energy is
required, since water is a very poor reducing
agent and is therefore not capable of reducing
CO 2 .
In the light-dependent part of photosyn-
thesis—the “light reactions ”—H 2 O molecules
are split into protons, electrons, and oxygen
atoms. The electrons undergo excitation by
light energy and are raised to an energy level
+
that is high enough to reduce NADP .The
+
NADPH+H formed in this way, in contrast to
H 2 O, is capable of “fixing” CO 2 reductive-
ly—i. e., of incorporating it into organic bonds. chrome b/f complex release protons from re-
Another product of the light reactions is ATP,
which is also required for CO 2 fixation. If
+
NADPH+H ,ATP, and theappropriate en-
zymes are available, CO 2 fixation can also
take place in darkness. This process is there-
fore known as the “dark reaction.”
The excitation of electrons to form NADPH
is a complex photochemical process that
involves chlorophyll, a tetrapyrrole dye con-
taining Mg 2+ that bears an extra phytol resi-
due (see p. 132).
B. Light reactions
In green algae and higher plants, photosyn-
thesis occurs in chloroplasts. These are organ-
elles, which—like mitochondria—are sur-
rounded by two membranes and contain their
ownDNA. Intheir interior, the stroma, thyla-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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koids or flattened membrane sacs are stacked
on top of each other to form grana.The inside
of the thylakoid is referred to as the lumen.
The light reactions are catalyzed by enzymes
the dark reactions take place in the stroma.
As in the respiratory chain (see p. 140), the
lightreactionscause electronstopassfrom
transport chain. However, the direction of
transport is opposite to that found in the res-
piratory chain. In the respiratory chain, elec-
+
trons flow from NADH+H to O 2 ,withthe
production of water and energy.
In photosynthesis, electrons are taken up
+
from water and transferred to NADP ,withan
expenditure of energy. Photosynthetic electron
transport is therefore energetically “uphill
work.” To make this possible, the transport
is stimulated at two points by the absorption
of light energy. This occurs through two pho-
tosystems—protein complexes that contain
large numbers of chlorophyll molecules and
other pigments (see p. 132). Another compo-
nent of the transport chain is the cytochrome
b/f complex, an aggregate of integral mem-
brane proteins that includes two cytochromes
(b 563 and f). Plastoquinone,which is com-
parable to ubiquinone, and two soluble pro-
teins, the coppercontaining plastocyanin and
ferredoxin, function as mobile electron car-
riers. At the end of the chain, there is an
+
enzyme that transfers the electrons to NADP .
Because photosystem II and the cyto-
duced plastoquinone into the lumen (via a Q
cycle), photosynthetic electron transport es-
tablishes an electrochemical gradient across
the thylakoid membrane (see p. 126), which is
used for ATP synthesis by an ATP synthase.
+
ATP and NADPH+H ,which arebothneeded
for the dark reactions, are formed in the
stroma.