SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 162

Box 9.5 Chlorophyll

We have seen the key role that nitrogen plays in the
synthesis of proteins. another compound essential to life,
in which nitrogen is a vital constituent, is chlorophyll , the
green pigment in plants upon which photosynthesis primar-
ily depends. Chlorophyll is actually a class of closely related
compounds that share the following characteristics:

N

N Mg N

(a) the fundamental architecture comprises four molecules
of pyrrole (C 4 h 5 N -Figure 9.5.1), linked together to form
a ring structure known as a porphyrin (Figure 9.5.2).
(b) at the centre of the porphyrin ring in chlorophyll
(Figure 9.5.3) lies a magnesium atom, forming a
co-ordination complex with the surrounding pyrrole
nitrogens.

C O

COOCH 3

COOC 20 H 40

Figure 9.5.3 Structure of chlorophyll a .

(c) Various alkyl and carboxyl groups are attached to the
porphyrin ring, whose identities differ with the variant
of chlorophyll under discussion.

N

Figure 9.5.1 Structure of pyrrole, using the condensed
notation of Box 9.4. remember that a hydrogen atom is
implicitly attached to each carbon atom ( i.e. at each apex
of the polygon) and to the nitrogen atom.

the alternation of double and single bonds in the porphy-
rin ring resembles that in graphite (Box 7.4) and benzene,
and in a similar way the bonds coalesce to form intercon-
necting molecular orbitals above and below the ring.
Chlorophyll absorbs light strongly at the blue and red ends
of the visible spectrum (hence its green colour -
Figure 8.3), photochemically releasing electrons from
these orbitals and setting in train a complex set of bio-
chemical reactions by which CO 2 is ultimately reduced to
carbohydrate (reaction 9.3).

N

Figure 9.5.2 Structure of porphyrin. the complement of
h atoms associated with one of the pyrrole groups is
shown in orange as a reminder. the porphyrin ring
structure is found in other biologically essential
compounds such as haemoglobin and vitamin B 12 .

than 120 km (Box 2.2). The derivation of diamond
from deep in the mantle, and geochemical evidence
for CO 2 -rich fluids in peridotite nodules brought up
from similar depths, indicate that carbon is an impor-
tant minor constituent of the mantle. Carbonaceous
material is a major component of many meteorites
(Box 11.1).

In recent decades several novel structural forms of
elemental carbon have been discovered (Box 9.6) that
hold the promise of major technological innovations.
Carbon occurs in the Earth's crust chiefly as carb-
onate, most of which is biogenic. Limestones make
up about 25% of the total mass of Phanerozoic sed-
imentary rocks.