Chemistry Reference
In-Depth Information
Chapter 18
Non-metals in Biology
Introduction
343
The Major Biogeochemical Cycles
343
INTRODUCTION
In a departure from the first edition we include here a brief account of selected non-metals and their multiple and
various activities in biological systems. We pointed out in Chapter 1 that organic chemistry is the chemistry of
hydrocarbons, but that if we restricted ourselves to just hydrogen and carbon, it would be impossible to construct
the molecules we know to be essential for life as we know it. To construct proteins, nucleic acids, carbohydrates,
and lipids, we also need oxygen, nitrogen, phosphorus, and sulfur. Indeed, we also require, as we saw in Chapter 1,
a number of other elements, notably a not-inconsiderable number of metals.
We begin this overview of the role of non-metals in biology by focusing on these six essential elements, H, C,
N, O, S, and P, which constitute the building blocks of all biological macromolecules. As we will see in the next
section, the fluxes of these elements constitute what is called the earth's biogeochemical cycles, that is the
pathways by which these chemical elements move through the biotic compartment (the biosphere) and the abiotic
compartments (the lithosphere, the atmosphere, and the hydrosphere) of the Earth. A good example of such a cycle
is the water cycle, which is illustrated in
Figure 18.1
.
Water undergoes evaporation, condensation, and precipi-
tation, falling back to the surface of the planet clean and fresh. There are also places where it can be held for long
periods of time (reservoirs), which in the case of water are the oceans and lakes, in the ice and snow of the
mountains, in the clouds in the atmosphere, and in the ground as ground water storage.
THE MAJOR BIOGEOCHEMICAL CYCLES
We have already pointed out that there are two compartments which constitute the theatre within which the
biogeochemical cycles take place, the biotic and the abiotic. Whereas the vast majority of abiotic geochemical
reactions are based on acid
base chemistry, the chemistry associated with the biosphere is based on redox
reactions. The fluxes of protons and electrons associated with the six major elements, H, C, N, O, S, and P can be
combined to construct a global metabolic map of the Earth (
Falkowski, Fenchel, & Delong, 2008
)
.
Figure 18.2
presents this generalised model of the biosphere showing the major inputs and outputs of energy and materials.
The geochemical (abiotic) transformations are represented by the atmospheric, diagenesic,
1
tectonic, and
geothermal compartments, while microbially driven biochemical processes are represented by the biospheric
compartment and the sediments. Biological cycling of the elements is not completely closed due to losses through
sedimentation of organic carbon and nitrogen, carbonate, metal sulfides, sulfate, and phosphate, and losses to the
e
1. Diagenesis
e
the physical and chemical changes occurring in sediment during its deposition and after its compression and transformation
into rock.
