ABSTRACT

1
ABSTRACT

• GEOCHEMISTRY AND THE FOOD CHAIN
• Element Uptake by Plants

Before an element can be utilized by an
organism, that organism must first be able to
uptake it. Elements originating in geological
materials are transported through soils and
presented to plants in a convenient form for
uptake. This project examines the pathways as
nutrients are released from Earth materials and
utilized by human organs. Of the many elements
on the periodic table, living organisms need
about thirty of them. Eleven of these appear to
be roughly constant and abundant in biological
systems. Four elements (hydrogen, carbon,
nitrogen, and oxygen) account for 96 of the
total human body, as well as the bulk of living
organisms and are termed major elements. The
minor elements include sodium, magnesium,
phosphorus, sulfur, chlorine, potassium, and
calcium. The latter are termed electrolytes and
comprise 3.78 of the human body mass. Lastly,
there are eighteen essential trace elements.
These nutrients are present in humans at levels
that are orders of magnitude lower than found in
the Earths crust. Metals comprise the bulk of
essential trace elements available for uptake,
but other important examples include selenium and
iodine. Of the metals, iron is an element that
is essential for many metabolic processes
including oxygen transport, DNA synthesis, and
electron transport. As such, this fundamental
element is used as a case example to demonstrate
the biogeochemical pathways starting in the soil
and ending in human organs. The impact that
geology has on uptake of elements by humans and
other organisms is dependent on a number biotic
and abiotic variables. Understanding of the
geological controls on nutrient uptake in plants
and animals, including humans, is essential for
deciphering public health consequences associated
with deficiencies or toxicity.

• Uptake of elements by plants can involve
  several process including cation exchange by
  roots, transport into cells by chelating agents
  (molecules that help transport nutrients), and
  rhizosphere effects (Lindh, Ulf. 2005). Plant
  uptake is a key stage in the soil, plant, human
  pathway of elements, second only to intake from
  drinking water, except in areas where geophagy
  (ingestion of soil) occurs.
• The factors that affect the amount of elements
  absorbed through the roots are
• Factors that control concentration in soil
  solution
• Movement of an element from the soil to the root
• Transport of the element from the root surface
  into the root
• And transportation from the root to the shoot.
• Other factors that influence element uptake by
  plants include
• mycorrhizae (symbiotic fungi that increase the
  absorptive area of the root and assist in uptake
  of nutrient ions when concentrations are low)
  the rhizosphere (1-2mm zone between the root and
  soil with microbial and biochemical activity
  where acidification, redox changes, and organic
  complex formation take place) and transpiration
  (the flow of water into roots, up through the
  plant and out of pores in leaves which is an
  important factor in root uptake of elements)
• Elements are absorbed by plants both passively
  and actively. Passive uptake involves diffusion
  of ions through the soil solution to the root
  endodermis. One element that is absorbed
  passively is lead. Active uptake, on the other
  hand, takes place against a concentration
  gradient and requires energy. Elements that must
  be absorbed actively include copper, molybdenum,
  and zinc.

Table 2. Abundance of trace elements in human
body mass.
Table 1. Abundance of major and minor elements
in human body mass.
FOCUS ON IRON
Iron is an essential trace element that is vital
to all living organisms. The most well known
metabolic process involving iron is oxygen
transport, but other processes include DNA
synthesis and electron transport (Lindh, Ulf.
2005). Uptake by plants Iron is abundant in
nature but usually found as Fe3 hydroxide
(Fe(OH)3) which is insoluble. Plants must find a
way to make Fe3 soluble in order to take it up.
Plants, bacteria, and some fungi secrete
siderophores (low molecular weight molecules that
bind to iron) which can make Fe3 soluble for
uptake. Uptake by humans Humans, as well as
other mammals, use absorption of iron as a major
control point for altering the iron content of
the body and of individual cells. The major
absorption area of iron is the intestine. In
the intestinal lumen, as with in the soil, iron
exists as Fe2 Fe3 iron salts. Fe3 is insoluble
at pH values above three, therefore it must be
reduced, or chelated (electrostatic complexes
that aid in absorption) by amino acids or sugars
to be absorbed. The cells that absorb iron are
called enterocytes and are located in the
gastrointestinal tract. Enterocytes control the
passage of dietary iron into the intestine and
transfer iron into the circulation of the body.
Human uptake of elements takes place primarily
in the gastrointestinal tract (Lindh, Ulf. 2005).
Elements that are taken up must be transported
across the mucosal cells in the intestine to
reach the bloodstream. Once in the bloodstream
they are transported to the liver, where the
elements are isolated and delivered into the
bloodstream once again to be transported to the
organs that will utilize them. Lastly the
elements must enter the cells of these organs.
Similar to transportation in plants,
transportation across membranes in humans can
occur passively or actively.
Figure 3. Essential trace element response to
dose (Lindh, Ulf. 2005).
Figure 4. Iron is absorbed into the enterocytes
from dietary iron and blood plasma (Lindh, Ulf.
2005).
Figure 2. Periodic table of elements showing
major, minor, and essential trace elements.
ESSENTIAL ELEMENTS

• 96 of the human body consists of hydrogen,
  carbon, nitrogen and oxygen. These four elements
  are the major elements. Minor elements make up
  3.78 of the human body. These elements include
  sodium, magnesium, phosphorus, sulfur, chlorine,
  potassium, and calcium. Minor elements are also
  called electrolytes. Trace elements include all
  elements besides major and minor elements and the
  noble gases.
• A trace element is considered essential if
• It is present in all healthy tissues of all
  living things
• Its concentration from one animal to the next is
  mostly constant
• Its withdrawal from the system induces the same
  physiological and structural abnormalities
  regardless of species
• Its addition either reverses or prevents these
  abnormalities
• The abnormalities induced by deficiencies are
  always accompanied by specific biochemical
  changes, and
• These biological changes can be prevented or
  cured when the deficiency is prevented or cured
  (Lindh, Ulf. 2005).

REFERENCES CITED
Lindh, Ulf. 2005. "Bioavailability of Elements in
Soil." Essentials of Medical Geology Impacts of
the Natural Environment on Public Health. By O.
Selinus and B. J. Alloway. Amsterdam Elsevier
Academic. p. 362-372. Lindh, Ulf. 2005.
"Biological Functions of the Elements."
Essentials of Medical Geology Impacts of the
Natural Environment on Public Health. By O.
Selinus and B. J. Alloway. Amsterdam Elsevier
Academic. p. 115-160. Lindh, Ulf. 2005. "Uptake
of Elements From a Biological Point of View."
Essentials of Medical Geology Impacts of the
Natural Environment on Public Health. By O.
Selinus and B. J. Alloway. Amsterdam Elsevier
Academic. p. 87-114. Selinus O. 2003.
Biogeochemical Monitoring in Medical Geology.
Geology and Health closing the gap. By Skinner
H. C. W. and Berger A.R. Oxford Oxford
Univeristy Press. p. 135-138. Steinnes, Eiliv.
2003. Biogeochemical Cycling of Iodine and
Selenium and Potential Geomedical Relevance.
Geology and Health closing the gap. By Skinner
H. C. W. and Berger A.R. Oxford Oxford
Univeristy Press. p. 57-60.
Figure 1. Mechanisms of transport across a
membrane (Lindh, Ulf. 2005).
Essentiality of trace elements is difficult to
prove because it is not possible to eliminate
every bit of an element in food.