ABSTRACT

1. 6. Biogeochemical Cycling and Interactions: Implications for Human Health Kailey Clarno Western Oregon University ABSTRACT • GEOCHEMISTRY AND THE FOOD CHAIN • Element Uptake by Plants • Element Uptake by Humans 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 Earth’s 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.