EESA10H3 Lecture Notes - Bioavailability, Ionic Bonding, Covalent Bond

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24 Jul 2011
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Lecture 5: Metals and Human Health
Heavy Metals
Define: Based on atomic weight, based on density (all elements having density higher than 7gm/cm3).
Trace elements are elements found in a lower concentration. Very low sometimes hard to detect.
Naturally occurring, extracted from the earth (ground) in ore in different forms.
Used for different applications
Wide environmental dispersion (water, food, air etc)
Tendency to accumulate in select tissues
Toxic in even low concentrations
Classification of metals
1. Class A:
K, Na, Mg, Ca, Al
Macronutrients (essential for biological/physiological processes)
Usually cations.
Tend to form ionic bond. Opposite forces are attracted to each other. (Na can react with Cl form table
salt)
_ Low electro negativity (tendency to lose rather than acquire electrons)
_ Low toxicity
2. Class B:
_ Hg, Ti, Pb, Ag, Au
_ Nonessential elements (not needed by the body and can interfere in the body)
_ Tend to form covalent bond
_ High electro negativity (tendency to acquire rather than lose electrons)
_ Very toxic (form soluble organometallics)
3. Borderline:
_ Cr, Cu, As, Co, Ni, Zn, Mn, Fe
Contribute in many important physiological processes.
_ Micronutrients
Toxicity:
Class B > Borderline > Class A
Mechanism of toxicity
1. Blocking essential functional groups such as proteins or enzymes, proteins cant carry anything
2. Displace other metals (class B, borderline)
3. Modifying the active conformation of biomolecules (twisting of molecules) (class B): like what you see
in the mirror. Really harmful for humans.
Coping Mechanisms
_ Resistance species develop mechanisms not to uptake metal (example Pb)
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