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A Metals Primer
 What
are metals?
Nearly three-quarters
of the known elements are metals. Opaque and lustrous, metals
are distinguished by their hardness, high melting point, strength,
density and ability to conduct heat and electricity. Metals
also can be hammered into thin sheets or stretched into wires
- physical properties known as malleability and ductility. These
properties, however, vary from metal to metal. For example,
chromium, which is the hardest metal, is used to strengthen
certain other materials, while cesium, the softest metal, can
be cut with a butter knife. Gold, silver and copper are extremely
malleable and ductile, while tungsten is far less so. Mixtures
of metals are known as alloys and have properties that are unique
mixtures of the properties of their constituent metals.
Chemically, metals are distinguished from nonmetals by their
capacity to lose electrons, forming positively charged ions,
in a chemical process called an oxidation-reduction or redox
reaction. When an atom of a metal loses electrons it is being
"oxidized." The atom or molecule that picks up the lost electron
or electrons is being "reduced" (think of negatively charged
electrons as reducing the total electrical charge. This reaction
- the transfer of electrons between atoms or molecules - is
a fundamental chemical process. The most common example of a
redox reaction is the formation of rust. In moist air, iron
tends to lose three electrons in a redox reaction with oxygen.
When that happens, the iron (Fe) loses electrons (becoming the
Fe+3 ion) and oxygen picks them up. The resulting product of
this redox reaction is the compound ferric oxide (Fe203), better
known as rust. Many metals oxidize when they are exposed to
moist air and the resulting metal ions (such as Cu+2, Al+3,
Zn+2) are generally soluble in aquatic environments and available
to living organisms. Though oxygen itself often plays the role
of electron acceptor, other molecules can act as "oxidizing"
agents in redox reactions.
Elements on the border between metals and non-metals exhibit
some metallic properties and are known as metalloids. One example
is arsenic (As), which has few of the metallic properties mentioned
above, but can be oxidized to As+3 and As+5 under environmental
and biological conditions.
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Where are metals found?
| Mining
and smelting metal ore can create piles of waste
where metals are concentrated and then carried by
rain into watersheds or borne on the wind into the
air. |
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Metals account for a quarter of
the Earth's mass, but a lower percentage of its crust. Sea water
contains trace amounts of metals, as do all living organisms
and even dust particles in the air. Volcanoes and natural weathering
can release metals into the environment, but human activities
now play the major role in dispersing metals on the earth¬s
surface.
The first metals to be used by humans were copper, gold and
silver, which could be found in their elemental metallic state
in nature. Most metals, however, occur in nature in ores, which
are compounds with oxygen or sulfur, buried beneath the Earth¬s
surface. To be useful as aluminum cans, copper wire and steel
beams, these metals need to be mined and separated from their
ores through a process known as smelting. Iron and tin are among
the easiest to extract from their ores and were the first of
these metals to be used by humans.
Mining and smelting of metal ores can create piles of waste,
or tailings, which often still contain relatively high concentrations
of metals that can be carried into watersheds or transported
by the wind. Metals are also released into the atmosphere from
fossil fuel power plants, trash incineration and combustion
of leaded gasoline.
What are toxic metals?
Many metals have no known
biological function and some of these are capable of disrupting
essential physiological processes. Examples of this are cadmium,
lead and mercury. Other metals in the wrong form can be toxic.
For example, chromium as the Cr+3 ion is an essential trace
element important for maintaining correct blood sugar levels,
but as the Cr+6 ion is a known human lung carcinogen.
Are heavy metals the same as toxic metals?
The short answer is no, as "heavy" refers to the atomic weight
of an element, not its tendency to behave as a biological bully.
The heavy metals cadmium, lead and mercury are certainly toxic;
however, molybdenum is a heavy but essential metal, while beryllium
is a light but very toxic metal.
Heavy metals have always been present in the earth's ecosystem,
but since the Industrial Revolution there has been a massive
redistribution of metals on the surface of the earth. Not only
their relative availability but the forms in which they are
being dispersed has changed.
The problem with certain heavy metals is that they tend to form
very stable and long-lasting complexes with sulfur in biological
molecules, which can disrupt their biological function. In some
cases this allows these metals to become concentrated at higher
levels of the food chain.
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What role do metals play in living things?
Many metals play critical roles in maintaining life. Some are
important for the structure of biological materials, as calcium
is for bone. Other metals stabilize proteins in unique and active
conformations, or structures. Zinc often performs this function.
Magnesium in the form of Mg+2 plays a role in balancing the
negatively charged phosphates that serves as the backbone of
DNA and RNA.
Metals also serve a chemically important role as essential components
of many enzymes. These metalloenzymes are involved in the synthesis,
repair and degradation of biological molecules, the release
and recognition of certain biological signaling molecules, and
the transfer of small molecules and electrons in crucial process
such as photosynthesis and respiration. For example, iron-containing
hemoglobin transports oxygen in blood.
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How can metals harm living things?
The toxic effects of most metals can be traced to their ability
to disrupt the function of essential biological molecules, such
as proteins, enzymes and DNA. In some cases this involves displacing
chemically related metal ions that are required for important
biological functions such as cell growth, division and repair.
Biological molecules have specific structures and certain components
that are essential for their roles. For example, a protein is
a specific chain of amino acids that folds into a unique three-dimensional
structure. If this structure is altered or a specific part of
the protein becomes damaged, then it may no longer be able to
carry out its necessary role.
Proteins, in particular, play an astounding number and variety
of roles in living organisms. They are used as structural elements,
for sending signals both within and between cells, and as enzymes
for the synthesis and degradation of other biological molecules.
If a metal ion binds to the amino acids of a protein, the resulting
metal-protein complex may lack the protein's original biological
activity.
For example, certain enzymes contain a cysteine amino acid that
contains a sulfur atom necessary for its function. Certain toxic
metals have a high affinity for sulfur and will bind tightly
to the essential cysteine, inhibiting the enzyme from functioning.
One metal may also substitute for another similar metal. For
example, the toxic metal, cadmium, can substitute for the essential
metal, zinc, in certain proteins that require zinc for their
structure or function. This can lead to alterations in that
protein that can have toxic consequences. In the same way, lead
can substitute for calcium in bone, and in other sites where
calcium is required.
Metal ions can also remove an electron from the amino acids
of a protein in a redox reaction that disrupts its ability to
carry out its biological function. Metal ions can also remove
an electron from the bases of DNA. Such oxidative damage to
these biological molecules is implicated in the cumulative effects
associated with aging and in the mutations associated with cancer.
In some cases the disruption of a few biological molecules has
an amplified effect. One example is the transcription factor
proteins that, in response to a signal, bind to DNA and initiate
the synthesis of new proteins required for development, normal
cellular metabolism or response to some stress. Another example
is enzymes, the biological catalysts that are needed in only
small amounts but which play essential roles in all biological
processes. A third example is proteins that are involved in
the repair of damage to biological molecules. While most damaged
proteins are simply replaced, DNA must be repaired if the information
in an organism's genome is to remain intact. Disruption of DNA
repair leads to propagation of errors in an organism's blueprint.
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How can metals be both good and bad for human health?
Living organisms have evolved on the earth with a protective
system for maintaining a balance between sufficient and excessive
quantities of elements. This system involves complex mechanisms
for transporting, storing and discarding the essential metals
to ensure that they are delivered to the right organ, tissue
or cellular compartment at the right concentration at the right
time.
Living organisms have also developed mechanisms for dealing
with certain toxic metals and toxic levels of essential metals.
For example, when animals are exposed to cadmium, lead or mercury
or to high levels of copper or zinc, they synthesize the small
protein metallothionein that binds these metals very tightly
and prevents them from encountering other biological molecules.
For certain metals, such as nickel and arsenic, whose toxicity
is well known at high acute doses, the cumulative long-term
health effects of low chronic doses is not understood.
From the point of view of a cell, an element - whether it comes
in the form of pollutants or poisons or drugs or even food -
is good or bad depending on the dose.
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