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.
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.
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.
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.
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.
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.