Life on Earth is a constant recycling of 6 chemicals
EARTH continually recycles the chemicals of life, providing a supply for countless generations of the
billions of species that live on the planet. Except for minuscule amounts of space dust, every atom on
Earth has been here for 4 billion years or so and has been used time and again in various ways.
Six major chemical elements are necessary for life. They are typically called CHNOPS
(pronounced "ch-nops"), the initials of the chemical symbols for carbon, nitrogen, hydrogen,
oxygen, phosphorus and sulfur. With the exception of O and H, CHNOPS elements are not
among the top 10 in abundance in Earth's outer layers.
O atoms are the most abundant, comprising 50 percent of crustal rocks, 21 percent of the
atmosphere, 89 percent of water and 26 percent of the human body. With the exception of O,
the abundance of an element does not correlate with its importance in the life processes.
Carbon atoms form the backbone of all CHNOPS biochemicals. Although C is the fourth most
abundant element in the universe, it is scarce in Earth's outer layers, ranking at No. 15. It
accounts for only 200 parts per million (ppm), or 3.2 ounces for every 1,000 pounds of rock
on average.
If written in order of abundance, the chemicals of life would be OHPSCN, which is much more
difficult to pronounce. There is quite a range in abundance among CHNOPS elements in
Earth's crust, oceans and atmosphere.
O weighs in at about 500,000 parts per million. N comprises only 25 ppm of the crust, and
occurs mostly in soil, but it dominates the atmosphere at 79 percent.
The remaining 21 percent of the atmosphere is mostly oxygen gas, with about .03 percent
carbon dioxide.
H ranks No. 10 on the list for overall abundance in Earth's crust. Most of it is combined with O
in the water of the oceans, and it makes up about 60 percent of the human body. It bonds
chemically with C, N and S as well as with O.
The millions of different biochemicals that exist throughout the ecosphere fall into only four
broad categories: carbohydrates, lipids, proteins and nucleic acids. Carbohydrates contain
CHO. These occur in the form of sugars, starches and glycogen, all of which are primarily fuel
for cell metabolism. Most sugar molecules, such as glucose, are made from polygonal rings
of five or six C atoms with O, H, OH or CHHOH sticking up and down at angles from the
corners of the rings. The positions of the four different types of attachments, the number of C
atoms in the ring and how (or if) the rings are strung together are the differences between the
many different kinds of sugars.
Living cells are fueled by glucose, which is the simplest sugar. Polymers (molecules strung
together like beads) of glucose make starches or glycogen.
Glycogen is a highly branched polymer of 10,000 to 120,000 glucose molecules. It is the
body's immediate storage reservoir of energy.
Lipids, or fats, can be quite complicated in structure. The simplest are made from C and H
atoms built like a carbon-bodied centipede, with hydrogen legs and two antennae on one end
of the molecule. One antenna is OH, the other is H, giving the last carbon atom the form
COOH. The more complex lipids can also contain P or N atoms.
Proteins are made from only 20 amino acids that are joined into long chains. Each amino acid
has NHH on one end and COOH on the other. They can be assembled in any order to make
proteins. Genes store instructions for assembling the particular sequence of each unique
protein that then folds into a specific shape that characterizes it.
Life is the function of proteins. Proteins do practically everything in an organism, but there are
just a few especially important functions: enzymes, structure, regulation, signals and defense.
Many enzymes are catalysts for all of the various functions of metabolism.
There are structural proteins such as keratin (hair, nails, silk) and collagen (cartilage,
ligaments and tendons, and the main protein component of bones and teeth).
Some proteins perform regulatory functions, such as transcription factors that bind to DNA
while genes transcribe instructions to make other proteins. Other proteins selectively activate
or deactivate the transcription factors.
Hormones, such as adrenaline, are proteins that act as signal molecules or their receptors.
Defensive proteins can include everything from antibodies of the immune system, to toxins
from bacterial metabolism or the venom of stinging and biting critters.
A human body can contain as many as 90,000 different kinds of proteins.
Nucleic acids are complex macromolecules that convey genetic information.
They are composed of nucleotide chains. The most common nucleic acids are
deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
A nucleotide is a chemical compound that consists of ring-shaped base molecules, a sugar
and one or more phosphate groups. Nucleic acids are the structural units of RNA, DNA and
other molecules that play important roles in energy production, metabolism and signaling in
cells.
Because there is only a fixed supply of the CHNOPS atoms available, they must continually
cycle through the food chains and webs of the ecosphere from reservoirs of air, water, earth
and life. Each of the CHNOPS has its unique cycle, a complex web of many processes that
link many reservoirs via scores of pathways.
The cycles of each element overlap with the cycles of one or more of the others, making a
description of the complete set of interwoven cycles virtually impossible to represent in a
single visual model.
As a simple example, C and O cycles overlap in the production, use and storage of
carbohydrates, carbon dioxide and carbonate minerals on land and in water.
H cycles overlap with O as water, with C in virtually all biochemicals, and with N in ammonia
and amino acids.
S an P are present in various forms in organisms and in the physical environment. Two amino
acids have S atoms attached, while nucleotides contain phosphates, which are chemically
bonded P and O atoms.
N is abundant in the atmosphere but is not useful biochemically in that gaseous form. Various
chemical, physical and biological processes convert N to usable ammonia (chemically bonded
N and H atoms) or nitrates (bonded N and O atoms) in the atmosphere or in the soil.
The intricately complex and tangled webs of cycles of the CHNOPS elements are part of the
diversity of the ecosphere and the life forms that inhabit it. In that diversity, evolved over 4
billion years, lies its strength.