Life and the Earth's
nature is a stranger yet;
The ones that cite her most
Have never passed her haunted house,
Nor simplified her ghost.
To pity those that know her not
Is helped by the regret
That those who know her
Know her less
The nearer they get."
Dickinson, cited by Margulis and Sagan in MICROCOSMOS, 1986.
can characterize living organisms by various chemical, physical, and biological
parameters, the harmony that puts it all together and forms the spark
we call life is far from understood. We see live organisms that may be
stages toward more complete organisms. Viruses, for example, are essentially
a piece of DNA or RNA coated with protein. It is by inserting this unit
into cells of other animals that viruses reproduce and continue their
lives. Infective viruses have been synthesized from elements. Several
scientists have synthesized DNA and RNA molecules that replicate themselves
in a test tube. But while we can concoct many of the molecules that must
have been in the primeval soup in which life evolved, we cannot yet compose
from elements a group of "cells that crawl out of a test-tube"
on their own, to paraphrase Margulis and Sagan.
We do know
a lot about the environmental conditions that sustain life. We have begun
to understand that not only does the environment sustain life, but life
in turn has made environmental conditions what they are today. We now
discuss some peculiar features and compounds on Earth that make our environment
particularly "fit" for our kind of life. Much of this discussion
is based on three sources:
Four Billion Years of Microbial Evolution, by Lynn Margulis and
Dorion Sagan, 1986.
Arrow and Evolution, by Harold F. Blum, 1951.
by James Lovelock, 1979
the "fitness of the environment," a concept originally proposed
by Lawrence J. Henderson in 1913. <INSERT note
on Henderson--write email to Beth> Certain aspects of the environment
make the Earth particularly advantageous for living organisms to live,
develop, and evolve. The Earth's size and its distance from the sun (a
medium yellow star),
and the nature of the sun itself, determine the gravitational force of
the Earth and the amount and type of electromagnetic energy we receive.
These factors provide the conditions under which life evolved, and that
sustain life on Earth.
Exercise / Discussion Questions
List and draw a concept map of how Earth's gravity, its distance
from the sun, and the properties of sunlight affect factors critical
to a system of living organisms based on carbon, hydrogen, and oxygen.
List the characteristics of a "live" system. Which of
these are not attainable by artificially-created systems like a
What do you think we would have to look for on another planet to
determine if there are life forms on it similar to ours?
Think of some parts of the Earth with an extreme environment (extreme
temperatures, pressures, etc.). Do organisms live there?
for Life on Earth: Water, Carbon, Hydrogen, and Oxygen
<Might be good to put electron dot diagrams
or something representative of these elements here for a visual effect?
Needs some pictures to break the monotony...>
oxygen, and carbon make up over 93% of the weight of the human body. Water
is almost 80-90% by weight of all living organisms. Water has several
physical and chemical properties
that make it important in maintaining conditions fit for life on Earth.
<student exercise on water?>
The large amount of water on the Earth's surface and the fact that at
the average temperature over most of the Earth, water is in a liquid state,
are both important to life. Water constitutes the largest fraction of
evenly over the Earth, the water present on Earth could form a layer about
2.5 km (1.6 miles) thick. Water has a high heat capacity. It can absorb
quite a bit of heat (1 calorie for every gram for each degree rise in
temperature) before its temperature rises significantly. This provides
a moderating influence that prevents sudden rises in temperature, which
could be damaging to live organisms. Water bodies on Earth work to moderate
atmospheric temperature changes, and internal water helps organisms maintain
has a large latent heat
of vaporization because a lot of energy is needed to break hydrogen bonds
among water molecules.
It takes a lot of heat to change liquid water to vapor--560 calories/gram.
Thus organisms (plants and animals) can dissipate a lot of heat by having
some of the water in them evaporate. For example, we are able to evaporate
water from our body, as sweat, cooling the body because of the heat removed
by the evaporating water.
a high latent heat of fusion as well. Eighty calories are required to
convert 1 gram of ice to water. In addition, because of the peculiarities
of the hydrogen bond, ice is less dense than water. It rarely happens
that a solid material is less dense than its liquid state. Ice also does
not conduct heat well. The high latent heat, low heat conductivity, and
low density of ice causes ice to float on water, keeping the warmer water
sealed below the insulating ice layer on lakes and other bodies of water,
and keeping the water habitable for aquatic life.
a "universal" solvent. It is capable of dissolving a variety
of materials. Salts dissolve in water to form ions <INSERT
note on ions> and various ions play important roles especially
in the conduction of nerve impulses. Balance of ionic flow across cell
membranes (cell walls) are also an important mechanism of moving nutrients
as well as in several other cell functions. <link
to one of ornella's animations?>
water vapor is one of the greenhouse gases that keeps the Earth's atmosphere
at the temperatures suited to life.
Many of the special properties of water come from hydrogen.
The small size of the hydrogen atom makes it possible to fit into many
more molecular configurations than a bigger atom can. Thus hydrogen can
form numerous compounds. Hydrogen is light and, due to its weight, all
hydrogen gas could have escaped the Earth's gravitational pull when the
Earth was still very hot. However, its high chemical reactivity with nitrogen,
oxygen, and carbon, and the abundance of these elements on Earth made
it possible for the Earth to retain a large amount of hydrogen in combination
with these elements as ammonia, water, and methane during the primitive
days of the Earth.
The carbon compounds that
make up essential molecules such as proteins are described in the notes
on biological molecules. Carbon is second only to hydrogen in the number
of compounds it can form, oxygen being the third in this capability. Carbon
can form more than 2500 compounds with hydrogen. Then next elements that
form most hydrides are boron and nitrogen; each of which can form only
seven! The C-C bonds make possible a great variety of molecules with different
chains and rings. C, H, and O combine together to from even a richer variety
of compounds. The same number of atoms can yield completely different
compound depending on the arraignment of atoms. For example, C can from
butyl alcohol (the prefix butyl refers to 4 carbons) in two alternate
forms with slightly different but similar properties.
of methane to show the bonding orbitals>
group is the hallmark group of an alcohol. These same number of atoms
could also form an ether characterized by the -O- bond between carbon
groups. Thus these atoms can form diethyl ether, two ethyl (C2H5)
groups bridged by O, as in:
much larger molecules with many C atoms, the possible arrangements become
very large. Recall that carbon is
the middle element in the first period of the Periodic Table. It has four
electrons (1s22s22p2) in the outer shell,
needing four more to complete the outer shell. This capacity to form four
covalent bonds makes for the capability of carbon to form compounds. The
four bonding electrons and various spatial arrangements give carbon its
and the fact that carbon dioxide is a gas at ordinary temperatures are
two important aspects in carbon being the chemical basis of life. Photosynthesis
occurs because CO2 is a gas and is soluble in H2O,
so that this mixture, with energy from the violet part of sunlight, can
of carbon comes from its central position in the periodic table. People
have conjectured why silicon in an analogous position and, being one of
the most plentiful elements in the Earth's crust, did not become that
centerpiece. SiO2 is ****
form in plenty--as sand. But it becomes a gas at only 3000°, and it
is not soluble in water. The chemical versatility of silicon is indeed
the property we value for its use for computers.
With carbon and hydrogen, oxygen forms the third principal element in
living systems. As we see later in this unit, our atmosphere was not always
oxygen-rich. About 2000 million years ago, there was only about 0.0001%
oxygen in the atmosphere. During the Archean and Proterozoic ages (when
plants started to use photosynthesis), there was a radical increase of
the O2 concentration to the almost 20% that it is today. This
resulted in a major extinction of some bacteria as discussed in a later
is to carbon, so is sulfur to oxygen. So we could imagine a material and
life configuration where H2S instead of H2O was
the basic "liquid" of life. (H2S is actually a gas
at Earth's average temperatures.) However the H-O bond is stronger than
the H-S, and oxygen is 50 times more plentiful on Earth than sulfur.
The other elements that play a vital role in living systems are N, Ca,
P, Na, Fe, K, Cl, S, Zn, and Mg. Together these form about 1.72 atomic
percent of the human body. Along with H, O, and C, these elements account
for 99.96% of the human body. Lighter elements dominate this list and
these elements have more specific roles in the function of biological
molecules than the more general C, O, and H trio.
and sulfur are components of all proteins. Phosphorus is an essential
component for the storage and use of energy in all cells. Cellular energy
resides in phosphate bonds. Mg is a central component of chlorophyll,
and iron is a component of hemoglobin and other respiratory enzymes. These
elements serve very specialized but important functions.
as Fe and Na are rare in the body but play important roles either in very
specific molecules as Fe in hemoglobin, Zn in gene transcription proteins,
or Mg in chlorophyll; or with a specific function such as Na or K ions
providing the flow of ions for conduction of information along nerves.
Most metals, however, are toxic to most organisms. Examples of metal toxicity
that have become significant environmental problems in the last half-century
are cases of lead poisoning, mercury poisoning (Minamata
disease), and poisoning by metals such as chromium (Cr), aluminum
(Al), and cadmium (Cd). The amounts of chromium and cadmium in the environment
have increased due to numerous technological uses ranging from steel production
to household batteries.
Basic Parts & Functions
parts of the cell and the function of each are described in the notes
on cells for those who require such a review.
is defined as the gradual change of any object, in our case, the biological
system, through time. We discussed briefly in the previous section some
of the aspects of the complex intricate system of the chemistry of biomolecules.
We now shift to the level of the organism--cells organized into tissues,
organs, and organism--which have persisted and changed through time. Broadly
speaking, the first single-celled organism did not have a nucleus. These
prokaryotes persist today as microbes, such as bacteria and viruses.
synthesis was an early step in the evolution of organisms who were then
capable of reproducing themselves using their RNA. It is not clear when
the first cells in the form we know now were formed. Somewhere, in the
primordial soups, some molecules got enclosed inside a membrane to form
a cell. RNA molecules within this cell could then facilitate reproduction
of the cell. All we can surmise is that this aggregation of molecules
started about 3.5 billion years ago. It is believed that RNA came after
proteins and enzymes evolved. Once a cell was established, RNA evolved
into DNA, making a subcellular structure we now call the nucleus. The
DNA, which is a coil of two strands of RNA (with the bases A, C, G, and
T, whereas RNA has A, C, G, U), is the molecule that enables protein synthesis.
the prokaryotic bacteria we know like mycoplasms, different varieties
of **** e-coli, and staphylococcus
still abound on Earth. Bacteria live in an enormous variety of environments,
often symbiotic with other organisms. Thus benign forms of e-coli live
in our guts and numerous bacteria live in soil and in plants. Species
of bacteria can use a variety of organic molecules such as sugars and
polypeptides as their food. Bacteria are the most abundant type of cell
on earth. The early bacteria seemed to have used ATP to store the energy
of sunlight. Blue-green algae is a type of bacteria that converts CO2
and N2 into its food.
developed the capability of photosynthesis, the composition of the early
atmosphere (mostly CO2) began to change over billions of years to this
of 20% oxygen and only traces of CO2 as the plants "fixed"
the carbon. Many of the early bacteria were used to an oxygen-poor environment
and several of them then became extinct. Others formed a symbiotic association
with oxygen--using (aerobic) types of cells to form the present day cells
with nuclei (eucaryotes). The different organelles in the cell such as
the chloroplasts and mitochondria are now believed to have been separate
organisms that associate with each other to form mutually beneficial colonies
we now call cells.