At 11:19 +0200 04/02/97, Compagnia delle Foreste wrote:
>Dear List-members,
>I am trying to find updated information about the role of trees (and in
>particular: fast-growing tree plantations) in reducing atmospheric CO2
>concentration. These information will be used for an article in our
>monthly forestry magazine in italian language called "Sherwood - Foreste e
>Alberi Oggi".
>In the net I was able to find only two good hints: the home page of the
>Carbon Dioxide Information Analysis Center -CDIAC-
>(http://cdiac.esd.ornl.gov/cdiac/ ) and an interesting document: Tree
>Planting to Abate Carbon Dioxide Emissions: Options for Southern
>California Edison (By Frank Muller, Center for Global Change, College
>Park, Maryland USA) found in
>http://solstice.crest.org/environment/global_change/ctrforgc/cgcdoc8.htm
>
>Following a mail sent to the CDIAC, Gregg Marland (Environmental Sciences
>Division - Oak Ridge National Laboratory Oak Ridge, TN USA) kindly
>addressed me to two interesting documents :
>http://www.joanneum.ac.at/IEA-Bioenergy-TaskXV and
>http://www.joanneum.ac.at/GORCAM.htm .
>
>Is there somebody in the Net that can send me information, or some more
>www address ?
>I will try then to summurize the results of such survey in a future mail
>to the Forest-List.
>Thank you for your help
>
>Antonio Brunori, M.Sc.
>Vice-Editor Sherwood
Dear Mr. Brunori
Follows a very inspiring note written by Dr Devens Gust from
Arizona State University. This paper focuss on many issues and
one of them deals with your question.
It comes from his site at
http://photoscience.la.asu.edu/photosyn/study.html
It is not an exhaustive point of view on the subject but worth
to be read!
Hope this will help you!
J.-Robert Thibault
Professor at the Faculty of forestry and geomatic
Laval University, Quebec, Canada
WHY STUDY PHOTOSYNTHESIS?
What is photosynthesis?
Photosynthesis is arguably the most important biological process on earth.
By liberating oxygen and consuming carbon dioxide, it has transformed the
world into the hospitable environment we know today. Directly or
indirectly, photosynthesis fills all of our food requirements and many of
our needs for fiber and building materials. The energy stored in petroleum,
natural gas and coal all came from the sun via photosynthesis, as does the
energy in firewood, which is a major fuel in many parts of the world. This
being the case, scientific research into photosynthesis is vitally
important. If we can understand and control the intricacies of the
photosynthetic process, we can learn how to increase crop yields of food,
fiber, wood, and fuel, and how to better use our lands. The
energy-harvesting secrets of plants can be adapted to man-made systems
which provide new, efficient ways to collect and use solar energy. These
same natural "technologies" can help point the way to the design of new,
faster, and more compact computers, and even to new medical breakthroughs.
Because photosynthesis helps control the makeup of our atmosphere,
understanding photosynthesis is crucial to understanding how carbon dioxide
and other "greenhouse gases" affect the global climate. In this document,
we will briefly explore each of the areas mentioned above, and illustrate
how photosynthesis research is critical to maintaining and improving our
quality of life.
Photosynthesis and food. All of our biological energy needs are met by the
plant kingdom, either directly or through herbivorous animals. Plants in
turn obtain the energy to synthesize foodstuffs via photosynthesis.
Although plants draw necessary materials from the soil and water and carbon
dioxide from the air, the energy needs of the plant are filled by sunlight.
Sunlight is pure energy. However, sunlight itself is not a very useful form
of energy; it cannot be eaten, it cannot turn dynamos, and it cannot be
stored. To be beneficial, the energy in sunlight must be converted to other
forms. This is what photosynthesis is all about. It is the process by which
plants change the energy in sunlight to kinds of energy that can be stored
for later use. Plants carry out this process in photosynthetic reaction
centers. These tiny units are found in leaves, and convert light energy to
chemical energy, which is the form used by all living organisms. One of the
major energy-harvesting processes in plants involves using the energy of
sunlight to convert carbon dioxide from the air into sugars, starches, and
other high-energy carbohydrates. Oxygen is released in the process. Later,
when the plant needs food, it draws upon the energy stored in these
carbohydrates. We do the same. When we eat a plate of spaghetti, our bodies
oxidize or "burn" the starch by allowing it to combine with oxygen from the
air. This produces carbon dioxide, which we exhale, and the energy we need
to survive. Thus, if there is no photosynthesis, there is no food. Indeed,
one widely accepted theory explaining the extinction of the dinosaurs
suggests that a comet, meteor, or volcano ejected so much material into the
atmosphere that the amount of sunlight reaching the earth was severely
reduced. This in turn caused the death of many plants and the creatures
that depended upon them for energy.
Photosynthesis and energy. One of the carbohydrates resulting from
photosynthesis is cellulose, which makes up the bulk of dry wood and other
plant material. When we burn wood, we convert the cellulose back to carbon
dioxide and release the stored energy as heat. Burning fuel is basically
the same oxidation process that occurs in our bodies; it liberates the
energy of "stored sunlight" in a useful form, and returns carbon dioxide to
the atmosphere. Energy from burning "biomass" is important in many parts of
the world. In developing countries, firewood continues to be critical to
survival. Ethanol (grain alcohol) produced from sugars and starches by
fermentation is a major automobile fuel in Brazil, and is added to gasoline
in some parts of the United States to help reduce emissions of harmful
pollutants. Ethanol is also readily converted to ethylene, which serves as
a feedstock to a large part of the petrochemical industry. It is possible
to convert cellulose to sugar, and then into ethanol; various
microorganisms carry out this process. It could be commercially important
one day.
Our major sources of energy, of course, are coal, oil and natural gas.
These materials are all derived from ancient plants and animals, and the
energy stored within them is chemical energy that originally came from
sunlight through photosynthesis. Thus, most of the energy we use today was
originally solar energy!
Photosynthesis, fiber, and materials. Wood, of course, is not only burned,
but is an important material for building and many other purposes. Paper,
for example, is nearly pure photosynthetically produced cellulose, as is
cotton and many other natural fibers. Even wool production depends on
photosynthetically-derived energy. In fact, all plant and animal products
including many medicines and drugs require energy to produce, and that
energy comes ultimately from sunlight via photosynthesis. Many of our other
materials needs are filled by plastics and synthetic fibers which are
produced from petroleum, and are thus also photosynthetic in origin. Even
much of our metal refining depends ultimately on coal or other
photosynthetic products. Indeed, it is difficult to name an economically
important material or substance whose existence and usefulness is not in
some way tied to photosynthesis.
Photosynthesis and the environment. Currently, there is a lot of discussion
concerning the possible effects of carbon dioxide and other "greenhouse
gases" on the environment. As mentioned above, photosynthesis converts
carbon dioxide from the air to carbohydrates and other kinds of "fixed"
carbon and releases oxygen to the atmosphere. When we burn firewood,
ethanol, or coal, oil and other fossil fuels, oxygen is consumed, and
carbon dioxide is released back to the atmosphere. Thus, carbon dioxide
which was removed from the atmosphere over millions of years is being
replaced very quickly through our consumption of these fuels. The increase
in carbon dioxide and related gases is bound to affect our atmosphere. Will
this change be large or small, and will it be harmful or beneficial? These
questions are being actively studied by many scientists today. The answers
will depend strongly on the effect of photosynthesis carried out by land
and sea organisms. As photosynthesis consumes carbon dioxide and releases
oxygen, it helps counteract the effect of combustion of fossil fuels. The
burning of fossil fuels releases not only carbon dioxide, but also
hydrocarbons, nitrogen oxides, and other trace materials that pollute the
atmosphere and contribute to long-term health and environmental problems.
These problems are a consequence of the fact that nature has chosen to
implement photosynthesis through conversion of carbon dioxide to
energy-rich materials such as carbohydrates. Can the principles of
photosynthetic solar energy harvesting be used in some way to produce
non-polluting fuels or energy sources? The answer, as we shall see, is yes.
Why study photosynthesis?
Because our quality of life, and indeed our very existence, depends on
photosynthesis, it is essential that we understand it. Through
understanding, we can avoid adversely affecting the process and
precipitating environmental or ecological disasters. Through understanding,
we can also learn to control photosynthesis, and thus enhance production of
food, fiber and energy. Understanding the natural process, which has been
developed by plants over several billion years, will also allow us to use
the basic chemistry and physics of photosynthesis for other purposes, such
as solar energy conversion, the design of electronic circuits, and the
development of medicines and drugs. Some examples follow.
Photosynthesis and agriculture. Although photosynthesis has interested
mankind for eons, rapid progress in understanding the process has come in
the last few years. One of the things we have learned is that overall,
photosynthesis is relatively inefficient. For example, based on the amount
of carbon fixed by a field of corn during a typical growing season, only
about 1 - 2% of the solar energy falling on the field is recovered as new
photosynthetic products. The efficiency of uncultivated plant life is only
about 0.2%. In sugar cane, which is one of the most efficient plants, about
8% of the light absorbed by the plant is preserved as chemical energy. Many
plants, especially those that originate in the temperate zones such as most
of the United States, undergo a process called photorespiration. This is a
kind of "short circuit" of photosynthesis that wastes much of the plants'
photosynthetic energy. The phenomenon of photorespiration including its
function, if any, is only one of many riddles facing the photosynthesis
researcher.
If we can fully understand processes like photorespiration, we will have
the ability to alter them. Thus, more efficient plants can be designed.
Although new varieties of plants have been developed for centuries through
selective breeding, the techniques of modern molecular biology have speeded
up the process tremendously. Photosynthesis research can show us how to
produce new crop strains that will make much better use of the sunlight
they absorb. Research along these lines is critical, as recent studies show
that agricultural production is leveling off at a time when demand for food
and other agricultural products is increasing rapidly.
Because plants depend upon photosynthesis for their survival, interfering
with photosynthesis can kill the plant. This is the basis of several
important herbicides, which act by preventing certain important steps of
photosynthesis. Understanding the details of photosynthesis can lead to the
design of new, extremely selective herbicides and plant growth regulators
that have the potential of being environmentally safe (especially to animal
life, which does not carry out photosynthesis). Indeed, it is possible to
develop new crop plants that are immune to specific herbicides, and to thus
achieve weed control specific to one crop species.
Photosynthesis and energy production. As described above, most of our
current energy needs are met by photosynthesis, ancient or modern.
Increasing the efficiency of natural photosynthesis can also increase
production of ethanol and other fuels derived from agriculture. However,
knowledge gained from photosynthesis research can also be used to enhance
energy production in a much more direct way. Although the overall
photosynthesis process is relatively wasteful, the early steps in the
conversion of sunlight to chemical energy are quite efficient. Why not
learn to understand the basic chemistry and physics of photosynthesis, and
use these same principles to build man-made solar energy harvesting
devices? This has been a dream of chemists for years, but is now close to
becoming a reality. In the laboratory, scientists can now synthesize
artificial photosynthetic reaction centers which rival the natural ones in
terms of the amount of sunlight stored as chemical or electrical energy.
More research will lead to the development of new, efficient solar energy
harvesting technologies based on the natural process.
The role of photosynthesis in control of the environment. How does
photosynthesis in temperate and tropical forests and in the sea affect the
quantity of greenhouse gases in the atmosphere? This is an important and
controversial issue today. As mentioned above, photosynthesis by plants
removes carbon dioxide from the atmosphere and replaces it with oxygen.
Thus, it would tend to ameliorate the effects of carbon dioxide released by
the burning of fossil fuels. However, the question is complicated by the
fact that plants themselves react to the amount of carbon dioxide in the
atmosphere. Some plants, appear to grow more rapidly in an atmosphere rich
in carbon dioxide, but this may not be true of all species. Understanding
the effect of greenhouse gases requires a much better knowledge of the
interaction of the plant kingdom with carbon dioxide than we have today.
Burning plants and plant products such as petroleum releases carbon dioxide
and other byproducts such as hydrocarbons and nitrogen oxides. However, the
pollution caused by such materials is not a necessary product of solar
energy utilization. The artificial photosynthetic reaction centers
discussed above produce energy without releasing any byproducts other than
heat. They hold the promise of producing clean energy in the form of
electricity or hydrogen fuel without pollution. Implementation of such
solar energy harvesting devices would prevent pollution at the source,
which is certainly the most efficient approach to control.
Photosynthesis and electronics. At first glance, photosynthesis would seem
to have no association with the design of computers and other electronic
devices. However, there is potentially a very strong connection. A goal of
modern electronics research is to make transistors and other circuit
components as small as possible. Small devices and short connections
between them make computers faster and more compact. The smallest possible
unit of a material is a molecule (made up of atoms of various types). Thus,
the smallest conceivable transistor is a single molecule (or atom). Many
researchers today are investigating the intriguing possibility of making
electronic components from single molecules or small groups of molecules.
Another very active area of research is computers that use light, rather
than electrons, as the medium for carrying information. In principle,
light-based computers have several advantages over traditional designs, and
indeed many of our telephone transmission and switching networks already
operate through fiber optics. What does this have to do with
photosynthesis? It turns out that photosynthetic reaction centers are
natural photochemical switches of molecular dimensions. Learning how plants
absorb light, control the movement of the resulting energy to reaction
centers, and convert the light energy to electrical, and finally chemical
energy can help us understand how to make molecular-scale computers. In
fact, several molecular electronic logic elements based on artificial
photosynthetic reaction centers have already been reported in the
scientific literature.
Photosynthesis and medicine. Light has a very high energy content, and when
it is absorbed by a substance this energy is converted to other forms. When
the energy ends up in the wrong place, it can cause serious damage to
living organisms. Aging of the skin and skin cancer are only two of many
deleterious effects of light on humans and animals. Because plants and
other photosynthetic species have been dealing with light for eons, they
have had to develop photoprotective mechanisms to limit light damage.
Learning about the causes of light- induced tissue damage and the details
of the natural photoprotective mechanisms can help us can find ways to
adapt these processes for the benefit of humanity in areas far removed from
photosynthesis itself. For example, the mechanism by which sunlight
absorbed by photosynthetic chlorophyll causes tissue damage in plants has
been harnessed for medical purposes. Substances related to chlorophyll
localize naturally in cancerous tumor tissue. Illumination of the tumors
with light then leads to photochemical damage which can kill the tumor
while leaving surrounding tissue unharmed. Another medical application
involves using similar chlorophyll relatives to localize in tumor tissue,
and thus act as dyes which clearly delineate the boundary between cancerous
and healthy tissue. This diagnostic aid does not cause photochemical damage
to normal tissue because the principles of photosynthesis have been used to
endow it with protective agents that harmlessly convert the absorbed light
to heat.
Conclusions
The above examples illustrate the importance of photosynthesis as a natural
process and the impact that it has on all of our lives. Research into the
nature of photosynthesis is crucial because only by understanding
photosynthesis can we control it, and harness its principles for the
betterment of mankind. Science has only recently developed the basic tools
and techniques needed to investigate the intricate details of
photosynthesis. It is now time to apply these tools and techniques to the
problem, and to begin to reap the benefits of this research.
--Written by and Copyright ©1996 Devens Gust, Professor of Chemistry and
Biochemistry, Arizona State University
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