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 Photosynthesis
The primary source of energy for nearly all life is the Sun.
The energy in sunlight is introduced into the biosphere by a
process
known as photosynthesis, which occurs in plants, algae and some
types of bacteria. Photosynthesis can
be defined as the
physico-chemical process by which photosynthetic organisms use
light energy to drive the synthesis of organic compounds.
Virtually all oxygen in the atmosphere is thought to have
been generated through the process of photosynthesis It is a
very complicated biological system. Basically it is the process
that converts energy from sunlight to chemical forms of energy
that can be used.
Plants, algae, as well as cyanobacteria are responsible for
a major part of photosynthesis in oceans. These organisms convert
CO2 (carbon dioxide) to organic material by reducing this gas
to carbohydrates in a rather complex set of reactions. Electrons
for this reduction reaction ultimately come from water, which
is then converted to oxygen and protons. Energy for this process
is provided by light, which is absorbed by pigments (primarily
chlorophylls and carotenoids). Chlorophylls absorb blue and red
light and carotenoids absorb blue-green light , but green and
yellow light are not effectively absorbed by photosynthetic pigments
in plants; therefore, light of these colors is either reflected
by leaves or passes through the leaves. This
is why plants are green.
Other photosynthetic organisms, such as cyanobacteria,
known as blue-green algae, and red algae,
have additional pigments called phycobilins that are red or blue
and that absorb the colors of visible light that are not effectively
absorbed by chlorophyll and carotenoids. Yet other organisms,
such as the purple and green bacteria, contain bacteriochlorophyll
that absorbs in the infrared, in addition to in the blue part
of the spectrum. These bacteria do not evolve oxygen, but perform
photosynthesis under anaerobic (oxygen-less) conditions. All
plants, algae, and cyanobacteria which photosynthesize contain
the pigment chlorophyll "a." A second kind of chlorophyll
is chlorophyll "b", which occurs only in green algae"
and in the plants. A third form of chlorophyll which is common
is called chlorophyll "c", and is found only in the
photosynthetic members of the Chromista as well as the dinoflagellates.
Photosynthetic pigments come in a huge variety. Some are
chlorophyll, carotenoids, and phycobilins, and they differ from
each other in their precise chemical structure. Light energy
is
absorbed by individual pigments, but is not used immediately
by these pigments for energy conversion. Instead, the light energy
is transferred to chlorophylls that are in a special protein
environment where the actual energy conversion event occurs,
the light energy is used to transfer an electron to a neighboring
pigment. The purpose is to maintain a high rate of electron transfer
in the reaction center, even at lower light intensities.
Plants have developed means to convert some of the
absorbed light energy to heat rather than to use the absorbed
light necessarily for photosynthesis. Because they interact with
light to absorb only certain wavelengths, pigments are useful
to plants and other autotrophs--organisms which make their own
food using photosynthesis. White light is separated into the
different colors (wavelengths) of light by passing it through
a prism. Wavelength is defined as the distance from peak to peak.
Energy is inversely proportional to the wavelength: longer wavelengths
have less energy than do shorter ones.
Without photosynthesis, the oxygen in the atmosphere would
be depleted within several thousand years. It should be emphasized
that plants respire just like any other higher organism, and
that during the day this respiration is masked by a higher rate
of photosynthesis.
In eukaryotes, photosynthesis takes place in the chloroplast,
which has long been known to have prokaryotic features.
Chloroplasts are thought to have evolved from a cyanobacterium
(or close relative) that was in a symbiotic relationship with
a
eukaryotic, non-photosynthetic cell and was engulfed inside this
cell. The cyanobacterium and the eukaryotic cell presumably
were in a mutually beneficial relationship (endosymbiosis), with
the photosynthetic organism sharing some of its produced
carbohydrates with the host, and the host providing the photosynthetic
bacterium with other compounds. The prokaryote
slowly gave up its independence as well as its cell wall, and
some of its genetic information was transferred to the nucleus
of its
eukaryotic host. The resulting chloroplast maintains a small,
prokaryote-like circular DNA of its own (DNA is material
carrying genetic information); this DNA contains the genetic
blueprint to make many of the membrane proteins needed in the
chloroplast, which apparently are not easily targeted to and/or
transported into the chloroplast. Occasionally, photosynthetic
organisms are found where the chloroplast has retained a little
more of the original cyanobacterial features. For example, in
algae such as Cyanophora paradoxa plastids (called cyanelles)
are found that resemble cyanobacteria in their overall
morphology as well as in the fact that they are surrounded by
a cell wall.
Even though plants are the most visible representatives of
photosynthetic organisms, it should be emphasized that many other
types of photosynthetic organisms exist.
We credit a lot of the information contained above in thanks
to Wim Vermaas, from the Department
of Plant Biology at Arizona.
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