Light : One of the most useful
scientific study...
Besides learning about all you want to know, care to see how
light can be used in many of the processes? From photosynthesis to lasers, light has
proven itself to be one of the most beneficial scientific studies of all times. It is
through the study of light that man learns to explore and open up new insights to fully
unlock the potential of light. This section will, thus let you know new processes and more
in-depth understanding of some of light miraculous processes...
Photosynthesis
Photosynthesis is the process, which by chlorophyll,
green plants, algae, and some bacteria together capture the light energy from the sun and
convert it to chemical energy. Chlorophyll is a green substance that is necessary for
capturing the sunlight. Virtually all the energy available for life in the earth's
biosphere—the zone in which life can exist — is made available through
photosynthesis.
A quite generalized, unbalanced chemical equation for
photosynthesis is
CO2 + 2H2A + light
energy ± (CH2) + H2O + H2A
The formula H2A represents a compound that can be oxidized,
that is, from which electrons can be removed; CO2 is carbon dioxide; and CH2 is a
generalization for the hydrocarbons incorporated by the growing organism. In the vast
majority of photosynthetic organisms—that is, algae and green plants—H2A is
water (H2O); in some photosynthetic bacteria; however, H2A is
hydrogen sulfide (H2S). Photosynthesis involving water is the most important and best
understood and, therefore, will be discussed here in detail.
Photosynthesis consists of two stages: a series of
light-dependent reactions that are temperature independent and a series of
temperature-dependent reactions that are light independent. Increasing light intensity
(within certain limits) but not by increasing temperature can increase the rate of the
first series, called the light reaction. In the second series, called the dark reaction,
the rate can be increased by increasing temperature (within certain limits) but not by
increasing light intensity.
Light Reaction
The first step in photosynthesis is the absorption of light
by pigments. Chlorophyll is the most important of these because it is essential for the
process. It captures light energy in the violet and red portions of the spectrum and
transforms it into chemical energy through a series of reactions. Different forms of
chlorophyll and other pigments known as carotenoids and phycobilins absorb slightly
different wavelengths of light and pass the energy to chlorophyll a for the completion of
the transformation process. These accessory pigments thus broaden the spectrum of light
energy that can be fixed through photosynthesis.
Photosynthesis takes place within cells, in organelles
called chloroplasts that contain the chlorophyll and other chemicals, especially enzymes,
necessary for the various reactions. The chemicals involved are organized into units of
the chloroplasts called thylakoids, and the pigments are embedded in the thylakoids in
subunits called photosystems. The pigments, raising their electrons to higher energy
levels absorb light. The energy is then transferred to a special form of chlorophyll a
called a reaction center.
Two photosystems, numbered I and II, are now recognized.
Light energy is first trapped by photosystem II, and the energized electrons are boosted
to an electron receptor. They are replaced in photosystem II by electrons from water
molecules, and oxygen is released. The energized electrons are passed along an electron
transport chain back to photosystem I, and energy-rich adenosine
triphosphate, or ATP is generated in the process. Light absorbed by photosystem I
is then passed to its reaction center, and energized electrons are boosted to its electron
acceptor. They are passed by means of another transport chain to energize the coenzyme
nicotinamide adenine dinucleotide phosphate, or NADP, resulting in its reduction to
NADPH2. The electrons lost by photosystem I are replaced by those passed along the
electron transport chain from photosystem II. The light reaction ends with the energy
yield stored in the ATP and NADPH2.
Dark Reaction
The dark reaction takes place in the stroma (matrix) of the
chloroplast, where the energy stored in the ATP and NADPH2is used to reduce carbon dioxide
to organic carbon. This is accomplished through a series of reactions known as the Calvin
cycle, driven by the energy in the ATP and NADPH2. At each turn of the cycle one molecule
of carbon dioxide enters and is initially combined with a five-carbon sugar called RuBP
(ribulose 1,5-biphosphate) to form two molecules of a three-carbon compound called PGA
(3-phosphoglycerate). Three turns of the cycle—each of which consumes one molecule of
carbon dioxide, two of NADPH2, and three of ATP—produce a three-carbon molecule,
glycer-aldehyde 3-phosphate, two molecules of which combine to form a six-carbon sugar, glucose. The RuBP is regenerated with each turn of the cycle.
Thus, the net effect of photosynthesis is the temporary
capture of light energy in the chemical bonds of ATP and NADPH2 through the light
reaction, and the permanent capture of the energy in glucose through the dark reaction.
Water is split during the light reaction to provide electrons, which transfer the light
energy to form ATP and NADPH2. Carbon dioxide is reduced in the dark reaction to provide
the basis for the sugar molecule. The complete, balanced equation for photosynthesis in
which water serves as the electron donor is
6CO2 + 12H2O ±
C6H12O6+ 6O2 + 6H2O
Artificial Photosynthesis
Were chemists able to duplicate photosynthesis by
artificial means, resulting systems would have enormous potential for tapping solar energy
on a large scale. Much research is now being devoted to this effort. An artificial
molecule that remains polarized sufficiently long to react usefully with other molecules
has not yet been perfected, but the prospects are promising.
