Photosynthesis

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PHOTOSYNTHESIS
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AIMS OBJECTIVES
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AIMS
At the end of the series of lessons you will be
able to
1. STATE the importance of photosynthesis, and
2. OUTLINE the mechanism of photosynthesis
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PRINCIPAL OBJECTIVES
At the end of the series of lessons, you will be
able to
1. DISTINGUISH between photosynthesis and
chemosynthesis.
2. NAME organisms which are associated with
photosynthesis and chemosynthesis.
3. INDICATE the criteria required for
photosynthesis in green plants.
4. WRITE an overall equation for the process of
photosynthesis.
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PRINCIPAL OBJECTIVES (cont..)
5. DESCRIBE the overall process of
photosynthesis in a single sentence.
6. STATE the importance of photosynthesis.
7. RECOGNISE the leaf as the photosynthetic
organ.
8. MAKE representative diagrams of whole
chloroplasts indicating the arrangement of
lamellae and membranes between lamellae.
9. DESCRIBE the structure of the chloroplast in
relation to its photosynthetic function.
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PRINCIPAL OBJECTIVES (cont.)
10. DESCRIBE and CONDUCT an experiment to
establish which photosynthetic pigments are
present in green leaves.
11. OUTLINE experiments which determine
experimentally the
a) site for photosynthesis, and
b) wavelengths of light which are most effective
for photosynthesis
12. DISTINGUISH between Action and Absorption
spectra.
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INTRODUCTION
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KEY QUESTIONS
1. Is it possible to sustain life in the absence
of photosynthesis?
Answer All forms of life are either directly or
indirectly dependent on photosynthesis
2. What is meant by the statement directly or
indirectly dependent on photosynthesis?
3. What are the two modes of nutrition?
Answer AUTOTROPHIC and HETEROTROPHIC
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AUTOTROPHIC NUTRITION which is the synthesis of
organic compounds from inorganic sources, takes
place by
1. PHOTOSYNTHESIS in plants, and
2. CHEMOSYNTHESIS in certain bacteria.
3. If we say that photosynthesis occurs in higher
green plants, what are the criteria for
photosynthesis to take place?
Answer Carbon Dioxide, Water, Chlorophyll and
Sunlight.
4. However, is it correct to say that
photosynthesis only occurs in higher green
plants?
Answer No, it is a misconception to say that
photosynthesis only takes place in higher
green plants.
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5. Research has shown that over 40 of all
photosynthesis on the surface of the Earth is
carried out in the oceans by
PHYTOPLANKTON
The PHYTOPLANKTON constitute such organisms as
Microscopic Algae, Diatoms, and Dinoflagellates
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6. Photosynthesis is a metabolic process that is
fundamental to all living organisms. Why?
Answer Solar energy is not only the immediate
source of energy for green plants and other
photosynthetic autotrophs but also the
ultimate source of energy for nearly all
heterotrophic organisms, through the operation
of food chains in the biosphere.
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Furthermore, Solar Energy captured by the process
of Photosynthesis is the source of most of the
energy used by Man for since fossil fuels such
as coal, oil and natural gas are all
decomposition products of biological materials
generated million of years ago by photosynthetic
organisms.
Heat, Light, and Power
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In summary, PHOTOSYNTHESIS is the process used
by all green plants, blue-green algae
(Cyanophyta) and certain bacteria. PHOTOSYNTHESIS
is the synthesis of organic compounds primarily
sugars from carbon dioxide and water (or other
hydrogen(electron) donors) using sunlight as the
source of energy and chlorophyll (or some other
closely related pigment) for trapping the light
energy.
This PROCESS can be summarised by the following
equation
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Sunlight


Chlorophyll
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Simple carbohydrates are given the empirical
formula That is, with the same ratio of
hydrogen to oxygen as in water. 3 is the smallest
value you can give to n in the empirical
formula. For Example Sugars, like
GLUCOSE are often considered as the major
products of photosynthetic activity, and the
process is often represented by the following
equation
(CH2O)n
When n 3, Then, (CH2O)n C3H6O3
Glyceraldehyde (triose sugar)
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Sunlight
6CO2 6H2O C6H12O6 6O2 Carbon
Dioxide Water GLUCOSE Oxygen.
Chlorophyll
Both equations are an oversimplification of a
complicated process made up of very many steps.
Additionally, the equations do not inform us how
the energy rich carbohydrates are produced
Also misleading is the impression that the oxygen
evolved comes from the carbon dioxide - which it
doesnt.
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The PHOTOSYNTHETIC PROCESS has been shown to be
divided into TWO major phases.
1. The LIGHT STAGE (Light-Dependent
Reactions) This stage represents the conversion
of radiant energy to chemical energy in the
form of ATP
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2. The DARK STAGE (Light-Independent
Reactions) This stage represents the enzymatic
synthesis of carbohydrate intermediates and
other compounds by the utilisation of the ATP
produced during the Light Stage.
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PHOTOSYNTHESIS commences by the absorption of
light by the green pigment CHLOROPHYLL a compound
which is abundantly found in leaves where it
is concentrated in disc-shaped structures called
CHLOROPLASTS
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BACKGROUND NOTES 1
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PHOTOSYNTHESIS
This is the process used by all green plants,
blue-green algae (Cyanophyta) and certain
bacteria. It is the synthesis of organic
compounds primarily sugars from carbon dioxide
and water (or other hydrogen electron donors)
using sunlight as the source of energy and
chlorophyll (or some other closely related
pigment ) for trapping the light energy.
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CHEMOSYNTHESIS
This involves the synthesis of organic compounds
from carbon dioxide and water but the energy
instead of coming from light (as in
photosynthesis ) is supplied by oxidising simple
inorganic compounds found in the environment,
such assulphur, hydrogen sulphide, ammonia,
nitrites and iron (II)
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Examples of chemosynthetic organisms are bacteria
associated with the nitrogen and sulphur cycles,
such as
Nitrosomonas (ammonia converted to nitrite)
Nitrobacter (nitrite converted to nitrate)
Thiobacillus (sulphur converted to sulphate)
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THE LEAF AS A PHOTOSYNTHETIC ORGAN
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DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE
SECTION OF A TYPICAL DICOTYLEDON LEAF
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Diagrammatic Representation of a Transverse
Section of a Dicotyledon Leaf
X
B
A
Y
X
A
B
Y
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STRUCTURE OF THE LEAF
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The structure of the leaf is highly adapted to
satisfy the criteria for PHOTOSYNTHESIS The leaf,
if it is to function successfully as a
photosynthetic organ, MUST
1. Provide a large surface area in addition to
being thin in order to allow quicker absorption
of carbon dioxide and sunlight.
2. Possess numerous stomata (mainly on
underside ) thus allowing rapid exchange of
carbon dioxide and oxygen with the atmosphere.
3. Receive an available source of carbon dioxide.
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4. Receive an adequate supply of water and
mineral salts (nitrates, sulphates and
phosphates).
5. Possess chlorophyll and be adapted to receive
sunlight.
6. Allow oxygen to escape as a waste product
via t intracellular spaces and stomata
7. Translocate sugars and other products of
photosynthesis to other parts of the plant to
be used or stored as required.
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DIAGRAMMATIC REPRESENTATION OF A VERTICAL
SECTION OF A TYPICAL DICOTYLEDON LEAF
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Diagrammatic Representation of a Vertical Section
of a Dicotyledon Leaf
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STRUCTURE FUNCTION OF CHLOROPLASTS
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In eukaryotes (e.g.. higher green plants)
PHOTOSYNTHESIS takes place in the specialised
organelles called CHLOROPLASTS which are found
chiefly in the mesophyll cells of leaves.
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1. SHAPE Chloroplasts of higher plants are
usually shaped like biconvex lenses (discoid)
varying in diameter from 4-10 µm (on average 5
µm) and in thickness from 2 - 3 µm. In
comparison, the chloroplasts of eukaryotic algae
vary in form from being an exotic ribbon-like
spiral structure in Spirogyra to cup-shaped in
Chlamydomonas.
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2. LOCATION NUMBER Chloroplasts are located in
the CYTOPLASM of cells and vary in number from
onein algae such as Chlamydomonas and Chlorella
to as many as 100 in the palisade mesophyll cells
of higher plants.
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3. STRUCTURE Viewed under the Electron
Microscope, chloroplasts are seen to be bounded
by a DOUBLE MEMBRANE. The outer continuous
membrane is smooth and rather fragile while the
inner membrane system whilst remaining continuous
is extended inwards as a system of paired folds
called LAMELLAE in which the photosynthetic
pigments like CHLOROPHYLL, ENZYMES and
ELECTRON CARRIERS are located.
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STRUCTURE (cont..)
The entire membranous structure runs through an
aqueous matrix called the STROMA which is
comparable to the mitochondrial matrix in
constitution. The STROMA contains proteins,
DNA, sugars, organic acids, starch granules and
the enzymes associated with the CALVIN CYCLE
such as CO2 reductase enzymes.
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STRUCTURE (cont..)
The LAMELLAE are composed of two membranes and
at regular intervals, widen to form flattened
membrane sacs called THYLAKOIDS (Greek for
baggy trousers) which form stacked arrangements
resembling pile of coins called GRANA. The
flattened sheets of lamellae which constitute
each granum can be divided into GRANAL and
INTERGRANAL regions
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4. FUNCTION of THYLAKOIDS The function of the
thylakoid membranes of the grana is to hold the
photosynthetic pigments in a suitable position in
order to trap the maximum amount of light
energy. The GRANA are thus the ACTIVE
PHOTOSYNTHETIC UNITS in the plant cell.
These QUANTOSOMES are believed to contain the
LIGHT REACTION ENZYMES required for catalysing
the SYNTHESIS of ATP.
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FUNCTION of THYLAKOIDS (cont..)
Further analyses under high resolution Electron
Microscopy has led to the identification of
globular units on the membranous surfaces
ofthylakoid lamellae constituting each granum.
These globular structures are called QUANTOSOMES
which can be further resolved into 4 sub-units of
protein.
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DIAGRAMMATIC REPRESENTATION OF THE INTERNAL
STRUCTURE OF A WHOLE CHLOROPLAST
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Diagrammatic Representation of the Internal
Structure of a Whole Cloroplast
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DIAGRAMMATIC REPRESENTATION OF A GRANUM
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Diagrammatic Representation of a Granum
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BACKGROUND NOTES 2 SUMMARY OF KEY POINTS
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1. The thin flat structures of CHLOROPLASTS allow
optimum exposure to light and minimum time for
diffusion.
2. LIGHT REACTIONS occur in QUANTOSOMES located
on the membranous surfaces of THYLAKOID
LAMELLAE. Solar energy is absorbed by
photosynthetic pigments and is used to produce
reduced nicotinamide - adenine dinucleotide
phosphate ( NADPH ) and adenosine triphosphate (
ATP ).
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3. DARK REACTIONS occur in the stroma whereby
under enzyme catalysed reactions
NADPH, CO2 and ATP
are used to synthesise energy rich organic
compounds such as SUGARS and STARCH
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ENERGY ABSORPTION BY CHLOROPHYLL ASSOCIATED
PIGMENTS
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CHLOROPHYLL ASSOCIATED PHOTOSYNTHETIC PIGMENTS
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AN EXPERIMENT TO ESTABLISH WHETHER CHLOROPHYLL IS
THE ONLY PHOTOSYNTHETIC PIGMENT PRESENT IN LEAVES
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To demonstrate the presence of photosynthetic
pigments in leaves of higher green plants you
only need to perform SIMPLE PAPER CHROMATOGRAPHY
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EXPERIMENTAL PROCEDURE
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1. GRIND chopped leaves such as spinach, in a
pestle and mortar with an organic solvent such
as propanone (acetone).
2. SEPARATE the pigments in the extract using
PAPER CHROMATOGRAPHY
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DIAGRAMMATIC REPRESENTATION OF ASCENDING PAPER
CHROMATOGRAPHY
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After the solvent front has progressed
sufficiently up the paper chromatogram stop the
process ( migration )by removing the cork and
taking the chromatogram out of the boiling tube.
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Since the PHOTOSYNTHETIC PIGMENTS are prone
to it is advisable to READ the
CHROMATOGRAM and MARK the SPOTS under Ultra
Violet light even before drying the paper.
PHOTODECOMPOSITION ( i.e fading )
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When viewed under U.V. light, PHOTOSYNTHETIC
PIGMENTS will be recognised on PAPER CHROMATOGRAM
as intense FLUORESCENT SPOTS
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Dry the CHROMATOGRAM and then NOTE THE COLOUR of
EACH PIGMENT as viewed in VISIBLE LIGHT
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PHOTOSYNTHETIC PIGMENTS DETECTED
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ON DETECTION 5 PIGMENTS can be identified
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PHAEOPHYTIN is in fact a BREAKDOWN of
CHLOROPHYLL
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ENERGY LIGHT
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LIGHT is a form of ELECTROMAGNETIC RADIATION VISIB
LE LIGHT forms only a small fraction of
the ELECTROMAGNETIC RADIATION that arrives at the
surface of the EARTH
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ELECTROMAGNETIC SPECTRUM
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WHITE LIGHT is a mixture of light of
DIFFERENT WAVELENGTHS
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The ENERGY of LIGHT is INVERSELY PROPORTIONAL to
its WAVELENGTH Thus, LIGHT of SHORT WAVELENGTH has
more ENERGY than LIGHT of LONG WAVELENGTH
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THE QUESTION STILL REMAINS WHICH OF THE VISIBLE
WAVELENGTHS ARE THE MOST EFFECTIVE
FOR PHOTOSYNTHESIS?
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RESEARCH BACKGROUND
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In 1880s a German Botanist name T.W. Engelmannin
attempting to identify the SITE of
PHOTOSYNTHESIS additionally demonstrated
experimentally the WAVELENGTHS of WHITE LIGHT
which were most effective for
PHOTOSYNTHESIS
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EXPERIMENT 1
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For the first experiment, Engelmann chose a
filamentous alga such as SPIROGYRA. The
filaments of Spirogyra are composed of
LARGE CYLINDRICAL CELL Splaced end-to-end with
each cell containing a RIBBON-LIKE CHLOROPLAST
spiralling round the perimeter of the cell.
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Engelmann mounted a filament of the GREEN ALGA on
a microscope slide in a drop of water containing
numerous aerobic bacteria PSEUDOMONAS
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In order to show that the process of
photosynthesis was taking place, Engelmann chose
to detect EVOLUTION OF OXYGEN For this, he used
AEROBIC BACTERIA which being MOTILE had the
ability to MOVE CLUSTER in AREAS where
OXYGEN CONCENTRATION was at its HIGHEST
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Initially the slide was kept in DARKNESSwhich
prevented PHOTOSYNTHESIS, STOPPED the
EVOLUTION OF OXYGEN, and IMMOBILISED THE BACTERIA
Afterwards, the filaments were then ILLUMINATED
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Engelmann the proceeded to OBSERVE for the
distribution of the AEROBIC BACTERIA in the
water. The MOTILE BACTERIA were seen to CLUSTER
round the edge of cells immediately adjacent to
the CHLOROPLASTindicating the EVOLUTION OF OXYGEN
at these sites
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QUESTIONS
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1. Explain why the distribution of the bacteria
could be used as an indication of the rate of
photosynthesis in different regions of
Spirogyra.
2. What evidence is there in the previous
diagrams for a relationship between
chloroplasts and photosynthesis?
3. Is there any evidence in the above diagrams
for red light being more effective than white
light?
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EXPERIMENT 2
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In 1882, performing the same experimental
technique, Engelmannconducted another experiment
but on this occasion chose another FILAMENTOUS
GREEN ALGA which has cells uniformly filled with
CHLOROPLASTS CLADOPHORA
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In a similar way he mounted filaments of the
green alga on a microscope slide in a drop of
water containing AEROBIC BACTERIA
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On this occasion, the filaments were ILLUMINATED
with LIGHT OF DIFFERENT WAVELENGTHS
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By observing the DISTRIBUTION OF BACTERIA in the
water, Engelmann was able to note that the
bacteria CLUSTERED near to the filaments when
BLUE LIGHT (450nm) or RED LIGHT (650nm) was used
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Knowing that the ALGA gave off OXYGEN as
it PHOTOSYNTHESISED Engelmann deduced that BLUE
and RED LIGHT are the most EFFECTIVE for
PHOTOSYNTHESIS
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RATE OF PHOTOSYNTHESIS
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With the advent of spectrophotometers,
researchers have been able to confirm Engelmanns
findings. This has been achieved by measuring the
RATE of PHOTOSYNTHESIS in the presence of LIGHT
of VARYING WAVELENGTH When the rate of
photosynthesis is plotted against wavelength of
light an ACTION SPECTRUM for PHOTOSYNTHESIS is
obtained
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ACTION SPECTRUM
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The spectrum broadly tells us which
BANDS OF WAVELENGTHSare the most effective for
PHOTOSYNTHESIS but doesnt tell us which
pigments are most effective.
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The next step can then be achieved by separating
extracts of each pigment (obtained from TLC,
column chromatography, etc.)
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Each pigment is in turn subjected to
LIGHT OF VARYING WAVELENGTH
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When the PERCENTAGE OF LIGHT ABSORPTION is
plotted against WAVELENGTH OF LIGHT
an ABSORPTION SPECTRUM is obtained for
EACH PIGMENT
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ABSORPTION SPECTRA OF CHLOROPLAST PIGMENTS
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This ABSORPTION SPECTRUM shows that CHLOROPLASTS
absorb both RED and BLUE LIGHT whereas the
CAROTINOIDS absorb mainly BLUE LIGHT
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By comparing the ACTION SPECTRUM for
Photosynthesis with the ABSORPTION SPECTRUM for
the combined Chloroplast extract it can be shown
that there is a close similarity between the two
traces providing sufficient evidencefor the ROLE
OF CHLOROPLAST PIGMENTS in PHOTOSYNTHESIS
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ACTION ABSORPTION SPECTRA
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