Photosynthesis by nikeborome

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									PHOTOSYNTHESIS
AIMS & OBJECTIVES
                     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
            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.
        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.
       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 .
INTRODUCTION
                     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?

    Answer:

3. What are the two modes of nutrition?

    Answer:     AUTOTROPHIC and HETEROTROPHIC
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.
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
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 .
Furthermore, Solar Energy captured by the process of Photosynthesis is
the source of most of the energy used by Man for:
                     Heat, Light, and Power
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.
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:
                          Sunlight



    CO2      +   H2O                 (CH2)n     +    O2
Carbon Dioxide   Water           Carbohydrate       Oxygen

                         Chlorophyll
Simple carbohydrates are given the empirical formula:
                             (CH2O)n
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:

                    When n = 3,
                    Then, (CH2O)n              = C3H6O3
                             Glyceraldehyde (triose sugar)
Sugars, like GLUCOSE are often considered as the major products
of photosynthetic activity, and the process is often represented by
the following equation:
                            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 doesn’t.
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
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.
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
BACKGROUND NOTES 1
                 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.
                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 as:sulphur, hydrogen sulphide,
ammonia, nitrites and iron (II)
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)
      THE LEAF
        AS A
PHOTOSYNTHETIC ORGAN
DIAGRAMMATIC REPRESENTATION
             OF A
     TRANSVERSE SECTION
             OF A
           TYPICAL
      DICOTYLEDON LEAF
   Diagrammatic Representation of a
Transverse Section of a Dicotyledon Leaf
                     X
  A                                B


                     Y
                           X
       A




            B

                          Y
STRUCTURE
OF THE LEAF
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 .
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.
DIAGRAMMATIC REPRESENTATION
            OF A
      VERTICAL SECTION
            OF A
          TYPICAL
     DICOTYLEDON LEAF
Diagrammatic Representation of a Vertical
      Section of a Dicotyledon Leaf
STRUCTURE & FUNCTION
         OF
    CHLOROPLASTS
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.
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.
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.
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.
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.
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
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.
  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.
DIAGRAMMATIC REPRESENTATION
           OF THE
     INTERNAL STRUCTURE
            OF A
           WHOLE
        CHLOROPLAST
  Diagrammatic Representation of the
Internal Structure of a Whole Cloroplast
DIAGRAMMATIC REPRESENTATION
       OF A GRANUM
Diagrammatic Representation of a Granum
  BACKGROUND NOTES
         2
SUMMARY OF KEY POINTS
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 ) .
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
ENERGY ABSORPTION
  BY CHLOROPHYLL
        &
ASSOCIATED PIGMENTS
 CHLOROPHYLL
      &
  ASSOCIATED
PHOTOSYNTHETIC
   PIGMENTS
    AN EXPERIMENT
 TO ESTABLISH WHETHER
     CHLOROPHYLL
      IS THE ONLY
PHOTOSYNTHETIC PIGMENT
   PRESENT IN LEAVES
To demonstrate the presence of photosynthetic pigments

in leaves of higher green plants you only need to perform

SIMPLE PAPER CHROMATOGRAPHY
EXPERIMENTAL PROCEDURE
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
DIAGRAMMATIC REPRESENTATION

       OF ASCENDING

   PAPER CHROMATOGRAPHY
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.
Since the PHOTOSYNTHETIC PIGMENTS are prone
to:
PHOTODECOMPOSITION ( i.e fading )
it is advisable to READ the CHROMATOGRAM
and MARK the SPOTS under Ultra Violet light
even before drying the paper.
When viewed under U.V. light, PHOTOSYNTHETIC PIGMENTS
will be recognised on PAPER CHROMATOGRAM as intense
FLUORESCENT SPOTS
Dry the CHROMATOGRAM
and then NOTE THE COLOUR
of EACH PIGMENT as viewed in
VISIBLE LIGHT
PHOTOSYNTHETIC
   PIGMENTS
   DETECTED
ON DETECTION
 5 PIGMENTS
can be identified
      PHAEOPHYTIN
        is in fact a
BREAKDOWN of CHLOROPHYLL
ENERGY & LIGHT
                 LIGHT
                is a form of

ELECTROMAGNETIC RADIATION
          VISIBLE LIGHT
     forms only a small fraction of the

ELECTROMAGNETIC RADIATION
 that arrives at the surface of the EARTH
ELECTROMAGNETIC SPECTRUM
     WHITE LIGHT
    is a mixture of light of

DIFFERENT WAVELENGTHS
     The ENERGY of LIGHT is

   INVERSELY PROPORTIONAL
       to its WAVELENGTH

Thus, LIGHT of SHORT WAVELENGTH

        has more ENERGY

 than LIGHT of LONG WAVELENGTH
   THE QUESTION STILL REMAINS
WHICH OF THE VISIBLE WAVELENGTHS
   ARE THE MOST EFFECTIVE FOR

    PHOTOSYNTHESIS?
RESEARCH BACKGROUND
In 1880’s 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
EXPERIMENT 1
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.
Engelmann mounted a filament of the GREEN ALGA on a
microscope slide in a drop of water containing numerous aerobic
bacteria PSEUDOMONAS
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
Initially the slide was kept in DARKNESS
which prevented PHOTOSYNTHESIS,


STOPPED the EVOLUTION OF OXYGEN, and


IMMOBILISED THE BACTERIA
Afterwards, the filaments were then ILLUMINATED
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 CHLOROPLAST
indicating the EVOLUTION OF OXYGEN at these sites
QUESTIONS
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?
EXPERIMENT 2
In 1882, performing the same experimental technique, Engelmann
conducted another experiment but on this occasion chose another


FILAMENTOUS GREEN ALGA which has cells uniformly filled
with CHLOROPLASTS CLADOPHORA
In a similar way he mounted filaments of the green alga on a microscope
slide in a drop of water containing AEROBIC BACTERIA
On this occasion, the filaments were ILLUMINATED with LIGHT
OF DIFFERENT WAVELENGTHS
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
Knowing that the ALGA gave off OXYGEN as it
PHOTOSYNTHESISED Engelmann deduced that
BLUE and RED LIGHT are the most EFFECTIVE for
PHOTOSYNTHESIS
RATE OF PHOTOSYNTHESIS
With the advent of spectrophotometers, researchers have been able to
confirm Engelmann’s 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
ACTION SPECTRUM
The spectrum broadly tells us which BANDS OF WAVELENGTHS
are the most effective for PHOTOSYNTHESIS but doesn’t tell us which
pigments are most effective.
The next step can then be achieved by separating extracts of each pigment
(obtained from TLC, column chromatography, etc.)
Each pigment is in turn subjected to LIGHT OF VARYING
WAVELENGTH
When the PERCENTAGE OF LIGHT ABSORPTION
is plotted against WAVELENGTH OF LIGHT an
ABSORPTION SPECTRUM is obtained for EACH PIGMENT
 ABSORPTION SPECTRA
         OF
CHLOROPLAST PIGMENTS
This ABSORPTION SPECTRUM shows that
CHLOROPLASTS absorb both RED and BLUE LIGHT
whereas the CAROTINOIDS absorb mainly BLUE LIGHT
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
  ACTION
    &
ABSORPTION
 SPECTRA

								
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