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Unit - I, Chapter - 4, Photosynthesis

      The synthesis of energetic carbohydrates from energy less CO2 and water in the presence of sun light by the green plants is called photosynthesis.
      It is the most important anabolic pathway.
      It is endergonic, Physico Chemical, Oxide reductive, Oxygenic (rarely anoxygenic) pathway that occurs during daytime. The equation showing Photosynthesis can be written as

      Glucose is the end product and oxygen is the by product in it.
      This reaction provides food and oxygen for the organisms to survive.
Historical Account 
* Joseph Priestley (1770) discovered that plants absorb toxic gases released by animals and give out oxygen. His famous experiment includes Mint, Mouse and Candle.
* Jan Ingenhousz proposed that light is essential for the plants to purify the air. He also proved that only the green parts evolve oxygen.

* Sachs (1854) found that green parts of the plants produce glucose and it is stored as starch. He proposed that starch is the visible product of photosynthesis.
* Hydrilla funnel experiment proves that oxygen is released during photosynthesis.
* Mohs 1/2 leaf experiment proved that CO2 is essential for photosynthesis.
* Ganong's light screen experiment proves that light is essential for photosynthesis.
* T.W. Engelmann worked on Cladophora (a green alga) and discovered that Red and Blue lights are useful in photosynthesis.
* Robert Hill (1937) an English biochemist worked on isolated chloroplasts. He proved that they release O2 in the presence of light and hydrogen acceptor (Hills Oxidant). He used potassium ferric oxalate as hydrogen acceptor.
* Cornelius Van Niel (1931) worked purple and green sulphur bacteria. He proved that they release sulphur instead of O2 in light reaction. He also proposed that O2 released by the green plants comes from water.

* Samuel Ruben and Martin Kamen (1941) used 18O and proved that O2 released during light reaction comes only from water.
* Robert Emersion (1950) worked on Chlorella (a unicellular, eukaryotic, green alga) and discovered 2 pigment or photosystems (PS I & PS II) based on his famous Emerson's enhancement effect.
* Robert Emersion while working on Chlorella also discovered Red drop. It says that when plants are continuously exposed to red light with longer than 680 nm wave length the rate of photosynthesis decreases.
* Dutrochet was the first to recognise the importance of Chlorophyll in photosynthesis.
* Blackman (1905) was the first to report that there are 2 Phases in the mechanism of photosynthesis.
1) Light reaction 2) Dark reaction.

 

Site of Photosynthesis 
* Photosynthesis occurs in Chloroplast.
* It is a double membrane bounded, self duplicating, semiautonomous organelle and next to nucleus in size. It is seen in eukaryotic, green, plant cells only.
* The envelope encloses periplastidial space.

* It has 2 important parts
   1) Grana    2) Stroma
* Grana are many in number. Each granum is a pack of grana thylakoids. They consists pigments, grouped as PS I and PS II. It is the site of light reaction.
* Stroma is colourless, jelly like and rich in enzymes. It is the site of dark reaction. Thus there is division of labour. Stroma also consist stroma thylakoids (stroma lamellae or Frets)
* They have PS I only and do not consist NADP reductase Chloroplast also consists circular, ds DNA and 70 S of ribosomes.


                                                      

In Photosynthesis CO2 is reduced and water is Oxidised

Pigments 
       The natural substances that have an ability to absorb light at specific wavelengths are called as pigments. The important pigments include
       1) Chlorophylls
       2) Carotenoids and
       3) Phycobilins
The technique that is used to separate the various kinds of pigments is called chromatography.


1) Chlorophylls
These are 5 types
1) Chl - a: It is the only Chl. that converts radiant energy to Chemical energy. It has 2 mutant forms (P700, P680) which act as reaction centres in Photosystems. It is universal & highly essential. Its formula is C55 H72 O5 N4 Mg. It is bright or blue green.
2) Chl - b: It is accessory pigment. It is present in higher plants and chlorophyceae only. Its formula is C55 H70 O6 N4 Mg. It is yellow green.

3) Chl - c: It is accessory pigment present in lower plants. It is seen in Bacillariophyceae (Diatoms) and Phaeo-Phyceae (Brown algae) only.
4) Chl - d: It is accessory pigment, present in Rhodophyceae (Red Algae) only.
5) Chl - e: It is accessory pigment present in Xanthophyceae (Yellow algae) only.
Carotenoids: 
       These are accessory pigments belonging to terpenoids. Structurally these are open chain compounds and lipid soluble.
These are two types
1) Carotenes

       These are accessory pigments, orange red in colour. Their chemical formula is C40H56. Their names end with -ene. Lycopene (Tomato), β − Carotene (Carrot, Sweet Potato), α − Carotene
2) Xanthophylls:
       These are yellow in colour. Their Chemical formula is C40H56 O2. Their names end with -in.
e.g.: Lutein, Zeaxanthin, Fucoxanthin.
Phycobilins 
      These are accessory, algal pigments soluble in water. These are 2 types.

1) Phycoerythrin: (Red)
    It is present in Rhodophyceae.
2) Phycocyanin: (Blue)
    It is present in Rhodophyceae and Cyanophyceae
The function of Accessory Pigments 
1. They absorb sunlight and convey it to Chl-a by resonance transfer or Inductive Resonance.
2. They protect Chl-a from photooxidation or solarisation or photobleaching when light intensity is more.
Structure of Chlorophyll 
       Plants Synthesise Chlorophyll in the presence of light and Fe (Iron). Plants grown in dark do not develop Chlorophyll in the leaves. They are Pale or colourless. It is called Etiolation. Structure of Chlorophyll molecule resembles tennis racket. It has 2 parts.
      1) Porphyrin head 2) Phytol tail.
      The head has circularly arranged 4 Pyrole rings each with a nitrogen. Mg is present in the centre to which 4 nitrogens are attached.

      The tail has a chain of alcohols with 20 carbons. It is attached to 4th Pyrole ring by ester bond. Chl-a has CH3 in the 3rd Carbon of 2nd Parole ring while Chl-b has CHO in the same place.


                                   
Photosystems 
These are 2 (PS I and PS II).

    Both are present in grana thylakoids while stroma thylakoids have PS I Only.
    A Photosystem has 2 parts.
1) Antenna: The light harvesting complex consisting, several pigment molecules bound to protein & act as Antenna. They absorb light and convey it to Reaction Centre.
2) Reaction Centre: PS I has P700 and PS II has P680 as reaction centres. These are special Chl - a molecules.
Light: 
     The Source of energy in this universe is Sunlight. Visible light (390 nm - 760 nm) is useful in Photosynthesis. It is called PAR (Photosynthetically Active Radiation)
    The spectrum of colours shown by it is VIBGYOR. Among them Blue and Red light are useful. Red light is more useful. The energy is termed as Radiant energy. The particulate unit is called Photon and energy in it is termed Quantum.
The energy in a photon is inversely proportional to its wave length.
                 So blue light has more energy than red light.

MECHANISM OF PHOTOSYNTHESIS 
Blackman divided mechanism of photosynthesis into 2 parts.
    1) Light reaction 2) Dark reaction
Light reaction: It occurs in grana thylakoids during day time. The factors required for it are
    1) PS I & PS II 2) H2O
    Radiant energy is converted to chemical energy. ATP and NADPH2 produced in it are together called Assimilatory Power.
    There are 2 kinds of light reactions.
(1) Non - Cyclic Photophosphorylation
(2) Cyclic Photophosphorylation
NON - CYCLIC PHOTOPHOSPHORYLATION: 
     It occurs in grana thylakoids.
     PS II and PS I participate in it. It occurs as follows.
* LHC II absorbs sunlight of 680 nm wavelength of red light and conveys it to the reaction centre P680 by resonance transfer.

* P680 becomes excited and looses a pair of energetic electrons and thus becomes oxidised.
* The initial acceptor of these electrons is Pheophytin (Chlorophyll without magnesium). The movement of electrons from Pheophytin to P700 is downhill (in terms of redox potential scale).
* Plasto Quinone accepts the electrons from Pheophytin and convey them to Plastocyanin through Cyt b6 - Cytf complex.
* Quinone Cycle occurs and for every electron 2H+ are shifted from stroma to lumen.
* Photolysis of water occurs in lumen and OEC (Oxygen Evolving Complex) transfers the electrons from water to P680. For every H2O two protons are added to lumen. Oxygen formed in it is released into atmosphere.
* LHC I absorbs sunlight of 700 nm wavelength of red light and conveys it to P700 of PS I. P700 becomes excited & oxidised by loosing a pair of energetic electrons.
* The electrons from Plastocyanin are accepted by P700. The electrons of P700 are accepted by Ferredoxin (Fe - S Protein) which conveys them to NADP through NADP reductase.
* Thus the transport of electrons is not cyclic. i.e., the electrons belonging to water finally reach NADP.

* Proton concentration gradient is established along the thylakoid membrane. According to Peter Mitchells' Chemi Osmotic model, the protons pass through F0 - F1 from lumen to stroma.
* For every 3 protons, one ATP is formed and it is called Photophosphorylation. For one H2O,
2 ATP and 1 NADPH2 are formed.


                    

CYCLIC PHOTOPHOSPHORYLATION 

        PS I alone participates in it. It occurs stroma thylakoids. Photolysis of water do not occurs. It occurs as follows.
* LHC I absorbs sunlight of 700 nm wavelength of red light and conveys it to P700 of PS I. P700 becomes excited & oxidised by loosing a pair of energetic electrons.
* The initial acceptor of electrons in it is Ferredoxin (Fe - S protein) from where they go to Plasto Quinone. Quinone Cycle occurs. For every electron, 2H+ are added to lumen.
* The electrons from PQ reach Plasto Cyanin through Cyt b6 −Cytf Complex. Plasto Cyanin conveys these electrons back to P700. Thus the transport of these electrons is Cyclic.
* Proton concentration gradient is established along the thylakoid membrane. According to Peter Mitchells' Chemi Osmotic model, the protons pass through F0 - F1 from lumen to stroma.
       For every 3 protons, one ATP is formed and it is called Photophosphorylation. For one H2O, 2 ATP and 1 NADPH2 are formed.
       Thus 1 ATP is produced for every pair of electrons released from P700.
       NADPH2 is not formed because
(1) Photolysis of water do not occurs.
(2) PS II and NADP reductase are absent.

                 

      Proton concentration gradient is established along the grana thylakoids (Non- Cyclic) and stroma thylakoids (Cyclic).
      Thylakoids are impermeable to protons. Proton concentration is more in lumen and less in stroma.
     Peter Mitchell (Nobel laureate) proposed chemiosmotic hypothesis (model).
     Chemiosmosis requires..
1) Membrane (grana thylakoid or stroma thylakoid).
2) Proton pump (Plastoquinone acts as proton pump).
    Radiant energy is used to pump electrons from stroma to lumen.
3) Proton concentration gradient. It is established due to the Quino Cycle.
4) ATP ase. It is F ° − F1. F ° is towards lumen and F1 protrodes into stroma. F° acts as proton channel. F1 shows confirmational changes by which ATP is formed. It is called Photophosphorylation.
Non-Cyclic Photophosphorylation: 
     Photolysis of every water molecule, 6H+ are accumulated in lumen. When they pass through
F ° and release energy in rotating F1, 2 ATP and one NADPH 2 are produced.


Cyclic Photophosphorylation: 

     For every pair of electrons a total of 4H+ are accumulated in lumen. They produce 1 ATP through ATP ase as follows.
DARK REACTION

     It occurs in the stroma of the chloroplast. Their process depends on the end products of light reaction. Hence it occurs during day time only. It is 3 types.
1) C3 Pathway
2) C4 Pathway and
3) CAM Pathway
C3 Pathway 
       Melvin Calvin, Andrew Benson and James Bassham worked on Chlorella and discovered the sequence of intermediate compounds. They found that PGA (Phospho Glyceric Acid) is the first stable, 3 − Carbon Compound formed in dark reaction. So it is called C3 pathway, or Calvin Cycle or PCR Cycle (Photosynthetic Carbon Reduction Cycle), Reductive Pentose Phosphate Pathway (RPP Cycle).

They used C14, Paper Chromatography, Autoradiography, and hot Methanol. It occurs in all kinds of plants.
It is divided into 3 parts.
    1) Carboxylation
    2) Reduction and
    3) Regeneration of RuBP
1) Carboxylation: It can also be called CO2 fixation. The initial acceptor of CO2 is RuBP (a 5 Carbon Compound). So CO2 is fixed to RuBP in the presence RuBP Case (or RuBISCO).

2) Reduction: It occurs in 2 steps.

Thus the end products of light reaction (assimilatory power) are utilised in Reduction.
One CO2 requires 2 ATP and 2
NADPH for reduction.


Out of 12 molecules of G−3−P, only 2 are (1/6) utilised in the glucose synthesis. The rest 5/6 G−3−P are utilised in the regeneration of RuBP. Glucose synthesis occurs in Cytosol.
3) REGENERATION Of RuBP
The initial acceptor of CO2 i.e. RuBP must be produced again to accept CO2 and the Cycle to continue.
The regeneration of RuBP requires one ATP.


   It occurs in 9 steps as follows..

Every CO2 participating in dark reaction requires 3 ATP + 2 NADPH


        6 Calvin cycles are required to produce one molecule of Glucose. 18 ATP and 12 NADPH are required to produce one glucose. 36 ATP and 24 NADPH are required to produce sucrose. Ratio between ATP and NADPH required to produce Glucose is 3 : 2. Ratio between G − 3 − P utilised to produce Glucose and to regenerate RuBP is 1 : 5.


                

KRANZ ANATOMY

        The leaves of grasses (Sugarcane, Maize etc) are isobilateral and show different anatomy. Chloroplasts are present in Mesophyll Cells and Bundle sheath cells. Chloroplasts show dimorphism. Mesophyll chloroplasts have PEP case and bundle sheath chloroplasts are agranel and have RuBP case. Bundle sheath cells are larger and arranged in the form of Wreath (German Wreath = KRANZ). So it is called Kranz anatomy. Prof. V. Ramdas worked on C4 pathway and proposed that dicotyledons like Euphorbiaceae, Amaranthacea, Portulaceae Chenopodiaceae etc, also shown Kranz anatomy.

C4 PATHWAY 
         H.P. Kortschak, C.E. Hartt and G.O. Burr worked on sugarcane and used C14. They found that the first formed stable compound in dark reaction is a 4 - Carbon Compound (Malic acid and Aspartic acid). These results were confirmed by M.D. Hatch and C.R. Slack of Australia. They also worked on sugarcane and used C14. They called it as C4 pathway. It is also called as C4 Cycle, Hatch - slack pathway, Dicarboxylic acid pathway or β-Carboxylation pathway. The C4 plants show division of labour- 2 kinds of cells (Mesophyll cells and Bundle sheath cells) participate in it. It occurs as follows.
I. Mesophyll Cells:
         Atmospheric CO2 unites with Phospho Enol Pyruvate in the presence of water & PEP case. It is called β-carboxylation. As a result Oxalo Acetic Acid is formed in Cytosol.


       
        It enters into Mesophyll Chloroplasts & undergoes either reduction to form Malic acid or transamination to produce Aspartic Acid. Thus C4 plants are two types Malate Plants or Asparpate Plants.

      Oxalo Acetic Acid + NADPH2  Malic Acid
      It goes to Bundle sheath Cells.
II.  Bundle Sheath Cells:
     Malic acid shows Oxidative decarboxylation.
     Malic Acid  CO2 + NADPH2 + Pyruvic acid
     CO2 participates in Calvin Cycle. Glucose is produced in Cytosol.
Regeneration of RuBP and PEP
    RuBP is regenerated in the bundle sheath Cells as in Calvin Cycle.
    Pyruvic Acid enters into Mesophyll Cells where it consumes 2 ATP to regenerate PEP.

C4 Pathway can be shown as follows.
C4 plants show C4 pathway in Mesophyll cells and C3 pathway in Bundle sheath cells. Thus C4 pathway and C3 pathway are separated by space.

              
CAM Pathway 
        Plants growing in arid climates show thick leaves. Generally they are called Succulents. They show a different kinds of mesophyll cells. The cells have unusually larger Vacuole and thin layer of Cytoplasm. They show a different kind of dark reaction which was first found in the members of crassulaceae. So it is called crassulacea acid metabolism and plants are called CAM Plants.

e.g.: Crassulaceae, Orchidaceae, Bromeliaceae. The cells have PEP case and RuBP case. A part of the dark reaction occurs during Night time. Malic acid is produced and stored in Vacuole. Thus CO2 is fixed in the night. In the day time Malic acid undergoes decarboxylation. CO2 participates in Calvin Cycle to produce Glucose. Thus C4 pathway (night) and C3 Pathway (day) are separated by time.
Differences between C4 Plants & CAM Plants


   

PHOTORESPIRATION

       Higher Oxygen concentration and lower CO2 concentration reduces the rate of photosynthesis. It is called Warburg effect. It is due to photorespiration. It occurs as follows.
         RuBP case has 2 reaction sites. One for CO2 and the other for O2. When O2 concentration is more it competes with CO2 and occupies some reaction sites on
RuBP case. Then it also participates in Oxidation. So RuBP case is also called RuBISCO.
    Three Organelles participates in Photorespiration.
    1) Chloroplast 2) Peroxisome 3) Mitochondria
   It is also called C2 Cycle or glycolate pathway or photosynthetic carbon Oxidation cycle. It occurs in C3 Plants. The ratio between CO2 fixation to photorespiration is 4 : 1.
    It causes considerable loss to the CO2 fixation.
    It is a wasteful process. No energy is produced more over ATP is used. But it protects C3 plants from photooxidative damage.
It occurs as follows

Posted Date : 26-07-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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