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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Photosynthesis Page 1

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Introduction

 

  1. Photosynthesis: A biochemical, anabolic process carried out by using carbon dioxide and water in the presence of chlorophyll and sunlight is called photosynthesis.
  2. The site for photosynthesis in plant cell is 'chloroplast'.

 

 

 

 

Chloroplast Ultrastructure and Function

 

 

  1. Under electron microscope, the chloroplast structure is seen clearly.
  2. Each chloroplast is a double-membraned structure, containing a ground substance inside it, called 'stroma'.
  3. Embedded in the stroma are small membranous disc-like structures called 'thylakoids' which are arranged one above the other to form a pile-like structure called 'granum'. Many such grana are present in the chloroplast.
  4. The grana are connected to each other by structures called 'intergranal lamellae'.
  5. On the surface of membranes of thylakoids, there are many granular structures called 'quantasomes', which are rich in chlorophyll.
  6. There are two types of pigments in the process of photosynthesis. They are 'essential' pigments and 'accessory' pigments. Essential pigments are chlorophyll-a and
    chlorophyll-b while accessory pigments are carotenes and xanthophylls.

 

 

 

Overall Equation of Photosynthesis

 

  1. The process of photosynthesis actually involves two main reactions, namely 'light' reaction and 'dark' reaction.
  2. Light reaction takes place in the presence of light while dark reaction does not require light.
  3.  

6CO2 + 12H2O

sunlight

chlorophyll

C6H12O6 + 6H2O + 6O2

 

 

 

Primary Process of Photosynthesis (Photochemical Phase)

 

  1. Light contains many particles called 'photons' which contain some energy called 'quantum'.
  2. Visible sunlight is a spectrum of various electromagnetic waves which range from violet to red.
  3. There are two photosystems which absorb specific photons having specific wavelengths. Example: Photosystem-I absorbs photon of 700 nm and is called P-700 while photosystem-II absorbs photons of 680 nm and is called P-680.
  4. The photochemical process, i.e. light reaction involves two main aspects as:
    (a) Photolysis of water and (b) Synthesis of ATP, i.e. Photophosphorylation.
  5. Photolysis of Water: Evolution of oxygen during photosynthesis was a puzzle. Robert Hill, in his experiment, showed that oxygen evolved during photosynthesis is due to splitting up of water molecule by light and not the CO2. Hill reaction is represented as:

2H2O + 2A

sunlight

chlorophyll

2AH2 + O2 where A is a hydrogen acceptor.

 

 

 

 

Photophosphorylation

 

 

  1. Photophosphorylation: Synthesis of high energy phosphate compound, i.e. ATP in the presence of light is called photophosphorylation.
  2. Cyclic Photophosphorylation: This process involves only photosystem-I. Here the photons hand over their energy to chlorophyll from PSI. From this chlorophyll, a pair of high energy electrons is released, which is accepted by several electron acceptors in a sequence and again comes back to chlorophyll. Since the electron returns back to chlorophyll, it is called cyclic photophosphorylation.
  3. The electron acceptors are: Ferrodoxin (Fd) Plastoquinone [PQ]
                                                                                               

                                   Plastocyanine [PC]
    Cytochrome - f Cytochrome b6 
    ATP is synthesised when electron is transferred from Cyto-b6 to Cyto.f.
  4. Non-cyclic Photophosphorylation: This process involves both the photosystems. In this, the electrons released from one photosystem are handed over to the other, but they do not return back to first photosystem. Thus it is called non-cyclic photophosphorylation.   
  5. During the process, chlorophyll from PS-II absorbs photons and releases high energy electrons. This loss of electrons is neutralised by accepting electrons released during photolysis of water.
  6. The electrons lost by PS-II are accepted by various electron acceptors as
    Plastoquinone
    Cyto. b6 Cyto.f Plastocyanine.                                              
  7. From plastocyanine, the electrons are handed over to PSI. From PSI, they are taken by primary acceptors, then to ferrodoxin. Ferrodoxin is oxidised by NADP and the end product is NADPH2. Thus non-cyclic phosphorylation has end products as O2, ATP and NADPH2.                                        

 

 

 

Secondary Process of Photosynthesis (Biochemical Phase)

 

  1. The second phase of photosynthesis process is the biochemical process, i.e. dark phase, which takes place in the absence of light.
  2. The process takes place in the stroma of chloroplast and since actual glucose formation takes place during this process, it is also called synthetic phase.
  3. Melvin Calvin was the scientist who described the dark phase. He divided the entire biochemical phase into three steps as carboxylation, reduction and regeneration.
  4. During carboxylation, fixation of carbon dioxide takes place. Six molecules of carbon dioxide react with six molecules of RuBP (Ribulose bi phosphate). This results in the formation of twelve molecules of phosphoglyceric acid (PGA).
  5. In the second phase, PGA is reduced step by step into PGAL.
  6. Third step is regeneration during which, as the name suggests, ribulose biphosphate (RuBP) is produced again. For this process, out of twelve molecules of PGAL, only ten molecules are used while two molecules are used to form glucose.
  7. Calvin cycle is also called C3 pathway because the first stable compound formed here, i.e. PGA, is a 3 carbon compound.

 

 

 

Diversity in Photosynthetic Pathway

 

  1. C4 pathway: In some plants like maize, sugarcane, amaranthus, jowar, etc., the dark phase is different from other normal plants.
  2. In these plants, instead of RuBP as a substrate, another substrate called phosphoenolpyruvic acid (PEP) is present for CO2 fixation.
  3. The first stable product of CO2 fixation is a 4 carbon compound called oxaloacetic acid (OAA).
  4. OAA gets converted to malic acid. Decarboxylation of malic acid then results into pyruvic acid and carbon dioxide.
  5. Pyruvic acid then re-enters the mesophyll chloroplasts and forms PEP.
  6. C4 pathway is also called HSK pathway in the honour of Hatch, Slack and Kortshak.
  7. CAM pathway: CAM, i.e. Crassulacean Acid Metabolism, is seen in members of family Crassulaceae, Cactaceae, Bromeliaceae and Orchidaceae.
  8. These plants are succulent plants and so during daytime the stomata are closed. Thus these plants do not get CO2 for photosynthesis from atmosphere. So they use malic acid as a source of CO2. This malic acid is produced during night.
  9. Since the stomata open during night, CO2 is absorbed and fixed.

 

 

 

Significance of Photosynthesis

 

  1. Life on earth can sustain only because of this process, because green plants prepare food for them and for others during photosynthesis.
  2. Oxygen is given out during this process which helps in replenishment of atmospheric oxygen. Some oxygen is also converted to ozone.
  3. This is a process during which solar energy is converted into chemical energy.
  4. Plants convert solar energy into organic molecules. These molecules are used as 'fuel' in the body of heterotrophs. After their death, the same is converted into fossil fuels.
  5. O2 - CO2 balance in the atmosphere is maintained.

 

 

 

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