Photosynthesis in Higher plants// Importance of photosynthesis//Light reaction///photosystem I and II//Photophosphorylation//photolysis///

 

Photosynthesis in Higher plants

The sun is the ultimate source of energy on the earth. Green plants show autotrophic nature due to this, plants are able to utilize sunlight for the photosynthesis.

What is the photosynthesis?

Photosynthesis is defined as a process by which green plants synthesize their own food by using water (H₂O) and carbon dioxide (CO₂) in presence of Chlorophyll and sunlight, oxygen(O₂) is given out as a byproduct.

Photosynthesis-I also know as light dependent reaction because reaction occurs in the presence of sunlight.

 

                                                          

                                                                            

Note:

Photosynthesis is a Anabolic process and it occurs in many kinds of Bacteria, Algae, in leaves and young stems of plants.

Only chloroplasts are able to carry out photosynthesis because having a green pigments called Chlorophylls.  Water and Carbon dioxide are the two raw materials required for photosynthesis.

Mechanism of photosynthesis:-

Photosynthesis is an oxidation–reduction process in which water is oxidized to form oxygen and carbon dioxide is reduced to form sugar.

Mechanism of photosynthesis takes place by two phases.

1.     Photochemical Reaction or Light Dependent Reaction

2.     Chemical Dark  Reaction or Light Independent Reaction

Light dependent reaction:-

Light dependent reaction is also known as Light Reaction  or Photochemical Reaction or Photochemical oxidation of water. This process occurs in chloroplast’s Grana. 

Light reaction include light absorption, water splitting, oxygen release and formation of high-energy chemical intermediates, ATP(Adenosine triphosphate) and NADPH (Nicotinamide adenine dinucleotide phosphate).

There are two photo acts or photosystems in light reaction which is called a Photosystem I and Photosystem II. 

Mechanism of Photosystem I and II:-

Phtosystem II (PS II) is a reaction center because it absorb 680nm wavelength of red light and causing electrons to become excited and picked up by an electorn acceptor which passes them to an electron transport system .

These electrons are available due to splitting of water. The electrons which is expelled from PS II accepted  by the Pheophytin where these electrons are accepted by the plastoquinone (PQ).

 Cyt-b₆-f complex become reduced by accepting electrons from reduced plastoquinone. In this conversion energy is released which is stored in form of ATP.  From Cyt-b₆-f complex electrons are accepted by Plastocyanin (PC) and then electrons are accepted by PS I(P700) from  Plastocyanin. These electrons moves in downhill manner.

Photosystem I (PS I):-

Electrons in the  PS I are also excited when they receive far red light of wavelength 700nm and are transferred to the A(Fe-S)  due to which  A (Fe-S) comes in reduced state.

These electrons are accepted by ferredoxin (Fd) from reduced A (Fe-S) and then electrons accepted by FAD from ferredoxin.

FAD become reduced and formed FADH₂ by taking Hydrogen molecule which comes from splitting of water.

Hydrogen molecules are accepted by NADP⁺ from reduced FAD  and comes in reduced state NADPH+H⁺. This whole scheme of transfer of electrons, from PSII to PSI  is called the Z Scheme, due to its characterstic shape.

                                     Z scheme of light reaction

 

Importance of Photosynthesis:- It is the ultimate source of food on the earth and the release of oxygen in our environment.

                                     

PHOTOPHOSPHORYLATION

It is called light reaction because it is light dependent process through which ATP (Adenosine Triphosophate) is synthesized from ADP (Adenosine Diphosphate) and inorganic phosphate.

The process of photophosphorylation  is of two types-

1.     1. Noncyclic Photophosphorylation

2.    2. Cyclic Photophosphorylation

 

1.     Noncyclic Photophosphorylation

 

Ø  In this type of photophosphorylation  both PS I and PS II are involved.

Ø  During this cycle, the electron released from PSII does not return back to it that is why it is known as Non-cyclic photophosphorylation.

Ø  Light energy is trapped by the reaction centre P₆₈₀ (PSII), due to which electrons gets excited and accepted by Pheophytin (Pheo) and then it moves downhill manner through an Electron Transport System comprising of  Platoquinone (Pq), Cytochrome- b₆-f complex and Platocyanin (Pc).

Ø  The electrons are now accepted by reaction centre P₇₀₀ (PS I) and light energy is also trapped by PSI and exicted electrons are accepted by A (Fe-S)  then ferredoxin (fd) and finally accepted by NADP⁺.

Now, ATP and NADPH₂  obtained during non-cyclic photophosphorylation are used up during dark reaction.

Note:  Mn⁺⁺ ion play a very crucial role in photolysis of water.                                  

  

                                     Noncyclic Photophosphorylation                             

                    

 2.     Cyclic Photophosphorylation 

• In this type of photophosphorylation only PSI is involved.
• Ø  During this cycle the excited electrons come back to the photosystem so it is known as Cyclic Photophosphorylation.
• Ø  Electrons  excited from PSI  accepted by A(FeS), ferredoxin (fd) and finally accepted by NADP⁺  through  one pathway and by another cyclic pathway the electrons from the A(FeS) go through ferredoxin (fd),  Cytochrome- b_6- f complex and Platocyanin (Pc) and return to P₇₀₀ again.
•  In this pathway only ATP is generated.

 

                            Cyclic Photophosphorylation