LIGHT-DEPENDENT REACTIONS

It is the first phase of photosynthesis and it occurs in grana (thylakoids or lamellae) of chloroplast.

It involves:

  1. Absorption of light energy by photosynthetic pigments.
  2. The conversion of light energy into chemical energy in the form of ATP i.e. photophosphorylation.
  3. Photolysis of water leading to formation of NADPH2 and liberation of O2.

Photophosphorylation

The process in which energy rich ATP molecules are synthesized using solar energy is called photophosphorylation.

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At the reaction centre, energy rich electrons are expelled on receiving photons and there is an electron transfer.

Dr. Arnon first suggested that there are two pathways for this electron transfer, one is non-cyclic and the other is cyclic.

Synthesis of ATP takes place during both the processes therefore; there are two types of photophosphorylation.

  1. Non-cyclic Photophosphorylation
  2. Cyclic Photophosphorylation

Non-cyclic photophosphorylation: (Z- Scheme)

This involves photolysis of water and a continuous unidirectional flow of electrons

  • from water to PS-II,
  • from PS-II to PS-I via cytochromes and
  • finally to NADP.

Electrons expelled from chlorophyll-a never come back to the same chlorophyll-a.

Photo-excitation of PS-II and flow of electrons from PS-II to PS-I

The pigments in PS-II absorb energy of different wavelengths of light which is funneled to the reaction centre, i.e. chl-a 680 or P680.

Energy rich electrons are expelled from P680.

These electrons are taken up by plastoquinone (PQ) through co-enzyme quinone (CO-Q) leaving the reaction centre ionized or positively charged and PQ gets reduced.

The energy-rich electrons from PQ roll down the chain of various electron carriers.

Components of electron transport chain or system (ETC or ETS) are cytochrome b6, cytochrome f and plastocyanin (PC).

The reduced plastocyanin transfers the de-energized electrons to the reaction centre of PS-I, i.e. P700. (Actually, PS-II replaces the lost electrons in PS-I)

During the transfer of electrons from cyt-b6 to cyt-f, the energy released is used in the synthesis of ATP.

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Photo-excitation of PS-I:

  1. The pigments in PS-I absorb the energy of different wavelengths of light and that is funneled to the reaction centre, i.e. chl-a 700 or P700.
  2. The energy rich electrons are expelled from excited chl-a molecule and
  3. The electrons are taken up by iron containing red protein called ferredoxin (Fd) through an unknown, electron acceptor which is called ferredoxin-reducing substance (FRS).
  4. From reduced ferredoxin, these electrons are taken up by co-enzyme NADP (Nicotinamide Adenine Dinucleotide Phosphate).
  5. NADP retains the electrons and becomes negatively charged.
  6. Due to loss of electrons P700 becomes positively charged or ionized.
  7. PS-ll replaces the electrons in PS-I as described above.

Photolysis of water

  1. P680+ (which is left ionized after donating electrons to PS-I) acts as strong oxidizing agent.
  2. It brings about splitting of water molecules to release oxygen (transfer of 4 charges initiates photolysis).
  3. This light- dependent splitting of water molecule is called photolysis of water.
  4. Manganese, calcium and chloride ions play an important role in photolysis of water.
  5. The four electrons generated in break-down of four water molecules are accepted by oxidized P680.
  6. The protons (4H+) are accepted by 2NADP to form 2NADPH2. An enzyme ferredoxin NADP reductase is required for this.

 

Significance of Non-cyclic Photophosphorylation

Non cyclic photophosphorylation is a significant process

  • photolysis of water is associated with it
  • It involves formation of ATP and NADPH2.
  • Dr. Arnon called it Assimilatory power as it is required for CO2 assimilation.
  • ATP acts as energy donor and NADPH2, acts as reducing power for reduction of CO2 to glucose during dark reaction.
  • It also involves evolution of oxygen, which is indispensable for life.
  • It occurs under aerobic conditions, high CO2 concentration and enough light intensity.
  • It is present in all O2 evolving photo-autotrophs and is absent in bacteria.

 

Cyclic Photophosphorylation

This involves only PS-1 and the flow of electrons is in cyclic manner as the same expelled electrons come back to the reaction centre. Photolysis of water is not associated with this.

Photo excitation of PS-I and flow of electrons

  • The pigments in PS-I absorb energy of different wavelengths of light which is funneled to the reaction centre, specific molecule of chlorophyll-a. i.e. P700.
  • It gets excited and expels energy-rich electrons.
  • The electrons are accepted by an iron containing red protein called ferredoxin (Fd) via an unknown electron acceptor called ferredoxin-reducing substance (FRS).
  • P700 , becomes positively charged or left ionized.
  • These electrons then roll down the energy gradient through a series of electron carriers such as cytochrome b6, cytochrome-f, and plastocyanin (PC).
  • From plastocyanin the electrons arc transferred back to P700.
  • Thus, the flow of electron is n closed circuit form.

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Diagrammatic representation of cyclic photophosphorylation. 

  • During the transfer of electrons from cytochrome b6 to cytochrome-f, energy from electrons is released, and is used in the synthesis of ATP.
  • ATP formation may also take place when the electrons are transferred from ferredoxin to cytochrome-b6.

 

Significance of Cyclic Photophosphorylation

  • Cyclic photophosphorylation is a significant process as it produces additional ATP molecules required for synthesis of glucose.
  • Synthesis of one glucose molecule requires 18 ATP and 12 NADPH2 molecules.
  • When CO2 fixation is stopped, or CO2 concentration is low and NADP is not available, electrons from Fd comes back to PS-I.