Light Reaction – Photosynthesis in Plants (PART II)

By | September 19, 2021
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Photosynthesis can be divided into two parts : Light dependent reactions or Light Reactions and Dark Reactions or Calvin Cycle.

Light reactions forms the “Photo” part of Photosynthesis and Dark reactions / Calvin Cycle is a separate process which accounts for “synthesis” aspect in Photosynthesis, where sugars are produced.

Light reaction involves the following events – absorption of light , splitting of water to release oxygen, electron transfer chain and synthesis of high energy molecules such as NADPH and ATP.

Light reactions begin when a photon hits a chlorophyll molecule present in the thylakoid membrane and the energy absorbed will excite an electron from ground state to excited state.

This starts a series of events which are necessary for converting light energy into chemical energy.

Photosystems – Reaction Centers :

  • Chlorophyll’s are not alone in this process of light absorption.
  • The pigments and several protein complexes form a Light harvesting Complex (LHC).
  • LHC are organized into two photosystems – Photosystem I (PSI) and Photosystem II (PS II).
  • These Photosystems are named in the sequence of their discovery and not in the order they function in light reactions.
  • LHC consists of many pigments molecules associated with proteins. However, only one member of the group, termed as the reaction-center chlorophyll a (in fact a dimer of chlorophyll a molecules) has the ability to actually transfer the electrons to an electron acceptor.
  • The remaining pigments present in LHC ( other than reaction center Chlorophyll a) are called as antennae, which help in making photosynthesis more efficient by trapping lights of different wavelengths and passing the same to reaction center.
  • The reaction center Chlorophyll a molecules in PS I has an absorption maxima at 700nm – Hence called P700 and P stands for Pigment.
  • In PS II – Reaction center chlorophyll a molecule absorption maxima at 680nm – Hence called P680.
The Light Harvesting Complex

The Electron Transport Chain (ETC) :

  • The reaction center Chlorophyll a molecule in PS II or P680 undergoes oxidation by losing electron upon excitation. This is called as “Photoact“.
  • These excited electrons (which are pushed from atomic nucleus to farther orbit) are picked by a primary electron acceptor ( Chlorophyll-like pheophytin molecule), which then pass them in sequential order to a Electron transport system consisting of different multi protein complexes. The electrons move downhill from Plastoquinone PQ A – PQ B – cytochrome b6 f – plastocyanin and then to P700+ .
  • PSII and PSI forms two major components of the photosynthetic electron transport chain along with cytochromes complex. These cytochromes are a group of reversibly oxidizable and reducible proteins that forms a core of electron transport chain between PSII and PSI.
  • Similar to PSII , the electrons in PS I or P700 are also excited by photon at the same time as in PS II, to form p700+ .
  • The excited electrons of PS I also moves down hill form primary acceptor, A0 to many iron-sulfur centers then to
    ferredoxin.
  • Finally electrons are transferred from ferredoxin to energy rich NADP+ which results in reducing NADP+ to NADPH. This is where the journey of that particular electron ends.
  • This whole journey of electrons in oxygenic photosynthesis where two photosystems act in concert, beginning from PSII, then uphill to primary acceptor, later- down the series of components of electron transport chain to PS I, further excitation of electrons from PS I to another acceptor uphill and finally downhill to form NADPH., is called Z scheme due to its characteristic shape of electron flow.
  • Z Scheme was first proposed by Robert Hill and Fay Bendall.

Electron Transport chain – Showing NADPH and ATP synthesis

Splitting of water or Photolysis:

  • In the above Z scheme or linear electron transport, PS II will be left oxidized( P680+) as it happens to the first member in the electron transport chain. All the members in ETC will keep on receiving the lost electrons from preceding member in the chain to get back to ground state.
  • The last member in the ETC, i.e., PS I will receive its lost electrons from plantacyanin (which eventually traces the electrons to PS II in the beginning of the chain).
  • The electrons lost from PS II must be replaced in order to excite electron again to start light reaction one more time.
  • PS II being the first member in the ETC, it requires electrons from a external source to replenish its lost electrons and reduce its oxidized chlorophyll a reaction center (P680+ to P680) by gaining electrons.
  • These electrons come from splitting of water molecules in plants, algae and cyanobacteria (blue-green algae).
  • PS II oxidizes two molecules of water in presence of light to yield a molecule of Oxygen, four Hydrogen ions (H+) and four electrons.
  • These electrons will be transferred to PSII, which will eventually resets the photosystem II to receive photons again and initiate light reaction by exciting electrons.
  • PSII is only known biological enzyme which can catalyze oxidation of water molecule in nature.
  • Photosynthetic water oxidation is carried out by Mn4Ca complex present in oxygen-evolving complex (OEC) of PS II . The OEC acts as a cofactor of the PS II enzyme, in oxidation of water.
  • The splitting of water in presence of light to give rise to protons, electrons and oxygen is termed as Photolysis of water.
  • This complex is composed of four manganese atoms (Mn) and one calcium atom (Ca), which are held together through a network of oxygen bridge.
  • The oxidation of water involves a complicated cycle at oxygen-evolving complex which releases electrons, hydrogen ions, and molecular oxygen.
  • Oxygen formed as a by product is used by all aerobic organism for cellular respiration, including photosynthetic organism such as plants.
Splitting of water at OEC and electron transport chain

What is Hill Reaction:

The Hill reaction is defined as the photoreduction of an electron acceptor by the hydrogens of water molecule, with the release of oxygen.

In 1937, Robert Hill demonstrated that the way plants produce oxygen is separate from the process that converts carbon dioxide into sugars.

Hill showed that isolated chloroplasts can evolve oxygen in the absence of CO2. This was first indication that source of electrons in light reaction comes from water rather than CO2.

In his experiments he used an artificial electron acceptor that changes color as it is reduced.

DCIP (2,6-dichlorophenolindophenol) is a dye which is blue in its oxidized form and colorless in its reduced form.

ATP Synthesis :

  • Due to the splitting of water ( electrons, protons and O2), which takes place inside the thylakoid lumen, leads to building up of protons or hydrogen ions in lumen.
  • The movement of electrons through ETC also accounts for increase in Hydrogen ions concentration and low pH in lumen when compared to stroma, which has less proton concentration and high pH .
  • This creates a proton and electrochemical gradient across the thylakoid membrane.
  • The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is used to generate ATP.
  • This diffusion will help in build up of energy ( as they have same electrical charge).
  • To release this energy, protons will try to push out of any possible opening. In thylakoid, this passage is provided by a specialized protein channel called the ATP synthase.
  • ATP synthase enzyme is a transmembrane protein allows passage of protons- is made up of two sub units.
  • The ATP synthase of Chloroplasts resembles closely to those of Mitochondria and Prokaryotes.
  • CF0 – embedded in thylakoid membrane and allows protons to move.
  • CF1 – protrudes out in stroma.
  • The released energy is used to activate ATP synthase enzyme by causing conformational change in CF1 subunit. This will catalyze the formation of ATP by attaching a third phosphate group to ADP.
  • This process of moving ions / protons across a semi-permeable membrane to create a electrochemical gradient, which drives synthesis of ATP is called Chemiosmosis.
ATP synthesis during light reaction

Non-cyclin and Cyclic Photo-Phosphorylation:

  • Living organisms have the tendency of extracting energy from oxidisable molecules. ATP, which carries energy in its chemical bonds are synthesized by cells by a process called phosphorylation.
  • When generated in Mitochondria its called oxidative phosphorylation and synthesis of ATP in presence of light in Chloroplasts is termed as photo phosphorylation.
  • In Chloroplast, when both PS I and PS II are employed ( as we have seen above) for flow of electrons through different carriers of ETC to produce both ATP and NADPH is called Non cyclic photo phosphorylation.
  • In case of cyclic photo-phosphorylation only ATP is formed. Only PS I is employed and electron is circulated within the photosystem.
  • The excited electron from PS I is not passed onto to NADP+ but is returned to PS I through a series of electron carriers to generate ATP.
  • Cyclin photo-phosphorylation occurs in stroma lamellae as they lack PS II and NADP reductase enzyme.
Cyclic Photo Phosphorylation leads to only ATP synthesis

The ATP and NADPH generated during light reaction will be used for synthesis of sugars in stroma by fixing CO2 in the next phase of Dark reactions or Calvin cycle.

Part I of Photosynthesis : Introduction

PART III – Calvin Cycle – Photosynthesis in plants

Image Credits : Electron transport chain , Oxygen evolving complex at PS II– Wikipedia commons.

ATP synthesis – Openstax biology

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