Plant Photosynthesis

The study of photosynthesis demonstrated the existence of two phases, a light phase and a dark phase. The reaction of light phase requires light and hence also called photochemical reaction whereas reactions of dark phase require no light (dark) and are purely chemical reactions.

Light Phase

Light phase: It can be explained under the following headings:

a) Transfer of energy

b) Emerson effect

c) Two pigment systems

d) Cyclic and non-cyclic photophosphorylation.

a) Transfer of energy

When photon of light energy falls on chlorophyll molecule, one of the electrons pair from ground or singlet state passes into higher energy level called excited singlet state. It comes back to hole of chlorophyll molecule within 10-9 seconds.

This light energy absorbed by chlorophyll molecules before coming back to ground state appears as radiation energy, while that coming back from excited singlet state is called fluorescence and is temperature independent. Sometimes the electron at excited singlet state gets its spin reversed because two electrons at the same energy level cannot stay; for some time it fails to return to its partner electron. As a result it gets trapped at a high energy level. Due to little loss of energy, it stays at comparatively lower energy level (triplet state) from excited singlet state. Now at this moment, it can change its spin and from this triplet state, it comes back to ground state again losing excess of energy in the form of radiation. This type of loss of energy is called as phosphorescence.

When electron is raised to higher energy level, it is called as single singlet state. It can lose its energy in the form of heat also. Migration of electron from excited singlet state to ground state along with the release of excess energy into radiation energy is of no importance to this process. Somehow when this excess energy is converted to chemical energy, it plays a definite constructive role in the process.

b) Emerson effect

Emerson and Lewis (1943) measured quantum yield at different wavelengths of light. Quantum yield can be defined as number of O2 molecules released per quantum of light absorbed. A sudden drop in rate of photosynthesis was noticed at 680 mµ (red region). This sudden fall in the photosynthesis yield beyond red region of spectrum is called red drop.

Emerson et al. further noticed that photosynthetic rate can be restored if simultaneously shorter wavelength is provided. This simultaneous giving of shorter and longer wavelengths gave photosynthetic rate higher than total rate from the beams separately. This photosynthetic enhancement is referred as Emerson enhancement effect or Emerson effect. The results obtained by Emerson were as under:

Photosynthesis at 700 nm = 10

Photosynthesis at 653 nm = 43.5

Photosynthesis at 653 + 700 nm = 72.5

c) Two photo systems (Pigment systems)

The discovery of Emerson effect has clearly shown the existence of two distinct photochemical processes, which are believed to be associated with two different specific groups of pigments called pigment or photo system I and pigment or photo system II.

PS I constitutes pigments, like chlorophyll b, PS II is absorbed by chlorophyll and P700. Light energy for PS II is absorbed by chlorophyll a 673, chlorophyll b, phycobilins and P680. While PS I is active in red and far red light, PS II fails to act in far red light.

d) Cyclic and non-cyclic photophosphorylation

Light phase includes the interaction of two pigment systems. PS I and PS II constitute various types of pigments. Arnon showed that during light reaction not only reduced NADP is formed and oxygen is evolved but ATP is also formed. This formation of high energy phosphates (ATP) is dependent on light hence called photophosphorylation.

ADP + Pi ——–> ATP

(where ADP = Adenosine diphosphate, Pi = Inorganic phosphate and ATP = Adenosine triphosphate).

When the light quantum is absorbed by various types of pigments (like chlorophylls, phycpbilins, carotenoids etc.), it is transferred to reaction centre i.e. P700 in PS I and P680 in PS II. Electrons excite from reaction centres due to funneling of energy. P700 gets photoexcited and comes under first excited singlet state. As a result electron is lost, which is accepted by an electron acceptor in the way. After absorbing light, excited electron is liberated from reaction centre interacts with water.

4H₂O ——–> 4H⁺ + 4OH^-

4OH^- + 4e^- ——–> 4OH

4OH ——–> 2H₂O + O₂

4H⁺ + 2A + 4e ——–> 2AH₂

Another important aspect of light reactions is the formation of ATP and NADPH₂ (Assimilatory power). H⁺ from water and electron from chlorophyll are made available to NADP to form NADPH₂. The electrons are accepted by NADP after passing through electron carriers. The carriers in the way undergo oxidation and reduction and are arranged in accordance with the redox potential value.

Non-cyclic photophsphorylation: When the same electrons released by two pigment systems PS I and PS II are not received back and are used at different places simultaneously producing the ATP with the help of light, process is called non-cyclic photophosphorylation. The electron excited reaction centers acquire a sufficient quantum of energy, This electron with high potential energy moves down through electron transport chain and in the way ATP is formed. In non-cyclic photophosphorylation electron lost from PS II i.e. P680 is finally accepted by P700 (PS I). P700 transfers its electron to NADP through ferredoxin. The oxidized P680 regains its electron by the photolysis of water into 2H⁺, 2e^- and oxygen. The excited electron from P680 flows down an electron transport chain to P700 (via plastoquinone, cytochrome complex and plastocyanin) generating ATP. Illumination of PS I boosts electrons to high energy state from which are passed to NADP reducing it to NADPH₂ H⁺ from H₂O.

The H⁺ (protons) collect inside the thylakoid membrane leading to proton gradient. The energy released by the protons, when they diffuse out of the thylakoid membrane into stroma of chloroplast (with H⁺ conc. gradient) is utilized for the formation of ATP. Formation of ATP is thus similar to that of F₀-F_s particles of mitochondria during oxidative phosphorylation.

The hydrogen ions are accepted by NASP (released due to photolysis of water), which forms PS I. Process needs constant supply of water molecules to be oxidized and NADP to be reduced. It is important to note that during non-cyclic photophosphorylation following changes have occurred:

(a) Formation of ATP (b) Formation of NADPH₂ (c) Release of O₂

Cyclic photophosphorylation: In addition to non-cyclic photophosphorylation, there is another pathway of ATP formation which involves only PS I. It has been referred to as cyclic photophosphorylation. When the photons activate PS I, a pair of electrons are raised to high energy level. They are captured by primary acceptor which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction center of PS I i.e. P700. At each step of electron transfer, the electrons lose potential energy. Their trip downhill is caused by the transport chain to pump H⁺ across the thylakoid membrane. The proton gradient, thus established is responsible for forming ATP.

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