Photosynthesis-II (Light Phase) Notes

Photosynthesis-II (Light Phase)

Photosynthesis occurs in two phases: the light phase and the dark phase.

Light Phase (Photochemical Reaction)

The light phase is light-dependent, converting radiant energy into chemical energy in the form of ATP and NADPH₂. Oxygen is evolved during this phase.

Dark Phase (Thermochemical Reaction)

The dark phase does not require light, utilizing the products of the light reaction (NADPH₂ and ATP) to fix CO₂ into carbohydrates.

Evidences for Two Phases

1. Flashing Light Experiment (Warburg, 1919):

   • Chlorella algae were exposed to both continuous light and intermittent light (short flashes followed by dark periods).
   • Photosynthesis rate was higher with intermittent light, indicating two phases.

2. **Temperature Coefficient ($Q_{10}$):

   • $Q_{10}$ is the ratio of reaction rate at a given temperature ($t^\circ$) to the rate at $(t^\circ - 10^\circ)$.
   • For purely chemical reactions, $Q<em>{10} \geq 2$, but for photochemical reactions, $Q</em>{10} \approx 1$.
   • Blackman (1905) found $Q<em>{10} = 1$ at low light intensity (photochemical reaction) and $Q</em>{10} \geq 2$ at high light intensity (thermochemical reaction).

   $Q_{10} = \frac{\text{Rate of reaction at } t^\circ\text{C}}{\text{Rate of reaction at } (t^\circ-10^\circ\text{C})}$

3. Carbon Dioxide Reduction in Dark (Calvin, 1948):

   • Green plants exposed to light with CO₂ were transferred to darkness with radioactive $^{14}CO_2$.
   • $^{14}C$ was found fixed in carbohydrates, showing the dark reaction reduces CO₂ to carbohydrates.

Concept of Quantasome

A. Granum fret membrane bearing quantasomes
B. Quantasome as photosynthetic unit
C. Absorption of light by harvesting molecules and transferring it to trap center

Photosynthetic Unit

• A photosynthetic unit is the smallest group of pigment molecules participating in a photochemical act by absorbing light, termed a quantasome.
• Quantasomes are morphological structures on thylakoids of chloroplasts.
• About 230 chlorophyll molecules form a photosynthetic unit.

Each unit consists of:

• A trap center (reaction center).
• An antenna complex, containing ~200 light-harvesting pigment molecules that:
  • Absorb light based on their absorption spectra.
  • Transfer energy to the trap center via inductive resonance (excitation energy transfer without loss or chemical reaction).

Harvesting of Light Energy

1. Ground State: Normal state of an atom or molecule.
2. Excited State: When a photon strikes a chlorophyll molecule, an electron moves to a higher energy orbit.
   • First Singlet State ($S_1$).
   • Second Singlet State ($S_2$).
   • Triplet State.

• Blue light excites electrons to $S<em>2$, while red light excites them to $S</em>1$.
• Both states are unstable (life period of $10^{-9}$ seconds).
• $S<em>2$ electron drops to $S</em>1$ releasing heat.

Possible actions of S1 electron

1. Falling to ground state with red light emission (fluorescence).
2. Losing energy as heat and transition to the triplet state (metastable state, longer life of $10^{-2}$ seconds).
3. Falling from triplet state to ground state with delayed light emission (phosphorescence).

• Most absorbed energy is released as heat, phosphorescence, and fluorescence, while a portion is converted to chemical energy for CO₂ assimilation in dark reactions.

Emerson Effect and Two Photosystems

Emerson et al. (1957) measured quantum yield in Chlorella plants.

• Quantum yield: Rate of photosynthesis, measured as $O_2$ molecules produced per quantum of light absorbed.
• Quantum requirement: Number of light quanta needed to produce one $O_2$ molecule (reciprocal of quantum yield).

1. Emerson Red Drop

Robert Emerson (1917) observed that quantum yield decreases sharply beyond 680 nm (red region) when algae are exposed to monochromatic light.

2. Emerson Enhancement Effect (1957-58)

Superimposing shorter wavelength light with far-red light recovers the 'Red Drop' and enhances photosynthetic yield beyond the sum of individual yields.

Conclusion

Photosynthesis is governed by two photochemical processes with different pigment groups:

• Photosystem I (PS-I): Absorbs longer wavelengths.
• Photosystem II (PS-II): Absorbs shorter wavelengths.

Table 9.1: Differences between PS-I and PS-II

Hill Reaction and Photo-oxidation of Water

Robert Hill (1937) found that isolated chloroplasts in light evolve oxygen in the presence of water. Adding oxidants like ferric oxalate reduces them to ferrous forms.

• Hill reaction: Chloroplasts' ability to evolve $O_2$ from water in light (without CO₂ fixation) and reduce oxidants.

Photo-oxidation of Water

Breaking two water molecules into $H^+$ ions, electrons, and $O_2$ in illuminated chloroplasts, associated with PS II system.

$2H<em>2O \xrightarrow{\text{light}} 4H^+ + 4e^- + O</em>2 \uparrow$

The splitting requires enzymes and co-factors (Mn, Cl, and Ca ions). It occurs in four steps:

• Each step requires a photon and releases an electron.
• Molecular oxygen is released in the fourth state, and the system returns to the initial state.

Redox Potential

Redox potential (E) is the potential required for oxidation and reduction reactions at 25°C and pH 7.

• Substances with -ve redox potential are electron donors (high energy).
• Substances with +ve redox potential are electron acceptors (low energy).
• During electron transfer, energy is released and stored in ATP bonds.

Table 9.2: Redox potential of some important compounds used in electron transport chain in light phase.

Mechanism of Light Reaction

Light energy is converted into chemical bonds of NADPH₂ and ATP in $1 \times 10^{-4}$ seconds.

Components:

1. PS-I and PS-II for light absorption.
2. Water-oxidizing system (photolysis).
3. Ferredoxin-NADP oxido-reductase.
4. Electron transport system (Cyt b6-f complex, plastoquinone (PQ), plastocyanin (PC), ferredoxin (Fd)).
   • PQ acts as a shuttle between PS-II and Cyt b6.
   • PC between Cyt b6 and PS-I.
   • Fd between PS-I and NADP+ reductase.
5. ATPase (coupling factor) for ATP generation.

Electron transport occurs via cyclic and non-cyclic pathways.

Non-Cyclic Photo Electron Transport Path

Electrons do not return to their origin; Z-scheme represents this pathway.

Z-Scheme

Continuous flow of electrons from water to NADP through PS-II, PS-I, and cytochrome complex.

1. Light absorption transfers energy to reaction centers of PS-II (P680) and PS-I (P700).

PS-II Complex

1. Chl a680 gets excited, expelling an electron with 23 kcal/mole energy (quantum conversion).
2. Oxidized chl a680+ receives an electron from water photo-oxidation via acceptor 'Z'.
3. Expelled electron is received by pheophytin (Phaeo), a form of chl a, then passes to quinone (Q).
4. Q transfers two electrons to plastoquinone (PQ), which picks up 2H+ from the thylakoid outer medium and becomes plastohydroquinone (PQH2).
5. PQH2 transfers electrons to Cyt b6f complex, releasing 2H+ into the thylakoid inner space, creating a H+ gradient.
6. Reduced cyt f transfers electrons to plastocyanin (PC) freely along the lumenal surface.

PS-I Complex

1. PS-I is activated by light at 700 nm, exciting chl a 700, which expels an electron.
2. Oxidized chl700+ receives an electron from reduced PC.
3. From FeS, electrons are passed to Ferredoxin (Fd), which reduces NADP+ with ferredoxin-NADP-oxido-reductase, resulting in NADPH + $H^+$.

Cyclic Photo Electron Transport Path

Electrons from PS-I return to oxidized P700.

1. Involves only PS-I and electron carriers to produce ATP.
2. No photo-oxidation of water or NADPH + $H^+$.

Conditions:

• Additional ATP requirement.
• Blocked PS-II activity.
• Light > 680 nm.
• Faster pathway without PS-I and PS-II transfer.

Process

1. PS-I activated by 700 nm light.
2. Chl a700 is excited, expelling an electron with 23 kcal/mole energy.
3. Electron captured by acceptor 'A', then passes to FeS and ferredoxin.
4. Electrons flow to Cyt b6 unit of Cyt b6-f, synthesizing ATP.
5. From Cyt b6, electrons are transferred to cyt f (via PQ), releasing $2H^+$ in the inner space.
6. Reduced Cyt f transfers electrons to PS-I via PC.
7. Oxidized chl a700+ gets neutralized.

Assimilatory Powers

1. Production of NADPH

In non-cyclic electron transport:
$2NADP^+ + 4e^- + 4H^+ \rightarrow 2(NADPH + H^+)$

• PS-I (P700) supplies 4 electrons when 4 P700 molecules are excited.
• P700+ is filled by electrons from P680 of PS-II.
• P680+ is filled by photo-oxidation of two water molecules.
  $2H<em>2O \rightarrow 4H^+ + 4e^- + O</em>2 \uparrow$
• Protons are used to reduce NADP+.

8 photons of light and two water molecules produce two NADPH molecules.

2. Photophosphorylation

ATP generation from ADP and iP (inorganic phosphate) by light reactions.

Chemi-Osmotic Mechanism (Peter Mitchell, 1976)

Proton flow down the electrochemical gradient through ATPase complex results in ATP synthesis.


Hydrogen ion concentration increases in the inner thylakoid space in the light reaction:

• Photo-oxidation of water releases $4H^+$ in the inner space.
  $2H<em>2O \rightarrow 4H^+ + 4e^- + O</em>2 \uparrow$
• PQ reduction and oxidation transfer $4H^+$ from outer matrix to inner space in both cyclic and non-cyclic systems.

Table 9.3: Comparison between Non-cyclic and Cyclic photophosphorylation