Comprehensive Study Notes on Light-Dependent Reactions of Photosynthesis

Overview of Light-Dependent Reactions

The light-dependent reactions represent the first stage of photosynthesis.
Requirement for Life: Photons must be absorbed by the plant for these reactions to initiate and proceed.
Energy Source: The entire process is powered by radiant energy captured from the sun.
Primary Outcomes:
- Water molecules ( $H_2O$ ) are split (photolysis).
- Adenosine triphosphate ( $ATP$ ) is produced.
- Nicotinamide adenine dinucleotide phosphate ( $NADPH$ ) is produced.
- Oxygen ( $O_2$ ) is released as a byproduct.

Components of the Photosystem Complex

Photosystems are functional and structural units involved in photosynthesis that absorb photons. A photosystem complex consists of three main components:
1. Antennae Pigments: These include chlorophyll b and various other accessory pigments that capture light energy.
2. Reaction Centre: This is composed of chlorophyll a, which receives energy from the antennae pigments.
3. Primary Electron ( $e^-$ ) Acceptor: A molecule that accepts the high-energy electrons emitted from the reaction centre.
Specific Photosystems:
- Photosystem I (PSI): Also known as $P700$ because it optimally absorbs light at a wavelength of $700\,\text{nm}$ .
- Photosystem II (PSII): Also known as $P680$ because it optimally absorbs light at a wavelength of $680\,\text{nm}$ .

Photoexcitation and the Oxidation of Water in Photosystem II

The Photoexcitation Process:
- Photons are absorbed by antenna pigments, causing electrons to move from a ground state to an excited state.
- This excitation energy is transferred toward the reaction centre.
- Chlorophyll a in the reaction centre absorbs this energy and subsequently emits $2\,e^-$ .
- These $2\,e^-$ are captured by the primary electron acceptor.
Mechanism of Action in PSII (Figure 2):
- A photon energizes an electron in $P680$ , forming an excited state denoted as $P680^*$ .
- The energized chlorophyll ( $P680^*$ ) transfers the high-energy electron to an acceptor molecule ( $A$ ) in the reaction centre.
- Following the loss of the electron, the chlorophyll becomes a positively charged ion, $P680^+$ .
- Due to its high electronegativity, the $P680^+$ ion oxidizes water to regain an electron.
- The high-energy electron is then transferred from the reaction centre to the carrier molecule plastoquinone ( $PQ$ ).
- Net Reaction of Water Splitting: This process occurs at the water-splitting complex, releasing oxygen gas ( $O_2$ ) and protons ( $H^+$ ) into the thylakoid lumen.

Linear Electron Transport (Noncyclic Photophosphorylation)

Linear electron transport involves both Photosystem II and Photosystem I and occurs within the thylakoid membrane.
Step-by-Step Pathway:
1. Absorption in PSII: A photon is absorbed by the antenna complex of PSII (containing chlorophyll a $P680$ ).
2. Primary Acceptance: The reaction centre energizes an electron to move from chlorophyll to Pheophytin I (the primary electron acceptor, which is itself a chlorophyll molecule).
3. Water Splitting: Electrons lost by PSII are replaced by the oxidation of water. The resulting $P680^+$ ion removes an electron from a water molecule. For every $H_2O$ molecule, $2\,e^-$ are passed. The balanced reaction for the formation of one $O_2$ molecule is: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ .
4. Plastoquinone ( $PQ$ ): An electron shuttle that picks up electrons from Pheophytin I and passes them to the Cytochrome complex ( $Cyt$ ).
5. Cytochrome Complex: As electrons pass through, the complex pumps $2H^+$ into the thylakoid lumen.
6. Plastocyanin ( $PC$ ): This electron shuttle transports electrons from the Cytochrome complex to Photosystem I.
7. Re-energization in PSI: A second photon strikes PSI ( $P700$ ), re-energizing the electrons.
8. Ferredoxin ( $Fd$ ): The primary electron acceptor of PSI passes electrons to Ferredoxin, an electron carrier.
  - Fd Sol (Soluble): Ferredoxin without an electron.
  - Fd Bnd (Bound): Ferredoxin carrying an electron.
9. NADPH Formation: $Fd\,\text{Bnd}$ is oxidized by the enzyme $NADP^+$ Reductase, which transfers the electrons to $NADP^+$ to form $NADPH$ .
Reduction Stoichiometry:
- Two electrons are required to reduce $NADP^+$ into $NADPH$ .
- $NADP^+ + e^- \rightarrow \text{NADP}$
- $\text{NADP} + e^- + H^+ \rightarrow NADPH$
- The second electron and a proton ( $H^+$ ) are sourced from the stroma. $NADPH$ serves as a carrier for two high-energy electrons.

Stoichiometry and Mathematics of Linear Electron Transport

Photon Requirements:
- Moving $1\,e^-$ requires the energy of $2$ photons (one at $P680$ and one at $P700$ ).
- To move $2\,e^-$ (the amount needed for the reduction of $NADP^+$ to $NADPH$ ), $4$ photons are absorbed.
- For every $4$ photons absorbed, $1\,ATP$ and $1\,NADPH$ are produced.
Water Oxidation and Oxygen Evolution:
- $2\,e^-$ are passed per each $H_2O$ molecule split.
- To produce one full molecule of $O_2$ , $2\,H_2O$ must be oxidized: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ .
- Since $2\,e^-$ require $4$ photons, the evolution of one $O_2$ molecule requires the absorption of $8$ photons total.
Proton Pumping: As $2\,e^-$ pass through the Cytochrome complex ( $Cyt$ ), $2H^+$ are pumped across the membrane, contributing to the synthesis of $ATP$ from $ADP$ .

Establishment of the Proton Gradient and ATP Synthesis

The Proton Gradient: Three specific mechanisms create a high concentration of protons ( $H^+$ ) across the thylakoid membrane (inside the lumen compared to the outside stroma):
1. Plastoquinone ( $PQ$ ): Protons enter the lumen during the reduction and oxidation cycles of Plastoquinone.
2. Photolysis: Two protons are added directly to the lumen for every water molecule split at the water-splitting complex.
3. NADPH Production: One proton is removed from the stroma (outside the thylakoid) each time an $NADPH$ molecule is formed.
ATP Synthase and Chemiosmosis:
- the high concentration of protons inside the lumen creates a Proton-Motive Force (PMF).
- Protons move down their concentration gradient from the lumen to the stroma through the enzyme ATP Synthase.
- This movement drives the catalytic production of $ATP$ in the stroma.
- Photophosphorylation: The term for the light-dependent formation of $ATP$ via chemiosmosis.

Cyclic Electron Flow (Cyclic Photophosphorylation)

Purpose: This pathway is used when additional $ATP$ is required by the cell but $NADPH$ is not. It generates a proton gradient for $ATP$ production without reducing $NADP^+$ .
Mechanism:
- It involves Photosystem I (P700) only; Photosystem II does not operate in this cycle.
- PSI passes $2\,e^-$ to its primary electron acceptor.
- The primary electron acceptor passes the $2\,e^-$ to Ferredoxin ( $Fd$ ).
- Instead of being used to make $NADPH$ , Ferredoxin returns the electrons back to Plastoquinone ( $PQ$ ).
- Plastoquinone passes the electrons to the Cytochrome complex ( $Cyt$ ).
- The Cytochrome complex uses the energy from the electrons to pump $2H^+$ from the stroma into the thylakoid lumen.
- Low-energy electrons are then passed to Plastocyanin ( $PC$ ), which returns them to PSI to be reused.
Outcomes:
- Immediate and exclusive synthesis of $ATP$ .
- No water is split (no $O_2$ produced).
- No $NADPH$ is produced.
Biological Context: This process can be performed by some bacteria to generate energy in an autotrophic manner.

Comparative Summary

Linear Electron Transport:
- Involved: PSII ( $P680$ ) and PSI ( $P700$ ).
- Final Electron Acceptor: $NADP^+$ (produces $NADPH$ ).
- Water Splitting: Yes (produces $O_2$ and $H^+$ ).
- ATP Production: Yes, via chemiosmosis.
Cyclic Electron Transport:
- Involved: PSI ( $P700$ ) only.
- Final Electron Acceptor: The electrons return to PSI (recycled).
- Water Splitting: No.
- ATP Production: Yes, generates additional $ATP$ via the proton gradient.

Homework and Resources

Textbook Reference: Read page 228 and complete questions # 1, 2, 4, 5, 9.
Instructional Videos:
- Overview of Light Reactions: https://www.youtube.com/watch?v=CMIPYHNNg28
- Detailed Photosynthesis (Light-Dependent and Independent): https://www.youtube.com/watch?v=D2Y_eEaxrYo