Detailed Study Notes on Light Dependent Reactions

12U Bio Metabolic Processes L08: Light Dependent Reactions

Starter Questions

  • How does the intensity and color of light affect the rate of photosynthesis in plants?

  • Why are chloroplasts green, and how does this relate to their function in photosynthesis?

  • What is hydrolysis? What do you think photolysis is?

Overview of Light Dependent Reactions

  • Definition: The light dependent reactions are powered by the radiant energy of the Sun.

  • Products of Light Dependent Reactions: These reactions produce three main products:

    • ATP (Adenosine triphosphate)

    • NADPH (Nicotinamide adenine dinucleotide phosphate)

    • Oxygen (O₂) as a waste product

  • Next Topic: Light-independent Reactions

Description of the Light Dependent Reactions

  • Characteristic: The light dependent reactions are characterized by their need for light absorption through photons, which is essential for initiating the reactions.

Photosystems and Electron Transport

Photosystem II (PSII)

  • Components: PSII contains the pigment chlorophyll a (p680), which absorbs light energy.

  • Function: A photon absorbed by the antenna complex energizes an electron (e-).

Noncyclic Photophosphorylation

  • Process: Light energy is utilized to convert ADP (Adenosine diphosphate) into ATP through phosphorlyation.

Electron Supply and Water Oxidation

  • Replacement of Electrons: The electrons lost by PSII are replenished by the oxidation of water molecules: 2H2Oightarrow4H++4e−+O22H_2O ightarrow 4H^+ + 4e^- + O_2

    • This reaction is balanced to produce one molecule of O₂.

  • Photolysis: The process of splitting water molecules (H₂O) using light energy, resulting in the release of:

    • Oxygen gas (O₂)

    • Hydrogen ions (H⁺)

    • Electrons

Electron Transport Chain Components

  • Plastoquinone (PQ):

    • Function: Picks up the energized electrons from PSII and transfers them to the cytochrome complex (b6f).

    • Proton Pumping: As electrons are transferred, the complex pumps 2H+ ions into the thylakoid lumen.

  • Plastocyanin (PC):

    • Function: Transports electrons from the cytochrome complex to PSI.

Photosystem I (PSI)

  • Description: PSI uses chlorophyll a (p700) and absorbs light at a wavelength of 700 nm.

  • Electron Energization: A second photon reaches PSI, re-energizing the electrons before they are passed to the primary electron acceptor.

Ferredoxin and NADPH Production

  • Electron Transfer: The primary electron acceptor of PSI transmits the electrons to Ferredoxin (Fd).

  • NADP+ Reduction: The electrons are oxidized by NADP+ reductase enzyme (FNR), resulting in the transfer of electrons from Fd to NADP+:

    • First electron: NADP++e−<br>ightarrowNADPNADP^+ + e^- <br>ightarrow NADP

    • Second electron: NADP+e−+H+<br>ightarrowNADPHNADP + e^- + H^+ <br>ightarrow NADPH

ATP Synthesis via Chemiosmosis

  • Proton Gradient Utilization: The H+ gradient within the thylakoid lumen aids in the chemiosmotic synthesis of ATP.

  • Enzyme: ATP synthase catalyzes this reaction using energy derived from the proton motive force (PMF) of the H+ gradient.

  • ATP Production: For every 4H+ ions, sufficient PMF is created to synthesize 1 ATP molecule.

Summary of Noncyclic Photophosphorylation

  • Production Rate: 1 ATP and 1 NADPH are produced for every 4 photons absorbed during noncyclic photophosphorylation (2 absorbed by PSII, 2 by PSI).

Cyclic Photophosphorylation

Overview

  • Purpose: Cyclic photophosphorylation is specifically responsible for synthesizing ATP only.

  • Components: Utilizes Photosystem I (PSI) containing chlorophyll a (p700).

Electron Transport in Cyclic Photophosphorylation

  • Electron Pathway:

    • PSI energizes an electron that is passed to the primary electron acceptor.

    • The electron is then transferred to ferredoxin (Fd), which directs the electron to plastoquinone (PQ).

    • PQ sends electrons to the cytochrome complex (Cyt).

  • Proton Pumping: Cyt extracts energy to pump 2H+ from the stroma into the thylakoid lumen, returning the low-energy electron to plastocyanin (PC) for reuse.

ATP Synthesis via Chemiosmosis in Cyclic Pathway

  • Process: Just like in noncyclic phosphorylation, the H+ gradient in the lumen is exploited for the chemiosmotic synthesis of ATP via ATP synthase, utilizing the PMF.

  • Production Rate: As in noncyclic photophosphorylation, every 4H+ results in the production of 1 ATP.

Key Takeaways from Light Dependent Reactions (LDR)

Review Questions for Understanding

  1. Define Photophosphorylation:

    • The process of phosphorylation/addition of inorganic phosphate (Pi) to ADP to yield ATP utilizing the energy of sunlight.

  2. Products of the LDR Moving to LIR:

    • ATP and NADPH.

  3. Waste Product Released by Plants:

    • Oxygen (O₂).

  4. Reused Products from Photolysis:

    • H+ ions reduce NADP+ to NADPH.

    • Electrons replace the losses in photosystem II.

  5. Applications of Photosystem I:

    • Utilized in both cyclic and non-cyclic photophosphorylation.

Challenges and Extensions

  • Task: Draw a schematic representation of the LDR, including:

    • The thylakoid membrane

    • The arrangement of photosystems

    • Arrows to illustrate the process of electron transport and ATP synthesis.

  • Extension: Additional task includes detailing the cyclic photophosphorylation process in the diagram further.