Linear electron flow pathway during chloroplast light reactions (Z-scheme):

• Photolysis of water oxidizes at Photosystem II (PSII / P680) -> (b).
• Reduced plastoquinone carries electrons to form Plastoquinol ($\text{PQH}_2$) -> (d).
• Plastoquinol transfers electrons via the Cytochrome $b_6f$ complex to the mobile copper protein Plastocyanin -> (c).
• Plastocyanin reduces photo-oxidized Photosystem I (PSI / P700) -> (a).

Physiological functions of phytohormones:

• Gibberellins break seed dormancy and induce hydrolytic enzymes (alpha-amylase) to trigger Seed Germination -> (IV).
• Indole-3-acetic acid (Auxin) stimulates Cell Elongation via the acid growth mechanism -> (III).
• 6-Benzylaminopurine (synthetic Cytokinin) functions as a Cell Division factor -> (II).
• Abscisic Acid maintains seed dormancy and prevents precocious germination, thereby mediating Inhibition of Vivipary -> (I).

Role of Plastocyanin in thylakoid electron transport:

• Plastocyanin is a water-soluble, peripheral copper-binding metalloprotein located in the thylakoid lumen.
• Its single catalytic copper ion alternates between $\text{Cu}^{2+}$ (oxidized, blue) and $\text{Cu}^+$ (reduced, colorless) as it shuttles electrons from the Cytochrome $b_6f$ complex to the reaction center of Photosystem I (P700).
• Cytochrome c oxidase is a mitochondrial enzyme, plastoquinone is a lipophilic quinone, and riboflavin is a vitamin cofactor.