5        Most sensitive part of Photosynthesis is
(1) PS-I                                       (2) PS-II
(3) Carboxylation reaction (4) Photolysis of water

Photosynthesis Sensitivity: Key Steps Under Stress

Introduction

Key phrase: most sensitive part of photosynthesis PSII photodamage

Identifying which step of photosynthesis is most sensitive to environmental stress or photodamage is vital for plant biology, crop improvement, and exam preparation. Learning why PS-II is considered the most delicate link can help in understanding photoinhibition, plant stress physiology, and efficiency in energy conversion.​

Detailed Explanation of Each Option

• (1) PS-I (Photosystem I):

  • PSI carries out electron transfer after PSII and is robust under most conditions, showing resilience to photodamage but can be inhibited under specific circumstances. However, it is less susceptible to rapid and direct damage from high light or other stresses compared to PSII.​

• (2) PS-II (Photosystem II):

  • Correct. PSII is the initial site for light absorption and photolysis of water, and is exceptionally sensitive to photoinhibition, high light intensity, heat, and various abiotic stresses leading to protein and complex breakdown. Repair mechanisms are regularly needed for PSII under stress to maintain photosynthetic efficiency.​

• (3) Carboxylation reaction:

  • Carboxylation, performed by Rubisco in the Calvin cycle, can be affected by temperature and CO2 levels, but it is generally more stable compared to PSII in terms of light-induced physical damage.​

• (4) Photolysis of water:

  • Photolysis of water occurs in PSII and is dependent on the functional integrity of PSII itself. While essential, the water-splitting reaction’s sensitivity is a reflection of PSII’s fragility rather than an independent vulnerability.​

Quick Facts About PSII Sensitivity

• PSII is the major site of light energy conversion and water splitting.

• It is prone to photodamage (photoinhibition) under high light, heat, or stress.

• Damage to PSII drastically restricts overall photosynthesis more than carboxylation or PSI.​

• PSII repair (including D1 protein turnover) is essential for maintaining photosynthetic function.​