2. A photosystem contains which of the following?
   (1) One photochemical reaction centre and multiple antennae
   (2) Multiple photochemical reaction centres and one antenna
   (3) One photochemical reaction centre and one antenna
   (4) Multiple photochemical reaction centres and multiple antennae

The correct answer is Option 1: One photochemical reaction centre and multiple antennae. A photosystem is a complex structure in plants and cyanobacteria that carries out the light-capturing steps of photosynthesis, and typically consists of a single reaction centre and many antenna complexes.

Photosystem Structure Explained

Introduction

Key phrase: photosystem contains one photochemical reaction centre and multiple antennae

Photosystems are integral to the photosynthetic machinery, helping plants capture solar energy with maximum efficiency. Understanding their components and organization clarifies how light energy is funneled and converted inside plant cells, which is essential knowledge for biology students and NEET/CSIR exam aspirants.​

Explanation of Each Option

• Option 1: One photochemical reaction centre and multiple antennae
  This accurately describes a photosystem. Each photosystem (PSI or PSII) has a single reaction centre, where photochemistry—the conversion of light energy to chemical energy—occurs. Surrounding this is an antenna complex made up of hundreds of pigment molecules (chlorophylls and carotenoids) which capture and transfer energy to the reaction centre. This arrangement maximizes light absorption.​

• Option 2: Multiple photochemical reaction centres and one antenna
  This does not describe photosystems correctly. A photosystem does not have more than one reaction centre. Instead, there can be multiple antenna complexes funneling energy to a single reaction centre.​

• Option 3: One photochemical reaction centre and one antenna
  While technically possible, in nature, photosystems almost always have multiple antenna pigments forming extensive complexes for efficient light harvesting. Limiting to one antenna would substantially reduce the photosystem’s light capture ability and is not biologically accurate.​

• Option 4: Multiple photochemical reaction centres and multiple antennae
  This describes a scenario with more than one reaction centre, which is incorrect for individual photosystems. Each photosystem is architecturally built around a single reaction centre with many associated antenna complexes.​

Key Takeaways

• Photosystems are described as having one photochemical reaction centre and multiple antenna complexes.

• Antenna complexes absorb and funnel light energy to the reaction centre, enabling high-efficiency energy conversion.

• Both PSI and PSII follow this structural principle, vital for photosynthesis in all plants and cyanobacteria.​