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Photosystem II

Photosystem II

The multi-protein complex photosystem II (PS II) harvests photons and transfers their energy through its bound pigments to its reaction center, and ultimately to photosystem I (PSI) through the electron transport chain. The pigments responsible for caputirng the light energy in photosystems include chlorophyll a, chlorophyll b, and carotenoids.
The pigment molecules are arranged across two photosystem domains — the antenna complex and the reaction center. The main aim of the pigment molecules...

Photosystems

Photosystems

Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.
Functioning of Photosystems
Photosystems contain many pigment molecules, such as chlorophylls and carotenoids, arranged in a particular organization across two domains — the antenna complex and the reaction center. The main aim of the pigment molecules...

Photosystem I

Photosystem I

Although structurally similar to photosystem II (PSII), photosystem I (PSI) is has a different electron supplier and electron acceptor.
Both these photosystems work in concert. An excited electron from PSII is relayed to PSI via an electron transport chain in the thylakoid membrane of the chloroplast, which is comprised of the carrier molecule plastoquinone, the dual-protein cytochrome complex, and plastocyanin. As electrons move between PSII and PSI, they lose energy and must be re-energized...

The Antenna Complex

The Antenna Complex

Plants and other photosynthetic organisms comprise pigments capable of absorption of direct sunlight. These pigments are present in the reaction center - the main site of photochemical reactions as well as in the antenna complex. Under average light conditions, the rate at which reaction center pigments absorb light is far below the electron transport chain's capacity. As a result, the reaction center alone cannot provide enough energy to drive photosynthesis. The photosynthetic efficiency can...

The Photochemical Reaction Center

The Photochemical Reaction Center

Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic...

Light as Energy

Light as Energy

The energy required to carry out photosynthesis is light— typically electromagnetic radiation from the sun. The range of all possible wavelengths is known as the electromagnetic spectrum.
Photons
A photon is a discrete electromagnetic particle or bundle of energy. Photons are characterized by their frequency, wavelength, and amplitude, similar to the properties of a wave. Waves with higher frequencies transmit more energy and have shorter wavelengths than longer wavelengths that transmit less...

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Related Experiment Video

Updated: Jun 19, 2026

Evaluation of Photosynthetic Behaviors by Simultaneous Measurements of Leaf Reflectance and Chlorophyll Fluorescence Analyses

Evaluation of Photosynthetic Behaviors by Simultaneous Measurements of Leaf Reflectance and Chlorophyll Fluorescence Analyses

Published on: August 9, 2019

THE CHLOROPHYLL UNIT IN PHOTOSYNTHESIS.

W Arnold¹, H I Kohn

¹Laboratory of General Physiology, Harvard University, Cambridge.

The Journal of General Physiology

|October 30, 2009

Summary

Researchers found that plants need 2,000 to 3,000 chlorophyll molecules to reduce one carbon dioxide molecule. This suggests a functional chlorophyll unit exists in photosynthesis.

Area of Science:

Plant Biology
Biochemistry
Photosynthesis Research

Background:

Photosynthesis is the process by which plants convert light energy into chemical energy.
Chlorophyll is the primary pigment responsible for capturing light energy.
Understanding the stoichiometry of photosynthesis is crucial for plant physiology.

Purpose of the Study:

To determine the minimum number of chlorophyll molecules required for carbon dioxide reduction in plants.
To investigate the potential existence of a functional chlorophyll unit in photosynthetic processes.

Main Methods:

Studied six plant species from four different phyla.
Quantified chlorophyll molecules and carbon dioxide reduction rates.

Main Results:

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Last Updated: Jun 19, 2026

Evaluation of Photosynthetic Behaviors by Simultaneous Measurements of Leaf Reflectance and Chlorophyll Fluorescence Analyses

Evaluation of Photosynthetic Behaviors by Simultaneous Measurements of Leaf Reflectance and Chlorophyll Fluorescence Analyses

Published on: August 9, 2019

High-Throughput, In-Field Screening of Photosynthetic Efficiency in Crop Plants Using an Autonomous Robot

High-Throughput, In-Field Screening of Photosynthetic Efficiency in Crop Plants Using an Autonomous Robot

Published on: January 9, 2026

A consistent ratio was observed across species: 2,000 to 3,000 chlorophyll molecules per carbon dioxide molecule reduced.
This stoichiometric relationship was found to be conserved across diverse plant groups.

Conclusions:

The findings strongly suggest the existence of a discrete chlorophyll unit involved in carbon dioxide fixation.
This unit likely represents a functional complex of chlorophyll molecules optimized for photosynthetic efficiency.