What is the Difference Between p680 and p700

p680 and p700 are two reaction-center chlorophyll a molecular dimers in the photosystem I and II that are involved in the light reactions of photosynthesis of plants, algae, and cyanobacteria. The light reaction is the first stage of photosynthesis. There are photosystems with pigment molecules in the light reaction to absorb the sunlight. Light reaction uses light energy to make two molecules needed for the next stage of photosynthesis: ATP and NADPH. Photosystems are large complexes of proteins and pigments that are optimized to harvest light in the light reaction.

The key difference between p680 and p700 is their nature. p680 is the reaction-center chlorophyll a molecular dimer in photosystem II in plants, algae, and cyanobacteria, while p700 is the reaction-center chlorophyll a molecular dimer in photosystem II in plants, algae, and cyanobacteria.

CONTENTS

1. Overview and Key Difference
2. What is p680  
3. What is p700
4. Similarities – p680 and p700
5. p680 vs p700 in Tabular Form
6. Summary – p680 vs p700
7. FAQ – p680 and p700

What is p680?

P680 is the primary donor in photosystem II and is the reaction-center chlorophyll a molecular dimer found in photosystem II of plants, algae, and cyanobacteria. P680 is central to oxygenic photosynthesis. P680 is the reaction-center chlorophyll associated with PS II (photosystem II), which is a multi-subunit membrane-protein complex participating in oxygenic photosynthesis. P680 got its name from the wavelength at which it captures the most light, which is 680 nm in this case.

Figure 01: p680

The structure of p680 consists of a heterodimer of two distinct chlorophyll molecules. These chlorophyll molecules are referred to as PD1 and PD2. However, this special pair forms an excitonic dimer that functions as a single unit when absorbing light. Furthermore, when p680 absorbs light energy, it loses electrons and becomes excited. The excitation of p680 leads to water splitting and the formation of oxygen. The loss of electrons passes through the transport chain, which ultimately leads to the formation of ATP and NADPH.

What is p700?

p700 is the photosystem I primary donor or the reaction center in photosystem I. It is a molecular dimer of chlorophyll a in plants, algae, and cyanobacteria. The structure of p700 has a heterodimer with two distinct chlorophyll molecules referred to as chlorophyll a and chlorophyll a. This special pair of p700 behaves as a single unit while absorbing light in the light reaction.

Figure 02: p700

The absorption of light by the p700 reaction center helps both cyclic phosphorylation and non-cyclic phosphorylation. Furthermore, p700 regenerates when p700+ recovers its lost electron by oxidizing plastocyanin after a light reaction that occurs in the photosystem I.

Similarities Between p680 and p700

1. p680 and p700 are two reaction-center chlorophyll a molecular dimers in the photosystem II and I involved in the light reactions of the photosynthesis of plants, algae, and cyanobacteria.
2. Both reaction centers consist of special pairs of chlorophyll a molecules that act as a single unit when absorbing light energy.
3. These reaction centers absorb light in the red range that has longer wavelengths.
4. They contribute immensely to generating ATP.

Difference Between p680 and p700

Definition

1. p680 is the reaction center of the photosystem II in plants, algae, and cyanobacteria.
2. p700 is the reaction center of the photosystem I in plants, algae, and cyanobacteria.

Absorb Light at Wavelength

1. p680 absorbs light at 680nm.
2. p700 absorbs light at 700nm.

Excitation

1. After light absorption, the resulting P680 with a loosened electron is designated as P680*.
2. After light absorption, the resulting P700 with a loosened electron is designated as P700+.

Charge Separation

1. Following excitation, the loosened electron of P680* is taken up by the primary electron acceptor, called pheophytin, in photosystem II.
2. Following excitation, the loosened electron of P700+ is taken up by primary electron acceptors, such as the phylloquinone molecule (known as A1), and then transferred to three iron-sulfur clusters.

Recovery

1. P680 regenerates when P680+ regains its electron by oxidizing water via the oxygen-evolving complex in photosystem II.
2. P700 regenerates when P700+ regains its electron by oxidizing plastocyanin in photosystem I.

Function

1. Light absorption by P680 supports only non-cyclic phosphorylation, producing both NADPH and ATP.
2. Light absorption by P700 supports both cyclic phosphorylation (producing ATP) and non-cyclic phosphorylation (producing NADPH and ATP).

The following table summarizes the difference between p680 and p700.

Summary – p680 vs p700

Photosystems such as PSI and PSII are large complexes of proteins and pigments that are optimized to harvest light in the light reaction of photosynthesis. Both photosystems contain many pigments that help collect light energy and a special pair of chlorophyll a molecules found at the core of the photosystem. Therefore, p680 and p700 are two reaction-center chlorophyll a molecular dimers in the photosystem II and I. They are also known as reaction centers in these photosystems. p680 reaction center absorbs light at 680nm wavelength while p700 reaction center absorbs light at 700nm wavelength. This is the main difference between p680 and p700.

FAQ: p680 and p700

1. What is photosystem 1 and photosystem 2?

• Photosystem 1 and photosystem 2 are light absorbing centers in the light reactions of photosynthesis. Each photosystem has its own characteristic reaction center that absorbs light at specific wavelengths. Photosystem II has a reaction center known as p680, which absorbs light best at 680 nm, while photosystem has a reaction center known as p700, which absorbs light best at 700 nm.

2. How does p680 function within photosystem II during the light reaction?

• p680 functions as a primary pigment or reaction center within photosystem II by absorbing light at wavelength 680nm. p680 gets excited and loses electrons. This loss of electrons triggers a series of reactions that ultimately produce ATP and NADPH.

3. Describe the role of p680 in water splitting?

• When p680 gets excited and loses electrons, it is oxidized. This facilitates the splitting of water molecules into oxygen, photons, and electrons. This process contributes to a steady supply of electrons to the electron chain and contributes oxygen to the atmosphere.

4. Is p700 chlorophyll a or b?

• P700 is the primary donor in Photosystem I, consisting of a chlorophyll a dimer within the reaction center of Photosystem I in plants, algae, and cyanobacteria.

5. What is cyclic and non-cyclic photophosphorylation?

• Cyclic photophosphorylation occurs only in photosystem I, but non-cyclic photophosphorylation occurs in both photosystems I and II. In cyclic photophosphorylation, only ATP is produced, while in non-cyclic photophosphorylation, both NADPH and ATP are produced.

Reference:

1.“P680 – An Overview.” ScienceDirect Topics.
2. Andrew N Webber a, et al. “P700: The Primary Electron Donor of Photosystem I.” Biochimica et Biophysica Acta (BBA) – Bioenergetics, Elsevier.

Image Courtesy:

1. “PSII new design” By Gmaxwell (CC BY-SA 3.0) via Commons Wikimedia
2. “Cyclic photosynthesis miguelferig” By Miguelferig – Own work (CC0) via Commons Wikimedia

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