The key difference between plastoquinone and plastocyanin is that plastoquinone is a lipophilic carrier molecule that transfers electrons to plastocyanin through cytochrome b6f protein complex. But, plastocyanin is a small water-soluble blue-copper protein that accepts a pair of electrons from the cytochrome b6f complex and passes it to photosystem I in the thylakoid space.
Photosynthesis has two types of reactions as light-dependant reactions and light-independent reactions. These two types of photosystems play an important role in light-dependent reactions. They are photosystem I and II. Photosystems are large complexes of proteins and pigments or light-absorbing molecules. They capture energy from sunlight. P700 is the reaction centre of photosystem I while P680 is the reaction centre of photosystem II. Each photosystem absorbs light and produces ATP via photophosphorylation. They also produce NADPH. Several types of electron acceptors participate in the light dependant reactions. Plastoquinone and plastocyanin are two types of molecules involved in the electron transport chain.
CONTENTS
1. Overview and Key Difference
2. What is Plastoquinone
3. What is Plastocyanin
4. Similarities Between Plastoquinone and Plastocyanin
5. Side by Side Comparison – Plastoquinone vs Plastocyanin in Tabular Form
6. Summary
What is Plastoquinone?
Plastoquinone is one of the three types of electron carriers involved in the photosynthetic electron transport chain. It is a lipophilic carrier molecule and an obligatory functional component of photosynthesis. Plastoquinone is localized in the inner thylakoid membrane of chloroplasts. Structurally, plastoquinone is composed of nine isoprenyl units with 45 C atoms in the side chain and it is structurally similar to ubiquinone.
Figure 01: Plastoquinone
Plastoquinone is found within photosystem II. Once plastoquinone accepts electrons from photosystem II, it is reduced into plastoquinol by transferring electrons to plastocyanin through cytochrome b6f protein complex.
What is Plastocyanin?
Plastocyanin is another electron carrier involved in light-dependent photosynthesis. It is a water-soluble copper-containing compound that accepts electrons from cytochrome b6f complex and passes to photosystem I in the thylakoid space and reduces the P700+. Plastocycnin is dependent on copper and it is a vital nutrient for plants. Copper deficiency may affect plants.
Figure 02: Plastocynin
Plastocyanin is localized in the thylakoid lumen. Structurally, it is a metalloprotein consisting of an eight-stranded antiparallel β-barrel with one copper atom at the center.
What are the Similarities Between Plastoquinone and Plastocyanin?
- Plastoquinone and plastocyanin are specific to plants.
- They are electron carriers involved in the photosynthetic electron transport chain.
- They are movable compounds.
- These compounds transport electrons over fairly extensive distances and play a unique part in photosynthetic energy conversion.
- Cytochrome b6f conveys electrons between plastoquinone and plastocyanin.
- Both plastoquinone and plastocyanin mediate electron transfer from PS II to PS I.
What is the Difference Between Plastoquinone and Plastocyanin?
Plastoquinone is an electron transporter that transports electron from photosystem II to cytochrome b6f. Plastocyanin, on the other hand, is a copper-containing electron carrier protein that accepts electrons from cytochrome b6f and passes to P700+ of photosystem I. So, this is the key difference between plastoquinone and plastocyanin.
Moreover, plastoquinone is present in the inner membrane of thylakoid, while plastocyanin is present in the lumen of the thylakoid. Also, plastocyanin is dependent on copper, while plastoquinone is not dependent on copper.
Below infographic summarizes the difference between plastoquinone and plastocyanin.
Summary – Plastoquinone vs Plastocyanin
There are three different mobile electron carriers involved in the photosynthetic electron transport chain. They are plastocyanin, plastoquinone, and ferredoxin. Of these, plastoquinone is a lipophilic molecule that accepts electron from photosystem II and passes to cytochrome b6f. Meanwhile, plastocyanin is a copper-containing water-soluble protein that accepts electrons from cytochrome b6f and passes to P700+ of photosystem I. Moreover, plastoquinone is present in inner thylakoid membrane, while plastocyanin is present in the thylakoid lumen. So, this is the summary of the difference between plastoquinone and plastocyanin.
Reference:
1. Gross, E L. “Plastocyanin: Structure and Function.” NCBI, U.S. National Library of Medicine, Aug. 1993, Available here.
2. “Plastocyanin.” Wikipedia, Wikimedia Foundation, 17 Dec. 2019, Available here.
Image Courtesy:
1. “Plastoquinone” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Plastocyanin-from-3BQV-3D-cartoon-balls” By Ben Mills – Own work (Public Domain) via Commons Wikimedia