Photosystems- Introduction, Characterstics and Difference

Introduction

Energy flow in ecosystem 

Life is propelled by energy. The energy cycle is based on the movement of energy across various trophic levels in an ecosystem. Our ecosystem is sustained by the cycling of energy and nutrients from many external sources. Primary producers at the first trophic level utilize solar energy to make organic material via photosynthesis. The food chain and food web facilitate the movement of energy. Plants collect sunlight with the aid of chloroplasts during the process of energy flow in the ecosystem, and a part of it is turned into chemical energy in the process of photosynthesis. 

Herbivores of the second trophic level consume plants as food, which provides them with energy. A considerable part of this energy is spent by these animals for metabolic tasks such as breathing, digesting food, sustaining tissue development, maintaining blood circulation, and body temperature. Carnivores at the next trophic level feed on herbivores to get energy for sustenance and development. When huge predators are present, they represent a higher trophic level and feed on carnivores to obtain energy. As a result, different plant and animal species are linked to one another via food chains. 

Photosystem  

A photosystem is a protein complex or a combination of two or more proteins that are required for photochemistry. Photosystem I and Photosystem II are the two photosystems. Photosystem II is the first to undergo photosynthesis’s light-dependent processes. The protein structures in plant chloroplasts that absorb light energy are known as photosystems.  

Photosystem I (PS I)  

Photosystem I (PS I) is a multi-subunit protein complex found in the thylakoid membranes of green plants and algae, where it begins one of the earliest stages in the conversion of solar energy via light-driven electron transport. Photosynthesis light absorption processes take place in large protein complexes known as photosystems. P700, a chlorophyll dimer with an absorption peak at 700 nm, is found in Photosystem I. 

An antenna complex is used by Photosystem I to capture light energy for the second step of non-cyclic electron transport. It takes energetic electrons from the first stage process, which is driven by Photosystem II and utilizes light energy to further raise the energy of the electrons toward the eventual aim of giving energy to the Calvin cycle in the form of reduced coenzymes. 

Photosystem II (PS II) 

The light reaction takes place in two photosystems (units of chlorophyll molecules). Light energy absorbed by photosystem II induces the creation of high-energy electrons, which are transmitted to photosystem I through an electron transport chain composed of acceptor molecules. Water molecules are divided into hydrogen ions (H⁺) and oxygen atoms by Photosystem II, which derives replacement electrons from them. When oxygen atoms unite and are released into the atmosphere, they combine to create molecule oxygen (O₂). 

Hydrogen ions are discharged into the lumen. Electron acceptor molecules pump more hydrogen ions into the lumen. This results in a high ion concentration inside the lumen. The transport of hydrogen ions back across the photosynthetic membrane supplies the energy required to promote the production of the energy-dense molecule adenosine triphosphate (ATP). 

High-energy electrons are needed to fuel the production of nicotine adenine dinucleotide phosphate, which are released as photosystem I absorbs light energy (NADPH). The electron transport chain provides replacement electrons to Photosystem I. ATP supplies the energy, whereas NADPH provides the hydrogen atoms required to power the subsequent photosynthetic dark reaction, also known as the Calvin cycle.  

Characteristics of photosystem 

Photosystem I and photosystem II are the two types of photosystems that absorb light energy. Chlorophyll is the primary pigment in both photosystems. 

Characteristics of photosystem (PS) I: 

• It is found on both the appressed and non-appressed parts of grana thylakoids. 
• PS1 has greater chlorophyll a, less chlorophyll b, and fewer carotenoids. 
• It has the ability to absorb light with wavelengths greater than 680 nm. 
• It is denoted as P700. 
• It is involved in both cyclic and non-cyclic photophosphorylation. 
• The primary function is the production of ATP. 

Characteristics of photosystem (PS) II: 

• It is found on the inside of the grana thylakoid membrane. 
• It can absorb light with wavelengths as low as 680 nm. 
• It is denoted as P680. 
• It only takes part in non-cyclic photophosphorylation. 
• The primary functions are ATP production and water hydrolysis.  

Difference between PS I and PS II 

Photosystem (PS) I: 

• The photo center is P700.  
• PS1 occurs on the leaf’s grana thylakoid membrane’s outer surface. 
• It is rich in chlorophyll a and chlorophyll b. 
• It has nothing to do with water photolysis.  

Photosystem (PS) II:

• The photo center is P680. 
• PS2 occurs at the inner surface of the leaf’s grana thylakoid membrane.  
• It is richer in chlorophyll b than chlorophyll a. 
• It has something to do with water photolysis. 

 Summary

• A photosystem is a protein complex or a combination of two or more proteins that are required for photochemistry.
• Photosystem I and Photosystem II are the two photosystems.
• PS II is the first to undergo photosynthesis’s light-dependent processes.
• PS I is a multi-subunit protein complex that begins one of the earliest stages in the conversion of solar energy via light-driven electron transport .
• PS I is the light energy complex that drives the cyclic electron transport mechanism in some photosynthetic prokaryotes.
• Light energy absorbed by PS II induces the creation of high-energy electrons, which are transmitted to PS I through an electron transport chain.
• Water molecules are divided into hydrogen ions (H+) and oxygen atoms by PS II, which derives replacement electrons from them.
• Molecular oxygen (O2) is formed when oxygen atoms combine and are discharged into the environment.
• Hydrogen ions are discharged into the lumen. Electron acceptor molecules pump more hydrogen ions into the lumen.
• This results in a high ion concentration inside the lumen. •
• The transport of hydrogen ions back across the photosynthetic membrane supplies the energy required to promote the production of ATP.