12 Difference Between Photosystem I And Photosystem II (With Diagram)

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Light Reaction Process

The light reaction of photosynthesis. The light reaction occurs in two photosystems (units of chlorophyll molecules). Light energy (indicated by wavy arrows) absorbed by photosystem II causes the formation of high-energy electrons, which are transferred along a series of acceptor molecules in an electron transport chain to photosystem I. Photosystem II obtains replacement electrons from water molecules, resulting in their split into hydrogen ions (H+) and oxygen atoms. The oxygen atoms combine to form molecular oxygen (O2), which is released into the atmosphere. The hydrogen ions are released into the lumen.

Additional hydrogen ions are pumped into the lumen by electron acceptor molecules. This creates a high concentration of ions inside the lumen. The flow of hydrogen ions back across the photosynthetic membrane provides the energy needed to drive the synthesis of the energy-rich molecule adenosine triphosphate (ATP). High-energy electrons, which are released as photosystem I absorbs light energy, are used to drive the synthesis of nicotine adenine dinucleotide phosphate (NADPH). Photosystem I obtains replacement electrons from the electron transport chain. ATP provides the energy and NADPH provides the hydrogen atoms needed to drive the subsequent photosynthetic dark reaction, or Calvin cycle.

PHOTOSYNTHESIS

What Is Photosystem I?

Photosystem I (PSI, or plastocyanin-ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein-complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the high energy carrier NADPH. The combined action of the entire photosynthetic electron transport chain also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II.

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What You Need To Know About Photosystem I

  • Photosystem I is located on the outer surface of the thylakoid membrane towards stroma.
  • Photosystem I is present in unstacked thylakoid membrane and causes light induced reduction of NADP+.
  • Photosystem I has an iron-sulphur type reaction centre.
  • Photosystem I is involved in both cyclic and non-cyclic photophosphorylation.
  • Molecular oxygen is not evolved and Photolysis of water does not occur in Photosystem I.
  • The main function of Photosystem I is NADPH synthesis.
  •  Released high energy electrons are replaced by the releasing energy of photolysis.
  • The reaction center of Photosystem I is made up of P700.
  • The core of Photosystem I is made up of psaA and psaB subunits.
  • Photosystem I contains chlorophyll B, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700 and carotenoids.
  • It receives electrons from Photosystem II.
  • The pigments in Photosystem I absorb longer wavelengths of light which is 700 nm (P700).

What Is Photosystem II?

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen.

By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP+ to NADPH or are used in non-cyclic electron flow. DCMU is a chemical often used in laboratory settings to inhibit photosynthesis. When present, DCMU inhibits electron flow from photosystem II to plastoquinone.

What You Need To Know About Photosystem II

  • Photosystem II is located on the inner surface of the thylakoid membrane.
  • Photosystem II is predominantly present in stacked thylakoid membrane.
  • Photosystem II has a quinine type reaction centre.
  • Photosystem II is only involved in cyclic photophosphorylation.
  • Molecular oxygen is evolved and Photolysis of water occurs in Photosystem II.
  • The main function of the Photosystem II is ATP synthesis and hydrolysis of water.
  • Released high energy electrons are replaced by the electrons released from Photosystem II.
  • The reaction center of Photosystem II is made up of P680.
  • The core of Photosystem II is made up of D1 and D2 subunits.
  • Photosystem II contains chlorophyll B, chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, phycobilins and xanthophylls.
  • It receives electrons from photolytic reaction.
  • The pigments in the Photosystem II absorb shorter wavelengths of light which is 680 nm (P680).

Difference Between Photosystem I And Photosystem II In Tabular Form

BASIS OF COMPARISON PHOTOSYSTEM I PHOTOSYSTEM II
Description Photosystem I is located on the outer surface of the thylakoid membrane towards stroma.   Photosystem II is located on the inner surface of the thylakoid membrane.  
Presence Photosystem I is present in unstacked thylakoid membrane and causes light induced reduction of NADP+.   Photosystem II is predominantly present in stacked thylakoid membrane.  
Reaction Center Type Photosystem I has an iron-sulphur type reaction centre.   Photosystem II has a quinine type reaction centre.  
Photophosphorylation Photosystem I is involved in both cyclic and non-cyclic photophosphorylation.   Photosystem II is only involved in cyclic photophosphorylation.  
Photolysis Of Water Molecular oxygen is not evolved and Photolysis of water does not occur in Photosystem I.   Molecular oxygen is evolved and Photolysis of water occurs in Photosystem II.  
Electron Replacement Released high energy electrons are replaced by the releasing energy of photolysis.   Released high energy electrons are replaced by the electrons released from Photosystem II.  
Main Function The main function of Photosystem I is NADPH synthesis.   The main function of the Photosystem II is ATP synthesis and hydrolysis of water.  
Reaction Center The reaction center of Photosystem I is made up of P700.   The reaction center of Photosystem II is made up of P680.  
Core The core of Photosystem I is made up of psaA and psaB subunits.   The core of Photosystem II is made up of D1 and D2 subunits.  
Chlorophyll Photosystem I contains chlorophyll B, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700 and carotenoids.   Photosystem II contains chlorophyll B, chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, phycobilins and xanthophylls.  
Pigments The pigments in Photosystem I absorb longer wavelengths of light which is 700 nm (P700).   The pigments in the Photosystem II absorb shorter wavelengths of light which is 680 nm (P680).