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Light Dependent Reactions

Light Dependent Reactions

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Light Dependent Reactions

Light Dependent Reactions

What is Light Dependent Reaction?

The series ofbiochemicalreactions in photosynthesisthat requirelight energy that is captured by light-absorbing pigments (such aschlorophyll) to be converted intochemical energyin the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

What is Light Dependent Reaction?

Light Dependent Reaction is one of the two parts of photosynthesis, the other being the Light Independent Reaction (Calvin Cycle). It is the first stage of photosynthesis.

How Light Dependent Reaction Work :

The overall function of light-dependent reactions, the first stage ofphotosynthesis, is to convert solarenergyintochemical energyin the form of NADPH andATP, which are used inlight-independent reactionsand fuel the assembly of sugarmolecules.

Inphotosynthesis, thelight-dependent reactionstake place on thethylakoid membranes. The inside of the thylakoid membrane is called thelumen, and outside the thylakoid membrane is thestroma, where the light-independent reactions take place.

The thylakoid membrane contains someintegral membrane proteincomplexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane:Photosystem II (PSII),Cytochrome b6f complex, Photosystem I(PSI), andATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.

Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways.

These are the non-cyclic and cyclic.

In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH.

Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.

The Light Dependent Reaction Model

The Light Dependent Reaction Model

The Process

ATP and NADPH

ATP (Adenosine triphosphate) is the high energy molecule that stores the energy that we need. It is present in the cytoplasm and nucleoplasm of every cell, and essentially all the physiological mechanisms that require energy for operation.

ATP and NADPH

NADPH (Nicotinamide adenine dinucleotide phosphate) is used to help turn the carbon dioxide into glucose. It is used as reducing power for the biosynthetic reactions in theCalvin cycleto assimilate carbon dioxide.

Key Points

Light energy splits water and extracts electrons in photosystem II (PSII); then electrons are moved from PSII to cytochrome b6f to photosystem I (PSI) and reduce in energy.

Electrons are re-energized in PSI and those high energy electrons reduce NADP+to NADPH.

In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump hydrogen ions from the lumen to the stroma; this energy allows ATP synthase to attach a third phosphate group to ADP, which forms ATP.

In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from both PSII and PSI to create more ATP and to stop the production of NADPH, maintaining the right proportions of NADPH and ATP.