Light-Dependent Reaction

The light-dependent reaction takes place within specialized membrane discs of the chloroplast, known as thylakoid. Four major protein complexes are located in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase that works together in carrying out the light reaction in plants.

The entire process occurs in three significant steps or stages.

Step 1: Excitation of Photosystems with Light Energy and Photolysis of Water

The function of the light-dependent reaction is to convert light energy into chemical energy within a multi-protein complex called the photosystem, found in the thylakoid membranes. There are two types of photosystems found in most plants: photosystem I (PSI) and photosystem II (PSII). Each photosystem is made of two components: 1) antenna complex that consists of 300-400 chlorophyll a and b molecules and other accessory pigments such as carotenoids and 2) reaction center that consists of one or more chlorophyll molecules with a primary electron acceptor. PSI was the first photosystem to be discovered and absorbs maximum light of wavelength 700 nm. Therefore, it is also known as P700. The second photosystem, PSII, absorbs maximum light of wavelength 680 nm and thus, called P680.

The light reaction starts when a photon, ‘packet of light,’ reaches the antenna pigments of photosystem II, which is then transferred to the reaction center. The electron in chlorophyll a molecule, present within the reaction center, is excited and released to the next carrier protein for transport on absorbing a photon. To replace the electron lost in the chlorophyll, a molecule of water splits into two atoms of hydrogen and one atom of oxygen. The two hydrogen atoms lose two electrons producing H+ ions in the thylakoid lumen of the chloroplast. The replacement of the electron enables chlorophyll to respond to another photon. The oxygen gas exits the leaf through the stomata and is released into the environment.

Step 2: Generation of ATP by Electron Transport Chain

The electrons released from photosystem II enter a chain of proteins known as electron transport chain (ETC).  They move from PSII to a small lipid-soluble molecule, plastoquinone (Pq), and then to a protein complex called cytochrome b6f. The electrons are finally transferred to a copper-containing protein called plastocyanin (Pc) before being accepted by PSI. As the electrons flow between PSI and PSII, they lose energy to translocate H⁺ ions from the stroma into the thylakoid lumen. Since they have lost energy before arriving at PSI, the electrons are re-energized at PSI to synthesize the reducing agent, called NADPH. This process allows the absorption of another photon by the PSI antenna pigments.

The accumulation of H⁺ ions in the thylakoid lumen creates a concentration gradient of H⁺ ions across the membrane, thus generating a force known as the proton motive force. A transmembrane protein called ATP synthase helps the H⁺ ions to return to the stroma, producing ATP from ADP + Pi. The flow of H⁺ ions from the lumen, an area of high concentration, to the stroma, an area of low concentration, of chloroplast through ATP synthase is known as chemiosmosis. This process of ATP synthesis is known as photophosphorylation, as light provides the energy to carry out the procedure.

Step 3: Formation of NADPH

This stage is the final step of the light-dependent reaction during which high energy electrons released from PSI travel a short second leg of the electron transport chain. Here, the electrons are first transferred to an iron-containing protein called ferredoxin (Fd) and then to a reducing agent, NADP, to form NADPH.

This type of electron transport involving both PSI and PSII is called non-cyclic photophosphorylation. It is named so because the electrons flow in a single direction and, after losing from PSII, do not return to the same photosystem. This predominant type of electron transport in plants is also called the Z-scheme.

Alternative Pathway

Sometimes plants follow an alternative path of electron transport called cyclic photophosphorylation. This term is named so because electrons released from PSI move along a circular path before returning to the same photosystem. Cyclic photophosphorylation does not involve PSII and produces only the ATP, stopping the production of NADPH.

Chemical Equation

2H₂O + 2NADP+ + 3ADP + 3Pi → O₂ + 2NADPH + 3ATP

Reactants

• H₂O
• NADP
• ADP + Pi

End Products

• O₂
• NADPH
• ATP

Fate of the Products

The energy-carrier molecules, ATP, and NADPH produced in the light reaction are used in the second phase of photosynthesis or the Calvin cycle to assemble sugar molecules.

• Plants absorb energy from the sun during photosynthesis.
• A photon of light strikes the antenna of photosystem II (PSII) and reaches the reaction center.
• Water splits up into two hydrogen atoms, which then become ions (H⁺) releasing two electrons. Oxygen gas is also released.
• The electrons flow from PSII to PSI and lose energy to translocate the H⁺ ions from the stroma into the thylakoid lumen.
• Hydrogen ions flow through ATP synthase via chemiosmosis to form molecules of ATP.
• Photosystem I releases electrons, which results in the formation of an NADPH molecule.

The following table compares and contrasts light-dependent and independent reactions.

Basis	Light Dependent Reaction	Light Independent or Dark Reaction
Occurrence	Takes place only in the presence of light	Can take place in both the presence and absence of light
Location	Grana of chloroplast	Stroma of chloroplast
End Product(s)	ATP and NADPH	Glucose