What is a Light-Dependent Reaction?
A light-dependent reaction or light reaction is a chemical reaction that takes place in the presence of light. It is also known as ‘photolysis,’ meaning occurring in the presence of light. For example, plants use sunlight to prepare their food through photosynthesis, which is discussed in this article.
Photosynthesis occurs in two phases: 1) in the presence of light, called light-dependent reaction, and 2) in the absence of light, called light-independent reaction or dark reaction, also known as the Calvin cycle. The light energy is trapped by a pigment in the chloroplast and converted into chemical energy. During this process, usable molecules like ATP and NADPH are generated as energy-carriers. The pigment is called chlorophyll and comes in two forms – chlorophyll a and chlorophyll b.
Where do Light-Dependent Reactions Occur?
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.
What Happens in the Light Dependent Reaction?
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.
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.
2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP
- ADP + Pi
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.
Difference between Light-dependent and Independent Reactions
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|
Ans. No. As the name suggests, the light-dependent reaction needs light to proceed and cannot occur in darkness.
Ans. The main similarity between light-dependent and light-independent reactions of photosynthesis is that they both occur in the chloroplast.
Article was last reviewed on Thursday, November 12, 2020