Light Reaction: Summary for NEET

To synthesise organic energy molecules, light processes are required (ATP and NADPH). Colored pigments, primarily green chlorophyll, initiate them. The first of two steps in the photosynthesis process, photosystem II and photosystem I, absorb light energy and create ATP, NADPH, and O₂ during light reactions.

The photolysis process is known as the light reaction. It occurs when light is present. The grana of chloroplasts are where it usually happens. An autotroph is an organism with metabolic activities that produce organic molecules by using energy from either inorganic chemicals or light. An organism that uses light energy to create organic compounds from inorganic sources is known as a photoautotroph.

Pigment molecules exist in photosystems. Chlorophyll is one of the major components in plants that carry out an operation in light responses, such as photosynthesis. Carotenoids are one of the additional pigments. Light energy is absorbed by the chlorophyll pigment, which is found in the thylakoid membrane of the chloroplast. This energy is used to generate ATP and NADPH through the electron transport chain (ETC). 

Light Reaction in Photosynthesis

• Photosynthesis is a chemical reaction through which plants, bacteria, and Protista use the energy from sunlight to make carbohydrates, which are then processed into ATP by cellular respiration. This is the "fuel" that all life forms need. 

• Photosynthesis is complete in two stages, which are light-dependent reactions and light-independent reactions (Calvin cycle).

 Photosynthesis in the chloroplast

• The direct energy of light helps the plant make molecules that carry energy for utilisation in the dark phase of photosynthesis in the light-dependent process that occurs in the grana, which are the stacked membrane structures within chloroplasts.

• The plants use light energy to produce the coenzyme, nicotinamide adenine dinucleotide phosphate, or NADPH, and the energy-carrying molecules, ATP. These compounds' chemical bonds store energy, and are used during the dark phase. A pigment is a molecule that is capable of absorbing light energy.
  

• Pigment molecules in thylakoids absorb light and convert it to chemical energy during light reactions.
  

Light Dependent Reactions

Light-dependent photosynthesis refers to reactions that use energy from light and water to produce ATP and oxygen; oxygen gets released into the atmosphere, and the plant utilises ATP to produce sugars through light-independent reactions.

Light-Independent Reactions

• The light-independent reactions come next in the photosynthesis hierarchy. Light-independent reactions are also understood as the Calvin Cycle, named after Melvin Calvin, who revealed these reactions. 

• The products of the light-dependent reaction are used to form carbohydrates during these reactions. Sugar is made up of energy from ATP, electrons from NADPH (both generated by light processes), and carbon from CO₂ (taken in from outside).

• This process of photosynthesis is also known as carbon fixation and is critical for maintaining stable atmospheric carbon dioxide levels.

• The energy outputs from the light reactions (ATP and NADPH) are used to power the conversion of CO₂ into the sugar, G3P, in the Calvin cycle. When ATP and NADPH are consumed, they produce ADP and NADP⁺, which are then returned to the light processes to produce new ATP and NADPH.

What Happens in Light Reaction

Light from the Sun is captured during photosynthesis via a series of reactions, involving the chemical chlorophyll. This results in the synthesis of two high-energy chemical compounds: ATP and NADPH, the latter of which has chemical energy that can be easily transferred to other compounds. This series of reactions necessitates the use of water,  from which oxygen is released during the process. In the following phase of photosynthesis, the dark reactions, ATP and NADPH, are used to produce glucose from Carbon dioxide. 

• The light reaction uses water, light (brought in from outside the cell), NADP⁺, phosphorus, and ADP (all created by the Calvin cycle) to produce ATP (both used in the Calvin cycle), and oxygen (a waste product).

• The light excites the electrons, and the excited electrons’ energy is then used to join ADP and phosphate together to form ATP.

• NADP⁺ then joins with the excited electrons.

• Water is split in the process, and oxygen is generated as waste and is discharged.

• In light reactions, sunlight energy is used to oxidise water (an electron donor) to oxygen, and then to pass these electrons to NADP⁺, resulting in NADPH. The conversion of ADP to ATP requires some light energy. The NADPH and ATP created, are then used to run the Calvin cycle, which produces sugar.
  

Site of Light Reaction

Mesophyll cells are the photosynthetic sites within a plant leaf's interior tissue. Chlorophyll is a green pigment that is found in the chloroplasts of plants, algae, and some bacteria that are photosynthetic.

Chloroplast

• The chloroplast is surrounded by a pair of envelope membranes (inner and outer) that separate the chloroplast's interior from the cell's cytoplasm. 

• The light processes occur inside the chloroplast on the chlorophyll-containing thylakoid membranes. 

• The thylakoid membrane, within the chloroplast, is a membrane that forms many flattened, fluid-filled tubules that encircle a single circuitous chamber. It possesses chlorophyll and serves as the site for photosynthesis' light-dependent reactions.

The Calvin cycle reactions occur in the aqueous stroma, which is located between the inner envelope membrane and the thylakoid membranes. The thylakoid space is located within the thylakoid membranes and is where protons collect during ATP synthesis in light reactions.

Light Dependent Phase of Photosynthesis

Photosystems are complex combinations of chlorophyll as well as other pigments, such as chlorophyll b, xanthophyll, and carotenoids, that capture light energy and use it to energise an electron, which is removed from a water molecule. Photosystems in plants are found within the chloroplast's thylakoid membrane. Photosystem I and photosystem II are the two types of photosystems.

Photosystem II

Between the two photosystems, photosystem II oxidises water and reduces the electron transport chain.

• P680 is oxidised (which oxidises water) in PS II (the first photosystem in the sequence), and the PS II main electron acceptor is reduced (which reduces the electron transport chain between the photosystems).

Photosystem I 

Between the two photosystems, photosystem I reduce NADP⁺ and oxidises the electron transport chain.

• The PS I primary electron acceptor is reduced (which reduces other molecules, reducing NADP⁺ to NADPH), and P700 is oxidised in PS I (which in turn, oxidises the electron transport chain between the photosystems).

Electron Transport Chain

• A series of membrane-bound carriers that transfer electrons from one to the next. 

• It gradually reduces the energy of the excited electrons so that some of it can be captured in the form of ATP

• High-energy electrons are delivered, while low-energy electrons are expelled.

ATP Synthase

• As a result of water splitting and being pumped in from the stroma, H⁺ ions build up within the thylakoid region. 

• The stroma becomes negatively charged in relation to the space within the thylakoids when H⁺ ions accumulate.

•  H⁺ ions are unable to pass through the thylakoid membranes directly.

• In contrast, the thylakoid membrane possesses a protein, called ATP synthase, that crosses the membrane, and permits H⁺ ions to pass through.

• The gradient propels H⁺ ions through ATP synthase, which causes it to rotate.

• As it rotates to make ATP, ATP synthase joins ADP and a phosphate group together. 

Light Reaction Equation 

Water is oxidised, electrons from water are transported to NADP⁺, and ATP is generated during the light processes. Equations can be used to summarise the three elements of the light reaction.

1. Water oxidation is a process that occurs when water is exposed to oxygen.

$2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{O}_{2}+4 \mathrm{e}^{-}+4 \mathrm{H}^{+}$

2. NADP reduction

$2 \mathrm{NADP}^{+}+4 \mathrm{e}^{-}+4 \mathrm{H}^{+} \rightarrow 2 \mathrm{NADPH}+2 \mathrm{H}^{+}$

3. Synthesis of ATP

$\mathrm{ATP} \rightarrow \mathrm{ADP}+\mathrm{Pi}$

The general equation for light reactions is obtained by combining these equations as given below:

$2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{NADP}^{+}+\mathrm{nADP}+\mathrm{nPi}+\mathrm{hv} \rightarrow \mathrm{O}_{2}+2 \mathrm{NADPH}+2 \mathrm{H}^{+}+\mathrm{nATP}$

Conclusion

The light reaction of photosynthesis, which takes place on the chloroplast's thylakoid membranes and transforms solar energy into the chemical energy of ATP and NADPH while also releasing oxygen. These products are used for carbon fixation in plants. This article has given all the useful details about the light reaction with respect to the NEET syllabus. These also help in clearing the doubts about this topic and help in qualifying for the exam.