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What Is Light Dependent Reaction?

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Green plants, synthesize the food they need, and all other organisms depend on them for their needs. Green plants carry out photosynthesis, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds. Photosynthesis may be defined as the synthesis of carbohydrates by the green organs of a plant in the presence of sunlight from CO2 and H2O. Photosynthesis is a sensitized physiochemical oxidation and reduction mechanism, in which water is oxidized and CO2 is reduced. Carbon and oxygen present in the carbohydrates are derived from carbon dioxide and hydrogen from water. Oxygen evolved In photosynthesis comes from water and not from CO2. Oxygen present in product water comes from CO2, and hydrogens of it come from substrate water. About 170 billion tonnes of dry matter in terms of carbon are produced by this process, of this 55 billion tonnes are formed in the oceans annually. Photosynthesis is the only natural process by which oxygen is liberated into the atmosphere. Chlorophyll (green pigment of the leaf), light, and CO2 are required for photosynthesis to occur.

Light Reaction

The light reaction needs light to produce organic energy-rich molecules (ATP and NADPH) Which are initiated mainly by green-colored pigment chloroplast. Light reaction or the photochemical phase includes light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. Several complexes are involved in the process. The light-absorbing pigments are located in the thylakoid membranes. Stroma lamellae lack NADP reductase and PS2, and the outer walls and margins of end thylakoids of grana represent non-appressed regions and the remaining grana thylakoids’ membranes represent appressed regions. There are protein complexes in thylakoid membranes to which pigment molecules are attached.


There are two types of photosystems in the thylakoid membrane. Photosystem 2 (PS2) functions first (the number reflects the order of discovery) and is best at absorbing a wavelength of 680 nm. The reaction center chlorophyll of PS2 is Called P680. Photosystem 1 (PS1) is best at absorbing a wavelength of 700 nm. The reaction center chlorophyll of PS1 is called P700. The thylakoid membranes of chloroplasts have two kinds of Photosystems, each with its own molecules. The pigments are organized into two discrete photochemical light-harvesting complexes (LHC) within Photosystem 1 (PS1) and Photosystem 2 (PS2). These are names in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC is made up of hundreds of pigment molecules bound to proteins. Each Photosystem has all the pigments (except one molecule of chlorophyll a) forming a light-harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll molecule forms the reaction center. The reaction center is different in both the photosystems. In PS1 the reaction center chlorophyll a had an absorption peak at 700 nm, hence is called P700, while in PS2 it has absorption maxima at 680 nm, and is called P680. All oxygen-evolving photosynthetic organisms contain Chl-a and two types of photosystems (PS1 and PS2). Photosynthetic bacteria which do not release oxygen lack Chl-a and PS2.

Red Drop & Emerson’s Effect 

Emerson and his co-workers exposed chlorella plants to only one wavelength of light at a time and measured the quantum yield. Such light with one wavelength is called monochromatic light. He plotted a graph of the quantum yield in terms of O² evolution at various wavelengths of visible light. He made this study to determine the wavelength of visible light. Furthermore, he made this study to determine the wavelength at which the photochemical yield of oxygen was maximum. The yield was almost constant in the region of 600-680 nm (red region), they fall in photosynthetic yield beyond the red region of spectrum Is called Red Drop or Emerson’s first effect. This is assumed due to non-functioning PS-2. Emerson and his co-workers modified the previous experiments by supplying shorter wavelengths of light (red light) along with longer wavelengths of light (far-red light beyond 680 nm). They found that the monochromatic light of longer wavelength (far-red light) supplemented with a shorter wavelength of light (red light), enhanced the photosynthetic yield in comparison to sum of yield in comparison to the sum of the yield when two photosystems operate independently. This led to the concept of two photosystems. This enhancement of photosynthetic yield is referred to as Emerson’s enhancement effect or Emerson’s second effect. The number of photons (quanta) required to release one molecule of oxygen during photosynthesis is called the quantum requirement (8 quanta). The number of oxygen molecules released per photon in photosynthesis is called quantum yield. One oxygen molecule is released per eight photons of light absorbed, hence quantum yield is 1/8=0.025.

Key Points 

  • The light response traps the energy from the sun and converts it into compound energy that is put away in NADPH and ATP.
  • Oxygen is delivered as a side effect.

Difference between Light Reaction and Dark Reaction

Light Reaction

Dark Reaction

It takes place only in the presence of light It can happen in the presence or nonappearance of daylight
It is a photochemical stage It is a biochemical stage
NADP uses H+ particles to shape NADPH The hydrogen of NADPH consolidates with CO2
The water particles split into hydrogen and oxygen Glucose is delivered. CO2 is used in obscurity response
It happens in the grana of the chloroplast It happens in the stoma of the chloroplast
Photolysis happens in PS-2 Photolysis doesn’t happen
The final results are ATP and NADPH Glucose is the final result. ATP and NADPH help in the arrangement of glucose

Conceptual Questions

Question 1: Which variety of light is chlorophyll generally delicate to?


Blue and Red are the variety to which chlorophylls are generally delicate. Leaves are green thus they mirror the green frequency of white light. The chlorophyll shade most proficiently retains those frequencies which lie in the red and blue light districts of white light.

Question 2: What is LHC? Where is it present?


LHC means Light-Harvesting complex. It is present in photosystem 1 (PS-1) and photosystem 2 (PS-2).

Question 3: Explain the photolysis of water?


The splitting of water into hydrogen, oxygen, and electrons in photosystem 2 (PS-2).

Question 4: What are the retention maxima for the response communities of PS-1 and PS-2?


The photosystem 1 has the absorption maxima of 700 nm, and the photosystem 2 reaction center has the absorption maxima of 680nm.

Question 5: What is the light reaction?


The light response is additionally called the Photochemical stage. It incorporates light assimilation, water parting, oxygen discharge, and the arrangement of high-energy synthetic intermediates (ATP and NADPH).

Question 6: What are the 4 stages of light responses?


Light assimilation in PSII. At the point when light is consumed by one of the many colors in photosystem II, energy is passed internally from one shade to another until it arrives at the response place.

  • ATP combination.
  • Light assimilation in PSI.
  • NADPH development.
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Last Updated : 20 Oct, 2022
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