Light reaction And Dark reaction

Photosynthesis is the process of conversion of light energy into chemical energy which can be utilized by the living organisms. It is a complex process which is carried out through various stages.

Photosynthesis comprises of two phases:

  • The first phase is the photochemical phase or light dependent process. This phase is commonly known as the light reaction.
  • The second phase is the biosynthetic phase or dark reaction of photosynthesis. This phase is a light-independent process.

The whole process of photosynthesis takes place within the chloroplast.

Also Read: Light-dependent reaction

light and dark reaction

Steps Carried out in Light Reaction and Dark Reaction

Light Reaction and Dark Reaction

Light Reaction

The light reaction is a light-dependent process which includes a series of events such as light absorption, hydrolysis, the release of oxygen, formation of ATP and NADPH.

The light reaction of photosynthesis initiates only when it is supplied with light energy.

The photosystem is the arrangement of pigments including chlorophyll within thylakoids.

There are two photosystems in plants:

  • Photosystem I (PS-I)
  • Photosystem II (PS-II)

Photosystem I absorbs light at a wavelength of 700 nm, whereas Photosystem II absorbs light at a wavelength of 680 nm.

The light reaction occurs in the thylakoids of the chloroplast. When the light hits, chlorophyll a get excited to higher energy state followed by a series of reactions. This energy is converted into energy molecules ATP and NADPH by using PS I and PS II.  Also, hydrolysis occurs and releases oxygen.

Also Read: Cyclic and non-cyclic photophosphorylation

Dark Reaction

Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from the carbon dioxide and water molecules.

The dark reaction occurs in the stroma of the chloroplast where they utilize the products of the light reaction.

Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin cycle.

In the Calvin cycle, the ATP and NADPH formed during light reaction drives the reaction and convert 6 molecules of carbon dioxide into one sugar molecule i.e. glucose.

Difference between Light and Dark Reaction

Following are the important difference between light and dark reaction:

Light Reaction

Dark Reaction

It takes place only in the presence of light.

It can take place in the presence or absence of sunlight.

It is a photo-chemical phase.

It is a biochemical phase.

It takes place in the grana of the chloroplast.

It takes place in the stroma of the chloroplast.

NADP utilizes H+ ions to form NADPH.

The hydrogen of NADPH combines with CO2

The end products are ATP and NADPH.

Glucose is the end product. ATP and NADPH help in the formation of glucose.

The water molecules split into hydrogen and oxygen.

Glucose is produced. Co2  is utilized in the dark reaction.

Photolysis occurs in PS-II.

Photolysis does not occur.

Also Read: Difference between Cyclic and non-cyclic photophosphorylation

For more details on the light reaction, dark reaction and the difference between light and dark reaction, keep visiting BYJU’S website or download BYJU’S app for further reference.


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