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. 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.

light and dark reaction

Light reaction

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) and 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.

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.

 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. This 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.

Photosynthesis is a complex process which is carried out through various stages. For more details and videos on this topic, come and join Byju’s- The Learning app or visit our website

Practise This Question

Microtubules are absent in