Test: Light Reaction & Dark Reaction (June 18) - NEET MCQ

Photosynthesis takes place in the green leaves of plants and other green parts of plants like stem etc. The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts, which contain the specialized light-absorbing green pigments, the chlorophylls. When chlorophyll absorbs light, it gets excited and emits electrons. These chlorophylls are found in photosynthetic units called Photosystem I and Photosystem II. Each unit has a specific reaction centre which contains pigment molecules. These molecules absorb light of different wavelengths and emit electrons. Due to the photon of light, electrons of chlorophyll get excited. These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. The excitement of electrons of chlorophyll b photon of light is the first step of photosynthesis.

The chlorophyll pigments which are excited give up their electrons and to compensate for the loss of electrons, water is split to release four H⁺ ions and four electrons and O². The electrons that are lost from the PSII enter into an electron transfer chain or ETC.

Photosynthesis occurs in two stages- light reaction and dark reaction. During the light, reaction water is oxidized using energy from light and reduced NADPH₂ and ATP are synthesized. The oxidation of water is called photolysis. Oxygen is released as a by-product during the photolysis of water. The NADPH₂ and ATP are used subsequently to reduce carbon dioxide into sugars.

During the dark phase of photosynthesis, carbon dioxide combines with RUBP or RUDP (ribulose bi/di phosphate) in the carboxylation reaction catalyzed by enzyme RUBISCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). The carboxylation of RUBP generates 3 PGA or 3 phosphoglyceric acid, which is the first stable product of dark phase.

3-phosphoglycerate (PGA, a carboxylic acid) is first phosphorylated into 1,3-bisphosphoglycerate by enzyme 3-phosphoglycerate kinase. This is followed by reduction of 1,3-bisphosphoglycerate catalyzed by glyceraldehyde 3-phosphate dehydrogenase enzyme into glyceraldehyde 3-phosphate (aldehyde). NADPH serves as electron donor here. Thus, the correct answer is B.

In the majority of the plants, the first stable intermediate of carbon dioxide reduction is a three-carbon compound. Hence, this type of carbon reduction is called as C3 cycle. In C3 cycle the initial carboxylation is brought about by RuBisCO, which carboxylates Ribulose- 1,5 bisphosphate (RUBP) into phosphoglyceric acid. The phosphoglyceric acid is later on reduced to phosphoglyceraldehyde, which is used to synthesize sugars and recycle RUBP.