Botany MCQs for NEET — Practice Questions with Answers

The first reaction of photorespiration occur in stroma of chloroplast. In this reaction the RuBP (Ribulose 1-5 biphosphate) consume one oxygen molecule in presence of Rubisco. 

In peroxisome the glycolate transfered from chloroplast takes up O₂ and formed the glyoxylate whereas the H₂O₂ release as byproduct.

Photorespiration is a type of respiration which occurs in small organelles called peroxisomes, and because it occurs only in light, it is called photorespiration. In contrast to 'normal respiration' or 'dark respiration' which occurs in the mitochondria, no ATP or reduced NAD is produced by photorespiration. The energy released being lost as heat. Under conditions of low carbon dioxide and high oxygen concentration which are normal atmospheric conditions, as much as 30-50 percent of the photosynthetically fixed carbon may be lost as carbon dioxide by photorespiration and the energy that was expanded to fix it is wasted to the plant. 

In photosystem II there is a complex of proteins with Mn²⁺, known as the water splitting centre, which extracts the electrons from water with the release of oxygen and protons and passes then to P₆₈₀ molecules restoring them to ground state. In this process manganese ions have a special role to play but chloride and calcium ions are also helpful.

In photorespiration uptake of oxygen and evolution of carbon dioxide are light dependent. RuBP carboxylase acts as RuBP oxygenase. Photorespiration occurs in chloroplast and requries the help of peroxisomes and mitochondria.

 Photosynthetic bacteria do not contain quantasome. Photosynthetic bacteria are of two types oxygenic (cyanobacteria) and anoxygenic (H2S donor like purple sulphur and green sulfur bacteria and organic donor like purple non-sulfur and green non-sulfur bacteria). Bacteria only have one photosystem i.e. type I reaction centre (PS I - 700) in green sulphur bacteria and type II reaction center (PS II - 680) in non sulphur bacteria.

Photorespiration in C3 plants starts with the formation of phosphoglycolate from ribulose-1,5-bisphosphate (RuBP) when oxygen instead of carbon dioxide binds to the enzyme RuBP carboxylase/oxygenase (RuBisCO). This phosphoglycolate is then dephosphorylated to glycolate, initiating the photorespiratory pathway.

The Hill reaction is a light-driven oxidation of water that occurs in the presence of an artificial electron acceptor like potassium ferricyanide or benzoquinone. It was discovered by Robert Hill in 1937 and is used to study the light-dependent reactions of photosynthesis in isolation.

Cyclic photophosphorylation is a process pf photophosphorylation in which an electron expelled by the excited photocentre is returned to it after passing through a series of electron carriers. Cyclic photophosphorylation is performed by photosystem I only. The electron is circulated with in the photosystem and the phosphorylation occurs due to cyclic flow of electron. The excited electron does not pass on to NADP⁺ but it is cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H⁺.

Clavin pathway occurs in all photosynthetic plants which is CO₂ fixation cycle. Carbon dioxide combines with ribulose- 1, 5, biphosphate (RuBp) to produce a transient intermediate compound. The intermediate compound splits up immediately in the presence of water to form the two molecules of 3-phosphoglycerate or 3-PGA. It is the first stable product of photosynthesis.

Photosystem II mostly occurs in the appressed regions of granal thylakoids.