Biosynthetic Phase - NEET MCQ

Carbonation is the addition of carbon dioxide to an acceptor. Photosynthetic carboxylation requires ribulose-1, 5-biphosphate or RuBP as acceptor of carbon dioxide and RuBP- Carboxylase oxygenase as enzyme in C₃ plants In C₄ plants, initial fixation of CO₂ occurs in mesophyll cells, The primary accept or of CO₂ is phosphoenol pyruvate which combines with CO₂ in presence of PEP carboxylase to form oxaloacetic acid or oxaloacetate. So, the correct answer is 'RuBP carboxylase and PEP carboxylase'.

NCERT Topic: The primary acceptor of CO2

NCERT Line: "Carboxylation is the fixation of CO2 into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA."

Biosynthetic phase (dark or Blackman's reaction) catalyses assimilation of CO2 to carbohydrates. The reactions are called carbon reactions. They occur in stroma of chloroplasts. The reactions do not require light. Instead assimilatory power (ATP and NADPH ) produced during photo chemical phase is used in fixation and reduction of carbon dioxide. All the enzymes required for the process are present in the matrix or stoma of the chloroplast. So, the correct answer is 'it does not directly depend on light energy'.

NCERT Topic: Dark reactions in photosynthesis

NCERT Line: "However, this should not be construed to mean that they occur in darkness or that they are not light-dependent."

In C₃ cycle for the fixation of every carbon dioxide molecule, the reduction and regeneration of steps require around 3ATP & 2 NADPH2 molecules.
So, the correct option is '3 ATP and 2NADPH₂'.

NCERT Topic: Reduction and regeneration steps

NCERT Line: "Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required."

In C₃ plants, Rubisco is located in the stroma of all chloroplasts.
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is a bifunctional enzyme that catalyses both the carboxylation and oxygenation of ribulose-1,5-bisphosphate (RuBP), thus fixing carbon dioxide as the first step of the Calvin cycle.
The enzyme RuBisCO carries out photosynthetic carboxylation of RuBP. The enzyme was previously called as carboxydismutase.
Hence, the correct option is Option B.

Calvin used the radioactive isotope of carbon (14C) of Chlorella. He observed that after second exposure of Chlorella in presence of 14 CO2, the radioactive isotope of carbon was found mainly in 3-phosphoglyceric acid (PGA). This clearly indicated that the three carbon compound 3- phosphoglyceric acid was the first stable product photosynthesis and the carbon from the CO2 is fixed in the preparation of the photosynthates.
Hence, the correct answer is 'PGA'.

NCERT Topic: Calvin cycle

NCERT Line: "The first product identified was 3-phosphoglyceric acid or in short PGA."

In fixation, the first stage of the Calvin cycle, light-independent reactions are initiated; CO₂ is fixed from an inorganic to an organic molecule. In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP⁺, respectively.

NCERT Topic: Reduction

NCERT Line: "Reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH."

Calvin cycle begins with fixation of CO₂ into 3 C sugar by the process of carboxylation. RUBISCO, present in the chloroplast stroma catalyzes carboxylation of ribulose-1, 5-biphosphate (RuBP) followed by immediate splitting of unstable product into 3-phosphoglyceric acid. Carboxylation is followed by a series of reduction reactions wherein 3 C compound 3-phosphoglyceric acid is reduced into 3 C compounds glyceraldehydes 3-phosphate (G3P); the reduction reactions are energized by ATP and NADPH serve as an electron donor. Three turns of Calvin cycle obtains 6 molecules of G3P, out of which one is used for glucose formation and rest are used in regeneration phase.  The last phase of Calvin cycle, the regeneration phase ensures the continuous supply of RuBP for carboxylation and regenerates it using G3P. Thus, the correct answer is option D.

NCERT Topic: Calvin cycle stages

NCERT Line: "The Calvin cycle proceeds in three stages: (1) carboxylation, during which CO2 combines with ribulose-1,5-bisphosphate; (2) reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and (3) regeneration during which the CO2 acceptor ribulose-1,5-bisphosphate is formed again so that the cycle continues."

Melvin Calvin, a biochemist who detected the crucial phase of photosynthesis that now bears his name and won him a Nobel Prize in 1961, died on Wednesday at Alta Bates Hospital in Berkeley, Calif. He was 85.
So, the correct option is 'Calvin'.

NCERT Topic: Melvin Calvin

NCERT Line: "The mapping of the pathway of carbon assimilation in photosynthesis earned him Nobel Prize in 1961."

Carbon dioxide fixation occurs during Calvin cycle. There are three stages in the process of Clavin cycle as carboxylation, reduction, and regeneration. One turn of Calvin cycle fixes one molecule of carbon dioxide. There are 6 atoms of carbon required for the synthesis of one glucose molecule. Hence, to produce one molecule of hexose sugar, six turns of Calvin cycle are required. Thus, the correct answer is option A.

NCERT Topic: Calvin cycle turns for glucose generation

NCERT Line: "To make one molecule of glucose 6 turns of the cycle are required."

Melvin Calvin was the professor of chemistry. He was awarded the nobel prize for chemistry for the discovery of various pathways of photosynthesis. 3-phosphoglyceric acid as first CO₂ fixation product in algal photosynthesis was discovered by Calvin. He used radioactive 14C in algal photosynthetic studies. This led to the discovery that the first CO₂ fixation product was a three-carbon organic acid. He also helped to mark out the complete biosynthetic pathway, hence it is named as Calvin Cycle. The first stable product identified was 3-phosphoglyceric acid (PGA), hence it is named as C₃ pathway. Calvin cycle occurs in all photosynthetic plants whether they have C₃ or C₄ pathway.

NCERT Topic: Melvin Calvin

NCERT Line: "Melvin Calvin... received his Ph.D. in Chemistry from the University of Minnesota. He served as Professor of Chemistry at the University of California, Berkeley."