Answer :
The true statement regarding the Calvin cycle is that the enzyme rubisco combines carbon dioxide with RuBP to form a six-carbon unstable molecule, which immediately splits into two 3-PGA molecules. Hence, option A) the enzyme rubisco combines carbon dioxide with RuBP to form a six-carbon unstable molecule is the correct answer.
The true statement regarding the Calvin cycle is that the enzyme rubisco combines carbon dioxide with RuBP to form a six-carbon unstable molecule which immediately splits into two three-carbon molecules of 3-PGA. Further steps involve the reduction of 3-PGA into glyceraldehyde-3-phosphate (G3P) using NADPH and ATP supplied by the light-dependent reactions, which is then used to regenerate RuBP and to form glucose and other sugars. Hence, statements 1, 2, and 3 are correct. Additionally, part of the G3P produced in the cycle is indeed used to regenerate RuBP, and another part is used in the synthesis of glucose and other carbohydrates. This makes statement 4 correct as well.
The Calvin cycle is a key component of photosynthesis, taking place in the stroma of chloroplasts. It is a light-independent process, meaning it does not require sunlight to occur, although it relies on ATP and NADPH produced by the light-dependent reactions. The cycle operates as a metabolic pathway and is essential for carbon fixation, allowing plants to convert atmospheric CO2 into energy-rich organic compounds like glucose.
During the cycle, one molecule of carbon dioxide is added to a five-carbon RuBP molecule by RuBisCO (ribulose bisphosphate carboxylase/oxygenase), the most abundant enzyme on Earth. This leads to the formation of an unstable six-carbon intermediate that breaks down into two molecules of 3-PGA. Then, through a sequence of enzymatic reactions, 3-PGA is phosphorylated by ATP and reduced by NADPH to form G3P. A portion of the G3P molecules can be combined to form glucose and other sugars, while a portion is used to regenerate the RuBP substrate, allowing the cycle to perpetuate. Hence, option A) the enzyme rubisco combines carbon dioxide with RuBP to form a six-carbon unstable molecule is the correct answer.