Question

Assertion: Each molecule of ribulose-1, 5 -bisphosphate fixes one molecule of \(\mathrm{CO}_{2}\). Reason: Three molecules of NADPH and two ATP are required for the fixation of one molecule of \(\mathrm{CO}_{2}\)

Short answer

The assertion that each molecule of Ribulose-1,5-bisphosphate helps in fixing one molecule of CO2 is correct, as this is exactly how CO2 fixation (the core process of the Calvin-Benson cycle) operates. Furthermore, the reason that 'Three molecules of NADPH and two ATP are required for the fixation of one molecule of CO2' is also correct because these molecules provide the necessary energy for this process.

Step-by-step solution

Understand the roles of CO2 and Ribulose-1,5-biphosphate

CO2 is the source of carbon in plants which is taken from the air and is fixed through photosynthesis. Ribulose-1,5-bisphosphate is a 5-carbon compound in plants that captures CO2 during photosynthesis and starts the carbon fixation cycle, also known as the Calvin-Benson cycle.

Understand the role of NADPH and ATP

NADPH and ATP are molecules that store energy. In photosynthesis, the light-dependent reactions generate NADPH and ATP, which are then utilized in the Calvin-Benson cycle for CO2 fixation.

Evaluate the Assertion

Given their roles, it can indeed be said that every molecule of Ribulose-1,5-bisphosphate helps in fixing one CO2 molecule. This is because in the Calvin-Benson Cycle, CO2 gets fixed to Ribulose-1,5-bisphosphate to start the cycle.

Evaluate the Reason

In the fixation of each CO2 molecule, several ATP and NADPH molecules are needed to provide the required energy. Therefore, the reason that 'Three molecules of NADPH and two ATP are required for the fixation of one molecule of CO2' is accurate.

Key concepts

Calvin-Benson Cycle

The Calvin-Benson cycle, also commonly referred to as the dark reactions or the light-independent reactions of photosynthesis, plays a pivotal role in converting atmospheric carbon dioxide into glucose, which plants use as an energy source. Unlike the light reactions that require sunlight to produce ATP and NADPH, the Calvin-Benson cycle occurs in the stroma of chloroplasts and does not directly depend on light.

During this cycle, each CO2 molecule is attached to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), in a process catalyzed by the enzyme RuBisCO. This results in a six-carbon compound that is immediately split into two three-carbon molecules of 3-phosphoglycerate (3-PGA). These molecules are then converted into glyceraldehyde-3-phosphate (G3P) via a series of reactions using ATP and NADPH produced in the light reactions.

One important aspect to note is that to produce one G3P molecule, the cycle must take place three times, fixing three CO2 molecules. And since G3P is used to regenerate RuBP and create glucose, continuous operation of this cycle is critical for the production of carbohydrates necessary for plant growth and energy storage.

Globally, the Calvin-Benson cycle is a fundamental biological process, as it represents the primary path by which inorganic carbon enters the biosphere, ultimately supporting most of the life on our planet.

Ribulose-1,5-bisphosphate

Ribulose-1,5-bisphosphate (RuBP) is at the heart of the carbon fixation process during photosynthesis. This molecule is an essential component of the Calvin-Benson cycle, acting as the CO2 acceptor that initiates the carbon fixation pathway. RuBP is a five-carbon sugar that is regenerated continuously within the cycle to ensure a constant supply for CO2 fixation.

When CO2 is absorbed by plants, it diffuses into the stroma of the chloroplast where it encounters RuBP. The enzyme RuBisCO facilitates the carboxylation reaction, vigorously combining CO2 with RuBP. This reaction is crucial as it forms the unstable six-carbon intermediate, which quickly breaks down into two molecules of 3-phosphoglycerate (3-PGA), each containing three carbon atoms.

Because RuBP is so central to the Calvin-Benson cycle, its availability can significantly influence the rate of photosynthesis. A shortage in RuBP would slow down the cycle, resulting in less glucose production and overall energy conversion efficiency. On the other hand, an efficient regeneration of RuBP from the products of the cycle promotes a quicker turnover and higher photosynthetic capacity.

NADPH and ATP in Photosynthesis

In the grand scheme of photosynthesis, NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate) are the primary energy carriers. These molecules are produced during the light-dependent reactions where light energy is converted into chemical energy.

NADPH is an electron carrier involved in the reduction stages of the Calvin-Benson cycle, particularly in converting 3-PGA to G3P. It provides the necessary high-energy electrons that drive the synthesis of glucose from CO2.

ATP, on the other hand, is a sort of energy 'currency' for the cell. In the Calvin-Benson cycle, ATP is required for various steps, including the transformation of 3-PGA into G3P and the regeneration of RuBP, ensuring that the cycle can proceed.

The quantity of NADPH and ATP used in this cycle is substantial: to fix each CO2 molecule, the cycle consumes two molecules of ATP and one molecule of NADPH. Considering that three rounds of the cycle are needed to produce a single G3P molecule, which leads to six ATP and three NADPH molecules being required per G3P. This large energy investment underscores why the efficient generation of these molecules during the light reactions is crucial for sustaining the energy-intensive process of carbon fixation in plants.