Question

Reduction process of Calvin cycle requires how many ATP and NADPH for the reduction of one molecule of \(\mathrm{CO}_{2} ?\) (a) 2 mole ATP and 3 mole NADPH (b) 2 mole ATP and 2 mole NADPH (c) 1 mole ATP and 2 mole NADPH (d) 3 mole ATP and 2 mole NADPH

Short answer

(d) 3 mole ATP and 2 mole NADPH

Step-by-step solution

Revisit The Calvin Cycle

The Calvin cycle is a part of photosynthesis, the process by which plants convert sunlight into usable energy. The cycle is composed of three stages: carbon fixation, reduction, and regeneration of the CO2 acceptor. In the reduction phase, ATP and NADPH play a critical role by providing energy for the conversion of 3-phosphoglycerate into glyceraldehyde-3-phosphate, a carbohydrate molecule.

Determine the Number of ATP and NADPH required

During the reduction phase of the Calvin cycle, each molecule of 3-phosphoglycerate is phosphorylated by an ATP molecule to form 1,3-bisphosphoglycerate, which is subsequently reduced by NADPH to form a molecule of glyceraldehyde-3-phosphate. Therefore, to reduce one molecule of \(\mathrm{CO}_{2}\) into a carbohydrate molecule, 2 molecules of NADPH and 3 molecules of ATP are required.

Key concepts

Understanding Photosynthesis

Photosynthesis is the extraordinary biological process by which plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy in the form of glucose. It's a critical process for life on Earth, as it forms the base of the food chain and is also responsible for the oxygen we breathe.

During photosynthesis, chlorophyll within the chloroplasts of plant cells captures light energy, initiating a series of reactions that ultimately lead to the production of glucose and oxygen from carbon dioxide and water. This process can be divided into two main stages: the light-dependent reactions and the light-independent reactions, commonly known as the Calvin cycle.

In the light-dependent reactions, light energy is converted into ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), energetic molecules that store the energy needed for the second stage. Oxygen is also released as a byproduct during this phase. Afterward, the ATP and NADPH are used in the Calvin cycle to synthesize glucose from CO2. Understanding the integral role these molecules play in photosynthesis is key to appreciating the energy flow in ecosystems.

The Role of Carbon Fixation

The first phase of the Calvin cycle, known as carbon fixation, is where atmospheric carbon dioxide (CO2) begins its transformation into glucose. In this step, the enzyme RuBisCO catalyzes the attachment of CO2 to a five-carbon sugar called ribulose bisphosphate (RuBP), producing a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

Carbon fixation is critical as it's the step where inorganic carbon from the air is 'fixed' into an organic form that can be utilised within the plant and eventually by other organisms when they consume plant material. It's the bridge between the nonliving and living parts of our planet's carbon cycle.

Despite its efficiency, photosynthesis is not perfect. RuBisCO, for instance, sometimes catalyzes the attachment of oxygen to RuBP instead, leading to a wasteful process called photorespiration. Enhancing the understanding of carbon fixation can lead to better strategies for increasing crop productivity and mitigating climate change by improving the natural sequestration of CO2.

Energy Consumption in the Reduction Phase of the Calvin Cycle

The reduction phase is the second part of the Calvin cycle and is where the ATP and NADPH produced during the light-dependent reactions are utilized. Each 3-PGA molecule is phosphorylated by ATP, converting it into 1,3-bisphosphoglycerate (1,3-BPG). Subsequently, the 1,3-BPG is reduced by NADPH to form glyceraldehyde-3-phosphate (G3P).

Energy Investment

For one CO2 molecule to be fully converted into G3P, the cycle requires an investment of 3 ATP and 2 NADPH molecules. In the broad scheme, this phase is an energy investment leading to the creation of carbohydrate molecules that can later be used to fuel various plant functions. This highlights the cooperation between the light-dependent and light-independent reactions of photosynthesis.

From an educational standpoint, emphasizing the stoichiometry involved in this phase can aid students in grasping the balance of inputs and outputs in biochemical cycles. By understanding the specific energetic requirements—3 ATP and 2 NADPH per CO2 molecule—learners can appreciate the efficiency of the Calvin cycle in energy management and its pivotal role in sustaining plant life.