Question

The NADPH produced during the light reactions is necessary for a. the carbon fixation phase, which incorporates carbon dioxide into an organic molecule during the Calvin cycle. b. the reduction phase, which produces carbohydrates in the Calvin cycle. c. the regeneration of RuBP of the Calvin cycle. d. all of the above. e. a and b only.

Short answer

The correct answer is option (b). NADPH is necessary for the reduction phase, which produces carbohydrates in the Calvin cycle.

Step-by-step solution

Understanding the function of NADPH

First, it's important to understand what NADPH does. NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is a coenzyme used in anabolic reactions, which build large molecules from small molecules. It's crucial in photosynthesis for its role in the 'reduction power'. It provides the high-energy electrons needed to convert carbon dioxide to glucose.

Making connection with each option

Next, evaluate how this function maps onto each answer choice: (a) refers to the carbon fixation phase; the role of NADPH isn't in the fixation of CO2 but in the subsequent reduction. (b) refers to the reduction phase - this is where NADPH comes into play, reducing 3-PGA into G3P, a carbohydrate. (c) Necessary for the regeneration of RuBP - while NADPH is crucial in the Calvin cycle, it doesn't directly participate in the regeneration of RuBP.

Finalising the answer

After evaluating, it's clear that NADPH is involved in the second step of the Calvin Cycle (reduction) to produce carbohydrates, so option B is correct. The term 'all of the above' in option D would be incorrect because NADPH is not necessary for the regeneration of RuBP, and option E is incorrect because it's not necessary for the carbon fixation phase either.

Key concepts

NADPH

NADPH, short for Nicotinamide Adenine Dinucleotide Phosphate, is a critical player in the world of photosynthesis. It acts as a "reduction powerhouse" by providing high-energy electrons for various chemical reactions. These electrons are essential for transforming carbon dioxide into glucose during photosynthesis.
NADPH is formed during the light-dependent reactions of photosynthesis. It plays a pivotal role in the Calvin cycle by acting as a reducing agent. In other words, it donates electrons that enable the conversion of precursor molecules into energy-rich sugars.

Carbon Fixation

Carbon fixation is the very first phase of the Calvin cycle, performing a crucial step by converting inorganic carbon dioxide into an organic molecule. This process initiates when carbon dioxide is attached to a five-carbon sugar named ribulose bisphosphate (RuBP).
The enzyme that facilitates this reaction is rubisco. Carbon fixation is the starting point, paving the way for subsequent transformations involving ATP and NADPH in later stages of the cycle. Although it is the initial step, NADPH doesn’t participate in fixing carbon; instead, it comes into play later in the cycle.

RuBP Regeneration

Regenerating RuBP is a vital phase in the Calvin cycle, ensuring the sustainability of the process by continually replenishing the carbon dioxide acceptor. This part of the cycle uses ATP to transform the triose phosphate molecules produced earlier back into RuBP.
This regeneration maintains a constant supply of molecules that can fix carbon dioxide, allowing the cycle to repeat. However, it's important to note that NADPH is not directly involved in this phase. The requirement here focuses more on energy supplied by ATP.

Photosynthesis

Photosynthesis is the remarkable process by which plants, algae, and some bacteria convert sunlight into chemical energy. This process consists of two main phases: the light-dependent reactions and the Calvin cycle.
In the light reactions, solar energy is captured and used to produce ATP and NADPH. These energy carriers are then utilized in the Calvin cycle to transform carbon dioxide into glucose. Photosynthesis not only fuels plant growth and sustenance but also produces oxygen, which is indispensable for life on Earth.

Reduction Phase

The reduction phase of the Calvin cycle is where the magic of carbohydrate formation occurs. In this phase, the molecule 3-phosphoglycerate (3-PGA) is converted into glyceraldehyde-3-phosphate (G3P), a simple sugar.
This transformation is powered by ATP and NADPH, where NADPH donates electrons, reducing the 3-PGA. This step is crucial for synthesizing glucose and other carbohydrates, making NADPH essential for the reduction phase. It's during this stage that the energy from light is effectively captured in chemical form, storing it as sugars the plant can use.