Question

In \(\mathrm{C}_{3}\) plants, the first stable product of photosynthesis during dark reaction is (a) 3-phosphoglyceric acid (b) Phosphoglyceraldehyde (c) Maleic acid (d) Oxaloacetic acid

Short answer

The first stable product of photosynthesis during dark reaction in C3 plants is 3-phosphoglyceric acid.

Step-by-step solution

Understanding C3 photosynthesis

In C3 plants, photosynthesis, specifically the dark reaction or the Calvin Cycle, happens in the stroma of the chloroplasts. This process is where carbon dioxide is transformed into glucose. The 'C3' term refers to the first stable product of this process, which is a compound consisting of 3 carbon atoms.

Identifying the initial product in C3 photosynthesis

The first stable product of photosynthesis during the dark reaction in C3 plants is a 3-carbon compound. The various options are examined to identify which one fits this description.

Select the correct answer

Among the given options, 3-phosphoglyceric acid is a 3-carbon compound and it is the first stable product of the dark reaction in photosynthesis of C3 plants. The other options are not the initial stable products of this process.

Key concepts

Understanding the Photosynthesis Dark Reaction

Photosynthesis is often associated with the vibrant green of leaves and the energy of sunlight. However, a critical component of this life-sustaining process occurs in the obscurity of the leaf's interior: the dark reaction, also known as the Calvin Cycle. This phase of photosynthesis is independent of light and is primarily concerned with the synthesis of glucose from carbon dioxide and water.

The dark reaction takes place in the stroma of chloroplasts, the fluid-filled interior space, and does not directly utilize light (hence 'dark'). However, it does rely on energy in the form of ATP and NADPH, which have been produced in the light-dependent reactions. During this phase, carbon fixation occurs, a process where atmospheric carbon dioxide is converted into an organic molecule, which then serves as a building block for glucose synthesis.

The importance of the dark reaction lies in its role in transforming inorganic carbon into organic compounds, which can then be used by the plant and, ultimately, by the organisms that consume the plant, including humans.

The Calvin Cycle

The Calvin Cycle, named after American chemist Melvin Calvin, forms the core of the photosynthesis dark reaction. It is a series of biochemical reactions that take place in the stroma and are crucial for the synthesis of sugar from carbon dioxide. The cycle operates in a circular manner and consists of three primary phases: carbon fixation, reduction and sugar creation, and regeneration of the starting molecule, ribulose bisphosphate (RuBP).

Your textbook exercise focuses on this cycle, where you learned that CO2 is fixed into a stable intermediate before being converted into glucose. The Calvin Cycle is a marvel of biological efficiency, as it not only creates sugars but also regenerates its starting material, allowing the cycle to continue indefinitely as long as the required substrates are present. It’s the silent workhorse of the plant world, turning inorganic carbon into organic molecules that eventually lead to the starches and sugars all life depends on.

What is 3-phosphoglyceric acid?

Going deeper into the textbook exercise, you learned that the first stable product of the Calvin Cycle is the three-carbon compound known as 3-phosphoglyceric acid (3-PGA). This compound forms immediately following the carbon fixation stage when the enzyme RuBisCO catalyzes the reaction of CO2 with RuBP. The resulting six-carbon compound is unstable and quickly splits into two molecules of 3-PGA.

3-PGA holds a pivotal role in the Calvin Cycle. It is the starting point for the subsequent reactions that eventually produce glucose. With the help of ATP and NADPH from the light-dependent reactions, 3-PGA is reduced and modified through a series of steps into the sugar molecule glyceraldehyde-3-phosphate (G3P). Some of these G3P molecules go on to form glucose, while others are used to regenerate RuBP, ensuring the cycle can continue.

First Stable Product of Photosynthesis

The solution to your textbook exercise reveals that the first stable product of photosynthesis in C3 plants during the dark reaction is indeed 3-phosphoglyceric acid (3-PGA). This fact is not just an interesting piece of trivia; it is fundamental to understanding how plants convert inorganic carbon from the air into energy-rich organic compounds. The 'C3' label indicates that the initial carbon fixation product in these plants is a three-carbon compound, which contrasts with C4 plants where the first stable product has four carbon atoms.

Knowing that 3-PGA is this first stable product also helps us appreciate the evolutionary biology of plants. C3 photosynthesis is considered the oldest and most common pathway for carbon fixation, representing a significant step in the Earth's biological history. The presence of 3-PGA, a simple molecule that initiates a complex cascade leading to life-sustaining sugars, underscores the elegance and intricacy of photosynthesis.