Question

The initial enzyme of Calvin cycle is (a) Ribulose 1,5 diphosphate carboxylase (b) Triose phosphate dehydrogenase (c) Phosphofructokinase (d) Cytochrome oxidase

Short answer

The initial enzyme of the Calvin cycle is (a) Ribulose 1,5 diphosphate carboxylase.

Step-by-step solution

Defining the Calvin cycle

The Calvin cycle, also known as the Calvin-Benson cycle, refers to the set of chemical reactions that take place in chloroplasts during photosynthesis. It is the phase where carbon dioxide is converted into glucose using ATP and NADPH. The cycle is driven by a series of enzymes.

Identifying the initial enzyme of the Calvin cycle

The first step in the Calvin cycle is the carboxylation of ribulose-1,5-bisphosphate, a reaction catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Thus, Rubisco is the initial enzyme of the Calvin Cycle.

Comparing the options given

Comparing the options given in the exercise, Ribulose 1,5 diphosphate carboxylase (option (a)) is another name for Rubisco, which is the initial enzyme of the Calvin cycle.

Key concepts

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)

The enzyme Rubisco serves as the gateway to carbon fixation in the process of photosynthesis. As the most abundant protein on Earth, it catalyzes a critical step in the Calvin cycle—capturing atmospheric carbon dioxide and integrating it into organic molecules. This biologically significant enzyme bonds carbon dioxide to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), which splits into two molecules of 3-phosphoglycerate (3-PGA) for further utilization in the cycle. The dual-function of Rubisco, in its interaction with oxygen leading to photorespiration, demonstrates the evolutionary adaptations of plants to fluctuating atmospheric conditions.
Despite its pivotal role, Rubisco is often considered inefficient due to its slow catalytic rate and tendency to also react with oxygen. Enhancing the efficiency of Rubisco is a significant area of research, aiming to boost agricultural productivity and address global food security challenges.

Photosynthesis

Photosynthesis is the extraordinary biochemical process that enables plants, algae, and certain bacteria to convert light energy into chemical energy. It primarily occurs in the chloroplasts, where light-dependent reactions convert solar energy into ATP and NADPH, powering the synthesis of glucose. This process is foundational to life as it not only creates food for plant-based organisms but also produces oxygen, essential for aerobic life-forms.
Understanding photosynthesis is crucial for grasping the underpinnings of the global carbon cycle and its impact on climate change. Improving photosynthetic efficiency is seen as a promising solution to increasing crop yields and sustainable bioenergy production, crucial for feeding a growing world population and transitioning to cleaner energy sources.

Chloroplasts

Chloroplasts are the photosynthetic powerhouses of plant and algal cells. These specialized organelles are encased by a double membrane and contain their own DNA. Within their innermost compartment called the stroma, the Calvin cycle takes place, utilizing ATP and NADPH produced in the light-dependent reactions across the thylakoid membranes. Chloroplasts are filled with a pigment called chlorophyll, responsible for the organelle's green color and crucial for capturing light energy.
Considering their evolutionary origin from ancient symbiotic cyanobacteria, chloroplasts demonstrate a fascinating example of endosymbiosis. This collaborative relationship has fundamentally shaped the path of life on Earth by enabling the efficient conversion of sunlight into usable energy forms.

ATP and NADPH

ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) are two essential molecules that serve as the energy currency and reducing power, respectively, in the Calvin cycle. ATP provides the energy required for the synthesis of glucose, while NADPH contributes the hydrogen atoms necessary for the reduction of carbon molecules.
During the light-dependent reactions of photosynthesis, the energy from sunlight is stored in these molecules, which are then transported to the Calvin cycle for sugar production. The continuous supply and regeneration of ATP and NADPH are critical for maintaining the cycle's momentum and, by extension, for sustaining life on Earth through photosynthesis.

Carbon fixation

Carbon fixation is the process of converting inorganic carbon dioxide into organic carbon compounds, essentially 'fixing' atmospheric carbon into a stable form that can be utilized by living organisms. This process is at the heart of the Calvin cycle, where CO2 is attached to the five-carbon sugar RuBP by Rubisco, forming an unstable six-carbon compound that quickly splits into molecules of 3-PGA.
Carbon fixation is a cornerstone of the biosphere, impacting everything from global food production to climate regulation. By transforming CO2 into glucose, the cycle provides the basis for the growth and energy requirements of plants and the entire food web dependent upon them. Enhancing carbon fixation efficiency is a major goal for reducing atmospheric carbon levels and mitigating climate change effects.