Photosynthesis enables plants to convert sunlight into energy in the form of glucose molecules to support plants and life on Earth. Sunlight transforms to glucose in the Calvin Cycle, a series of three phases.
The Calvin Cycle is a series of reactions in plant cells to convert carbon dioxide into glucose, a type of sugar that plants use for energy. It's also called the light-independent reactions or dark reactions.
The Calvin Cycle happens in the stroma of the chloroplasts, the organelles in plant cells responsible for photosynthesis. Light-dependent reactions require light energy to produce adenosine triphosphate (ATP) and NADPH.
The Calvin Cycle can happen in the dark, using ATP and NADPH (nicotinamide adenine dinucleotide phosphate) produced during the light-dependent reactions to power its reactions.
It's named after American scientist Melvin Calvin, who receives the Nobel Prize in Chemistry in 1961 for his groundbreaking work. Though the Calvin Cycle doesn't need direct light, it depends on products of light-dependent reactions.
The main purpose of the Calvin Cycle is to produce glucose, a vital energy source not just for the plant but also for nearly all living organisms on Earth. In fact, about 80% of the energy required by life comes from glucose derived from photosynthesis.
By synthesizing glucose, the Calvin Cycle effectively captures and stores solar energy in a form that can be used by various life forms.
The Calvin Cycle consists of three main stages: carbon fixation, reduction phase, and regeneration of ribulose bisphosphate (RuBP), the starting molecule.
Carbon Fixation
The first stage of the Calvin Cycle is carbon fixation. In this phase carbon dioxide is incorporated into an organic molecule. It's done with help of the enzyme rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase).
Rubisco catalyzes the reaction between carbon dioxide and ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar. The result of this reaction is a six-carbon intermediate.
It immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. This is the moment inorganic carbon from the atmosphere is brought into the organic molecule.
Around 90% of carbon fixation in terrestrial ecosystems happens through the Calvin Cycle.
Reduction
The second stage of the Calvin Cycle is reduction. ATP and NADPH produced during light-dependent reactions are used to convert 3-PGA into a usable form.
In this stage, 3-PGA is first phosphorylated by ATP to form 1,3-bisphosphoglycerate (1,3-BPG). Then, NADPH reduces 1,3-BPG to form glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
For every three turns of the Calvin Cycle, one molecule of G3P exits the cycle. The remaining molecules go back to regenerate RuBP, ensuring continuation of the cycle.
Once G3P is formed, it can be converted into glucose and other carbohydrates. It's used immediately by the plant or stored for later use.
Regeneration of the Starting Molecule
The final stage of the Calvin Cycle is regeneration of RuBP. Out of the six G3P molecules produced, only one exits to contribute to glucose formation.
Five are reconfigured back into three molecules of RuBP through a series of enzymatic reactions. The regeneration process needs additional ATP. It helps the cycle continue by replenishing substrate for new CO2 molecules.
The Calvin Cycle helps maintain ecological balance. By transforming CO2 into glucose, it helps regulate atmospheric carbon levels and supports respiration in animals and other organisms.
The plant packs the glucose into polysaccharide chains and compounds like starches and sucrose. A mature oak tree can store 11,000 kg of glucose as starch. Veg like carrots store extra sugars in roots underground.
Factors Affecting the Calvin Cycle
Several factors can influence the efficiency and rate of the Calvin Cycle. These include:
Light Intensity: The availability of light directly affects the production of ATP and NADPH in the light-dependent reactions, which in turn fuels the Calvin Cycle.
Temperature: Optimal temperatures can improve enzymatic reactions; however, extreme temperatures can reduce the cycle's efficiency.
Carbon Dioxide Concentration: Higher CO2 levels can enhance photosynthesis rates, showing a direct correlation with plant growth.
Under ideal conditions, the Calvin Cycle functions effectively. In stressful situations like extreme temperatures or drought, the cycle diminishes, with lower photosynthesis rates and stunted plant growth.
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