Photosynthesis is the biogeochemical process which occurs in all green plants or autotrophs producing organic molecules from carbon dioxide (CO2). These organic molecules contain many carbon-hydrogen (C–H ) bonds and are highly reduced compared to CO2.
There are two stages of Photosynthesis –
Light-dependent reactions – As the name suggests, it requires light and mainly occurs during the day time.
Light-independent reactions – It also called the dark reaction or Calvin cycle or C3 cycle. This reaction occurs both in the presence and absence of sunlight.
Let us have a detailed look at Calvin Cycle or C3 cycle along with its stages.
Also read: Photosynthesis
Calvin Cycle Definition
“Calvin cycle or C3 cycle is defined as a set of chemical reactions performed by the plants to reduce carbon dioxide and other compounds into glucose.”
What is Calvin Cycle?
Calvin cycle is also known as C3 cycle or light-independent or dark reaction of photosynthesis. However, it is most active during the day when NADPH and ATP are abundant. To build organic molecules, the plant cells use raw materials provided by the light reactions:
1. Energy: ATP provided by cyclic and noncyclic photophosphorylation, which drives the endergonic reactions.
2. Reducing power: NADPH provided by photosystem I is the source of hydrogen and the energetic electrons required to bind them to carbon atoms. Much of the light energy captured during photosynthesis ends up in the energy-rich C—H bonds of sugars.
Plants store light energy in the form of carbohydrates, primarily starch and sucrose. The carbon and oxygen required for this process are obtained from CO2, and the energy for carbon fixation is derived from the ATP and NADPH produced during the photosynthesis process.
The conversion of CO2 to carbohydrate is called Calvin Cycle or C3 cycle and is named after Melvin Calvin who discovered it. The plants that undergo Calvin cycle for carbon fixation are known as C3 plants.
Calvin Cycle requires the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase commonly called rubisco. It generates the triose phosphates 3-phosphoglycerate (3-PGA), glyceraldehyde-3P (GAP) and dihydroxyacetone phosphate (DHAP), all of which are used to synthesize the hexose phosphates fructose-1,6-bisphosphate and fructose 6-phosphate.
Stages of Calvin Cycle or C3 Cycle
Calvin cycle or C3 cycle can be divided into three main stages:
The key step in Calvin cycle is the event that reduces CO2 and attaches it to a very special organic molecule. Photosynthetic cells produce this molecule by reassembling the bonds of two intermediates in glycolysis; fructose 6-phosphate, and glyceraldehyde 3-phosphate, to form the energy-rich five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), and a four-carbon sugar.
CO2 binds to RuBP in the key process called carbon fixation, forming two-three carbon molecules of phosphoglycerate. The enzymes that carry out this reaction are ribulose bisphosphate carboxylase/oxygenase, which is very large with a four-subunit enzyme present in the chloroplast stroma. This enzyme works very sluggishly, processing only about three molecules of RuBP per second (a typical enzyme process about 1000 substrate molecules per second). This is mainly because of the enzymes works slowly, and many molecules of Rubisco are needed. In a typical leaf, over 50% of all the protein is rubisco. It is thought to be the most abundant protein on earth.
It is the second stage of Calvin cycle. The 3-PGA molecules created through carbon fixation that are converted into molecules of simple sugar – glyceraldehyde-3 phosphate (G3P).
This stage obtains energy from ATP and NADPH formed during the light-dependent reactions of photosynthesis. In this way, Calvin cycle becomes a pathway in which plants convert sunlight energy into long-term storage molecules, such as sugars. The energy from the ATP and NADPH is transferred to the sugars.
This step is known as reduction since electrons are transferred to 3-PGA molecules to form glyceraldehyde-3 phosphate. The reduction is the process of donating one electron.
It is the third stage of Calvin cycle and is a complex process which requires ATP. In this stage, some of the G3P molecules are used to produce glucose, while others are recycled to regenerate the RuBP acceptor.
Also read: Light reaction And Dark reaction
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