The Calvin cycle is a crucial part of photosynthesis that converts atmospheric carbon dioxide into carbohydrates, using energy and electrons supplied by the light-dependent reactions.
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The leaves of this plant have cells that carry out photosynthesis.
If we zoom in on this photosynthetic plant cell, we can see the chloroplasts
where the reactions of photosynthesis occur. Photosynthesis
consists of two primary steps: the light reactions and the
Calvin cycle reactions. In this tutorial, we'll focus exclusively
on the Calvin cycle reactions. The overall purpose of the Calvin cycle
is to convert carbon dioxide from the atmosphere into
carbohydrates (or sugars) which the plant needs to power its cellular activities
and build new plant structures. The Calvin cycle reactions occur
within the stroma of the chloroplast. The reactions use the ATP
and NADPH produced by the light reactions to convert the CO2
to carbohydrate. Let's zoom into the Calvin cycle to take a closer
look. The Calvin cycle can be divided into 3 phases:
carbon dioxide fixation, carbon dioxide
reduction, and regeneration of RuBP.
The first phase, carbon dioxide fixation, basically
captures the CO2 from the atmosphere, so it can be used in the reactions.
To do this, CO2 is attached to RuBP,
a five carbon molecule. The enzyme used in this reaction
is called Rubisco, and the result of the reaction is an unstable
6-carbon molecule that quickly splits into two 3-carbon molecules
called 3-phosphoglycerate, or 3PG.
Next, the process of converting the CO2 to carbohydrate
begins. This phase is called the carbon dioxide reduction phase,
because we're adding electrons and energy to the CO2 molecule.
During this phase, a sequence of reactions uses NADPH
and some of the ATP from the light reactions.
These molecules supply the needed electrons and energy for CO2
reduction. Electrons are added from NADPH
and through a series of reactions, 3PG is reduced to
form G3P, a carbohydrate. ADP and
NAD+ return to the thylakoids, to be converted back
ATP and NADPH by the light reactions.
One of the G3P molecules is set aside as a building block
for glucose. But the majority of the G3P molecules move
forward into the third phase of the Calvin cycle. In this phase,
ATP is used to combine the rest of the G3P molecules to
form RuBP molecules.
This RuBP can then combine with additional carbon dioxide molecules
continuing the carbon reactions.
To form a glucose molecule, the cycle actually has to turn 6 times,
because each turn of the cycle adds only one carbon atom from the incoming
carbon dioxide. It's important to remember that the
Calvin cycle is dependent on the light reactions, which provide
NADPH and ATP, which in turn provide the electrons
and energy needed to form carbohydrates.
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