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- Although the Calvin Cycle is not directly dependent on light, it is indirectly dependent on light since the necessary energy carriers (ATP and NADPH) are products of light-dependent reactions.
The Calvin cycle is not totally independent of light since it relies on ATP and NADH, which are products of the light-dependent reactions. The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.
It does not directly require light, hence it is also known as the light-independent or dark reactions. The Calvin cycle uses the ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into glucose, a type of sugar that the plant can use for energy.
- Overview
- Introduction
- Overview of the Calvin cycle
- Reactions of the Calvin cycle
- Summary of Calvin cycle reactants and products
How the products of the light reactions, ATP and NADPH, are used to fix carbon into sugars in the second stage of photosynthesis.
You, like all organisms on Earth, are a carbon-based life form. In other words, the complex molecules of your amazing body are built on carbon backbones. You might already know that you’re carbon-based, but have you ever wondered where all of that carbon comes from?
As it turns out, the atoms of carbon in your body were once part of carbon dioxide (CO2 ) molecules in the air. Carbon atoms end up in you, and in other life forms, thanks to the second stage of photosynthesis, known as the Calvin cycle (or the light-independent reactions).
In plants, carbon dioxide (CO2 ) enters the interior of a leaf via pores called stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions, where sugar is synthesized. These reactions are also called the light-independent reactions because they are not directly driven by light.
In the Calvin cycle, carbon atoms from CO2 are fixed (incorporated into organic molecules) and used to build three-carbon sugars. This process is fueled by, and dependent on, ATP and NADPH from the light reactions. Unlike the light reactions, which take place in the thylakoid membrane, the reactions of the Calvin cycle take place in the stroma (the inner space of chloroplasts).
The Calvin cycle reactions can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule.
Here is a general diagram of the cycle:
Diagram of the Calvin cycle, illustrating how the fixation of three carbon dioxide molecules allows one net G3P molecule to be produced (that is, allows one G3P molecule to leave the cycle).
3 CO2 molecules combine with three molecules of the five-carbon acceptor molecule (RuBP), yielding three molecules of an unstable six-carbon compound that splits to form six molecules of a three-carbon compound (3-PGA). This reaction is catalyzed by the enzyme rubisco.
In the second stage, six ATP and six NADPH are used to convert the six 3-PGA molecules into six molecules of a three-carbon sugar (G3P). This reaction is considered a reduction because NADPH must donate its electrons to a three-carbon intermediate to make G3P.
[See a diagram that shows the molecular structures]
Three turns of the Calvin cycle are needed to make one G3P molecule that can exit the cycle and go towards making glucose. Let’s summarize the quantities of key molecules that enter and exit the Calvin cycle as one net G3P is made. In three turns of the Calvin cycle:
•Carbon. 3 CO2 combine with 3 RuBP acceptors, making 6 molecules of glyceraldehyde-3-phosphate (G3P).
•1 G3P molecule exits the cycle and goes towards making glucose.
•5 G3P molecules are recycled, regenerating 3 RuBP acceptor molecules.
•ATP. 9 ATP are converted to 9 ADP (6 during the reduction step, 3 during the regeneration step).
•NADPH. 6 NADPH are converted to 6 NADP+ (during the reduction step).
Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO 2 from the environment. An enzyme, RuBisCO, catalyzes a reaction with CO 2 and another molecule, RuBP.
There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration. Though it is also called the "dark reaction", the Calvin cycle does not actually occur in the dark or during night time.
The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules (Figure 1). Figure 1 The light-dependent reactions harness energy from the sun to produce ATP and NADPH.
People also ask
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What are light-independent reactions?
The Calvin cycle, or the light-independent reactions, is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules (Figure 6.13).
