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- Because it's a waste of energy because there is no electron flow without light. The enzymes that power the Calvin cycle are therefore regulated to be light dependent even though the chemical reactions themselves don't require photons. At night, plants convert starch into sucrose and release it into the phloem.
www.thoughtco.com/what-is-the-calvin-cycle-608205
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.
- The Light-Dependent Reactions
The two reactions use carrier molecules to transport the...
- Photosynthesis in Prokaryotes
The two parts of photosynthesis—the light-dependent...
- The 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.
- 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).
Oct 4, 2019 · The Calvin cycle is also sometimes referred to as the “light independent” reactions of photosynthesis, since it is not powered directly by photons from the Sun. Instead, the Calvin cycle is powered by ATP and NADPH, which are created by harnessing the energy from photons in the light-dependent reactions.
The light-independent reactions or Calvin cycle are not really independent of light. They depend on the earlier reactions to supply ATP and NADPH in order to proceed. This pathway makes the storage and transport form of energy used by nearly every living organism, sugars.
Jun 21, 2024 · The light-independent stage, also known as the Calvin cycle, takes place in the stroma, the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light-independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like ...
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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).
