10 how many times does the calvin cycle turn Ideas

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n Cycle | Biology I

Learning Objectives

By the end of this section, you will be able to:

  • Describe the Calvin cycle
  • Define carbon fixation
  • Explain how photosynthesis works in the energy cycle of all living organisms

After the energy from the sun is converted and packaged into ATP and NADPH, the cell has the fuel needed to build food in the form of carbohydrate molecules. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. 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.

The Interworkings of the Calvin Cycle

This illustration shows that ATP and NADPH produced in the light reactions are used in the Calvin cycle to make sugar.

Figure 1. Light-dependent reactions harness energy from the sun to produce ATP and NADPH. These energy-carrying molecules travel into the stroma where the Calvin cycle reactions take place.

In plants, carbon dioxide (CO2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized. The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery (Figure 1).

The Calvin cycle reactions (Figure 2) can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end.

RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is “fixed” from its inorganic form into organic molecules.

ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.

One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.

In summary, it takes six turns of the Calvin cycle to fix six carbon atoms from CO2. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step.

Concept in Action

Check out this animation of the Calvin cycle. Click Stage 1, Stage 2, and then Stage 3 to see G3P and ATP regenerate to form RuBP.

Evolution in Action

Photosynthesis

This photo shows a cactus.

Figure 3. Living in the harsh conditions of the desert has led plants like this cactus to evolve variations in reactions outside the Calvin cycle. These variations increase efficiency and help conserve water and energy. (credit: Piotr Wojtkowski)

The shared evolutionary history of all photosynthetic organisms is conspicuous, as the basic process has changed little over eras of time. Even between the giant tropical leaves in the rainforest and tiny cyanobacteria, the process and components of photosynthesis that use water as an electron donor remain largely the same. Photosystems function to absorb light and use electron transport chains to convert energy. The Calvin cycle reactions assemble carbohydrate molecules with this energy.

However, as with all biochemical pathways, a variety of conditions leads to varied adaptations that affect the basic pattern. Photosynthesis in dry-climate plants (Figure 3) has evolved with adaptations that conserve water. In the harsh dry heat, every drop of water and precious energy must be used to survive. Two adaptations have evolved in such plants. In one form, a more efficient use of CO2 allows plants to photosynthesize even when CO2 is in short supply, as when the stomata are closed on hot days. The other adaptation performs preliminary reactions of the Calvin cycle at night, because opening the stomata at this time conserves water due to cooler temperatures. In addition, this adaptation has allowed plants to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.

Photosynthesis in Prokaryotes

The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis (Figure 4). It is here that organisms like cyanobacteria can carry out photosynthesis.

This illustration shows a green ribbon, representing a folded membrane, with many folds stacked on top of another like a rope or hose. The photo shows an electron micrograph of a cleaved thylakoid membrane with similar folds from a unicellular organism

Figure 4. A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis. (credit: scale-bar data from Matt Russell)

The Energy Cycle

Living things access energy by breaking down carbohydrate molecules. However, if plants make carbohydrate molecules, why would they need to break them down? Carbohydrates are storage molecules for energy in all living things. Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy. Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates, for example, plants have mitochondria in addition to chloroplasts.
You may have noticed that the overall reaction for photosynthesis:

6CO2+6H2O→C6H12O6+6O2

is the reverse of the overall reaction for cellular respiration:

6O2+C6H12O6→6CO2+6H2O

Photosynthesis produces oxygen as a byproduct, and respiration produces carbon dioxide as a byproduct.

In nature, there is no such thing as waste. Every single atom of matter is conserved, recycling indefinitely. Substances change form or move from one type of molecule to another, but never disappear (Figure 5).

This photograph shows a giraffe eating leaves from a tree. Labels indicate that the giraffe consumes oxygen and releases carbon dioxide, whereas the tree consumes carbon dioxide and releases oxygen.

Figure 5. In the carbon cycle, the reactions of photosynthesis and cellular respiration share reciprocal reactants and products. (credit: modification of work by Stuart Bassil)

CO2 is no more a form of waste produced by respiration than oxygen is a waste product of photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to break down carbohydrates in mitochondria. Both organelles use electron transport chains to generate the energy necessary to drive other reactions. Photosynthesis and cellular respiration function in a biological cycle, allowing organisms to access life-sustaining energy that originates millions of miles away in a star.

Section Summary

Using the energy carriers formed in the first stage of photosynthesis, the Calvin cycle reactions fix CO2 from the environment to build carbohydrate molecules. An enzyme, RuBisCO, catalyzes the fixation reaction, by combining CO2 with RuBP. The resulting six-carbon compound is broken down into two three-carbon compounds, and the energy in ATP and NADPH is used to convert these molecules into G3P. One of the three-carbon molecules of G3P leaves the cycle to become a part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be formed back into RuBP, which is ready to react with more CO2. Photosynthesis forms a balanced energy cycle with the process of cellular respiration. Plants are capable of both photosynthesis and cellular respiration, since they contain both chloroplasts and mitochondria.

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Additional Self Check Questions

1.Which part of the Calvin cycle would be affected if a cell could not produce the enzyme RuBisCO?

2. Explain the reciprocal nature of the net chemical reactions for photosynthesis and respiration.

Answers

1. None of the cycle could take place, because RuBisCO is essential in fixing carbon dioxide. Specifically, RuBisCO catalyzes the reaction between carbon dioxide and RuBP at the start of the cycle.

2. Photosynthesis takes the energy of sunlight and combines water and carbon dioxide to produce sugar and oxygen as a waste product. The reactions of respiration take sugar and consume oxygen to break it down into carbon dioxide and water, releasing energy. Thus, the reactants of photosynthesis are the products of respiration, and vice versa.

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Frequently Asked Questions About how many times does the calvin cycle turn

If you have questions that need to be answered about the topic how many times does the calvin cycle turn, then this section may help you solve it.

How many times per glucose does the Calvin cycle repeat?

Given that each glucose molecule contains six carbon atoms, the Calvin cycle will need to complete six turns in order to produce one molecule of glucose.

The Calvin cycle repeats itself six times.

In conclusion, the Calvin cycle requires energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step, and 6 ATP molecules in the regeneration step, for six turns to fix six carbon atoms from CO2.

The six turns of the Calvin cycle are why?

The remaining G3P molecules regenerate RuBP, allowing the system to get ready for the carbon-fixation step; however, because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed).

Do Calvin cycles repeat themselves?

The two molecules of G3P, a 3-carbon carbohydrate, will combine to form one molecule of glucose, a 6-carbon carbohydrate, OR another organic compound after the Calvin Cycle “turns” twice (well, actually six times).

Why is the Calvin cycle three turns long?

G3P, the initial product of photosynthesis, contains three carbon atoms, and each turn of the Calvin cycle requires one carbon atom in the form of carbon dioxide. This explains why it takes three turns of the cycle to produce G3P.

How many times must the Calvin cycle run to generate 5 molecules of glucose?

Six turns of the Calvin cycle are needed to produce one molecule of glucose (6C), as the Calvin cycle can only take in one carbon (as CO2) at a time. Therefore, 30 turns of the Calvin cycle are needed to produce a net gain of 5 molecules of glucose.

How many Calvin cycles are required to produce a unit of sucrose?

In order to form 2 G3P from 1 sucrose formation, the calvin cycle must occur 6 times, resulting in 12 calvin cycles for 2 sucrose molecules.

Why does the Calvin cycle have three turns?

It takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P because the G3P exported from the chloroplast has three carbon atoms, but each “turn” makes two G3Ps, so three turns make six G3Ps.

Does the Calvin cycle always occur?

Though it is called the “dark reaction”, the Calvin cycle does not actually occur in the dark or during night time. This is because the process requires NADPH, which is short-lived and comes from the light-dependent reactions.

Why does the Calvin cycle need to repeat itself twice?

Did you know that the kelvin cycle must repeat twice in order to produce one glucose molecule? It is said that one round of the kelvin cycle produces one g 3 p molecule, and we know that two g p 3 molecules are needed to form one glucose molecule.

What is another name for the Calvin cycle?

The Calvin cycle, also referred to as the C3 cycle or the dark or light-independent reaction of photosynthesis, is most active during the day when NADPH and ATP levels are high.

The Calvin cycle pauses at night; why?

Because the Calvin Cycle requires reduced NADP, which is transient and produced by light-dependent reactions, the plants stop the Calvin Cycle in the dark when the levels of ATP and NADPH drop.

How frequently is the C3 cycle repeated?

The Calvin cycle is a light-independent reaction in photosynthesis that occurs in three steps: carbon fixation, reduction, and regeneration. It repeats six times for the synthesis of one molecule of glucose.

Why does it take three turns to create a G3P?

Glyceraldehyde-3-phosphate, a three-carbon molecule, is the end result, and is used to create glucose, a six-carbon molecule. One carbon cycle is fixed, and three cycles of the cycle fix three carbons, producing one G3P molecule.

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