Calvin Cycle (Dark Reaction) — Equation & Steps

How Cells Make Glucose

Photosynthesis consists of two parts. The first requires sunlight and the second does not.

The Calvin cycle is also called the dark reactions or light-independent reactions because it is the part that doesn't need energy from the sun to happen.

The Calvin cycle takes place within the stroma of the chloroplast. This is where plants make sugar molecules that they can use to make other essential components, and that all other organisms can use for energy.

The cycle uses ATP and NADPH synthesized in the light-dependent reactions to drive its reactions forward. So although the cycle itself doesn't use light energy, it does depend on the light reactions to provide it with the ATP and NADPH.

Reactions in the Cycle

The cycle begins when carbon dioxide ($CO_{2}$) from the atmosphere enters plant cells. An enzyme called rubisco catalyzes the first reaction, where $CO_{2}$ binds to a specific 5-carbon molecule called ribulose-1,5-bisphosphate (RuBP). This reaction creates a 6-carbon molecule which then splits into two 3-carbon molecules. This part of the cycle is called carbon fixation. This just means that inorganic carbon is converted to organic molecules, like sugar.

The high-energy products from the light reactions are used in the next reaction. ATP and NADH donate electrons to the 3-carbon molecules, which are converted into a 3-carbon sugar called glyceraldehyde-3-phosphate (G3P).

Some of these G3P molecules leave the cycle to form glucose molecules. These will be used by the plant during cellular respiration. Three turns of the cycle are needed for one G3P molecule to exit the cycle. Glucose is made of six carbons, so two G3P molecules are needed to make one glucose molecule.

After three turns of the cycle, this leaves five more G3P molecules which are recycled. This allows the cycle to continue. ATP is used to convert the leftover G3P into the molecules that can bind incoming carbon dioxide and restart the cycle.

Light-Independent Reactions

The light-independent reactions make up the second part of photosynthesis.

You should note that the name "light-independent reactions" is a little misleading. These reactions do not directly require light, but they can't occur without the NADPH and ATP molecules produced in the light-dependent reactions. There are a few exceptions such as desert plants, who only open their pores at night to get carbon dioxide in order to avoid losing water during the day.

The light-independent reactions are also called the Calvin cycle (or the Calvin-Benson cycle) and occur in the stroma (fluid interior) of the chloroplast.

In the light-independent reactions, the plant uses carbon dioxide ($CO_2$) and the ATP and NADPH from the light-dependent reactions to produce a sugar called glucose.

Glucose ($C_6H_{12}O_6$) can then be converted by the plant into other molecules that the cell needs. It can also be used in cellular respiration.

Notice how the ATP and NADPH are recycled by the light-independent reactions. They return to the light-dependent reactions to be used again.

Key Vocabulary:

1. Photosynthesis
2. Chloroplast
3. Stroma
4. Glucose

What Is the Calvin Cycle?

After a plant traps sunlight energy in the light-dependent reactions, it needs to use the ATP and NADH it created to make long-term energy storage molecules like glucose. The Calvin cycle is used to do this. Since the cycle does not involve sunlight in any of its reactions, it is considered a dark reaction. You can also refer to dark reactions as light-independent reactions.

What Are the Steps of the Cycle?

The Calvin cycle has three main steps: carbon fixation, reduction, and regeneration.

What Happens During Carbon Fixation?

Carbon dioxide molecules from the environment are absorbed by plants through holes in their leaves called stomata. Stomata pores allow gases to go in and out of the leaves. The carbon dioxide makes its way to mesophyll cells and then diffuses into the stroma of their chloroplasts.

When carbon dioxide enters the cycle, it is joined together with ribulose bisphosphate, or RuBP. Carbon dioxide has one carbon while RuBP has five. Together, they create a 6-carbon molecule that is split into two 3-carbon molecules called 3-phosphoglyceric acid, or 3-PGA.

When carbon dioxide binds to the RuBP, the carbon in the $CO_2$ is fixed because it is no longer loose in the atmosphere as a gas.

What Is Rubisco?

The important thing to remember about carbon fixation is that it uses the enzyme Rubisco. Rubisco is what causes the carbon dioxide to bind to the RuBP and then split into 3-PGA. Rubisco is the most abundant protein on Earth since nearly all photosynthesizing organisms use it.

What Happens During Reduction?

During reduction, six ATP are reduced to ADP in order to be used again in the light reaction. Six NADH are reduced to NAD+ in order to convert the six 3-PGA into six glyceraldehyde 3-phosphate, or G3P. Just like the ADP, the NAD+ can now be used again in the light reaction.

Basically, reduction recycles ATP and NADH into ADP and NAD+ while turning 3-PGA into G3P.

How Do the Reactions Make Glucose?

The Calvin cycle itself does not make new glucose, but it makes the "building blocks" of glucose that are used in other reactions.

G3P is one of the building blocks of glucose, and this cycle generates six G3Ps. Only one G3P leaves the cycle to be used in glucogenesis. The other five G3Ps stay in the cycle. It generally takes six turns of the Calvin cycle to generate one glucose molecule.

What Happens During Regeneration?

During regeneration, the five G3P molecules are used to make three RuBP molecules. A very complex reaction occurs that involves three ATP releasing phosphates and becoming three ADP.

It is important to remember that the five G3P molecules have three carbons each. The three RuBP molecules have five carbons each. You can see that both sets of molecules have 15 carbons total. All of the carbons that are conserved as RuBP are regenerated from G3P.

When thinking about the phosphates, remember that the five G3P molecules have one phosphate each, or five phosphates total. However, the three RuBP molecules have two phosphates each, or six phosphates total.

When the three ATPs are used in this reaction, three phosphates release. Two of them leave the cycle to be used in other reactions. One is added to the RuBP. This is what allows the RuBP to have two phosphates each.

After regeneration, the newly made RuBP joins with new $CO_2$ molecules and takes part in the Calvin cycle once again.

Overall, the regeneration part of the cycle creates RuBP that can be used as the cycle repeats.

Congratulations. Just like that one lucky G3P, you are now free of the Calvin cycle.

Producing Glucose

(Video) Nature's Smallest Factory: The Calvin Cycle

by Cathy Symington on TED-Ed

Have you read about the Calvin cycle but are having trouble visualizing it? If so, check out this video by TED-Ed to see how chloroplasts work like a factory to produce glucose and recycle materials.