The second stage of photosynthesis is called the light-independent reaction. It happens after the light-dependent reaction and has two other names: the dark reaction and the Calvin cycle. Although it doesn't need light to occur, it still happens during the day. The light-independent reaction takes place in the stroma, a colourless fluid in the chloroplast. This is where carbon dioxide gets converted into glucose. The thylakoid discs' membrane surrounds the stroma, where the light-dependent reaction happens. Melvin Calvin discovered the light-independent reaction, and it's a self-sustaining cycle of reactions. The equation for the light-independent reaction is: [insert equation].

What are the reactants in the light-independent reaction?

The light-independent reaction uses three main reactants:

Carbon dioxide is used in the first stage, called carbon fixation. It's incorporated into an organic molecule and then converted into glucose.

NADPH acts as an electron donor in the second stage, called phosphorylation and reduction. It's produced during the light-dependent reaction and split into NADP+ and electrons during the light-independent reaction.

ATP is used twice during the light-independent reaction, in phosphorylation and reduction and regeneration. It's then split into ADP and inorganic phosphate (Pi).

The light-independent reaction has three stages:

1. Carbon fixation
2. Phosphorylation and reduction
3. Regeneration of the carbon acceptor

It takes six cycles of the light-independent reaction to create one glucose molecule.

Carbon fixation

Carbon fixation is the process by which living organisms incorporate carbon into organic compounds. In the light-independent reaction, carbon dioxide and ribulose-1,5-biphosphate (RuBP) are fixed into 3-phosphoglycerate (G3P) using an enzyme called ribulose-1,5-biphosphate carboxylase oxygenase (RUBISCO).

Phosphorylation

After carbon fixation, we have G3PiphBPG) during the phosphorylation stage. BPG has one more phosphate group than G3P, and we get this extra phosphate group from ATP that's produced in the light-dependent reaction.

Reduction

After phosphorylation, BPG is converted into glyceraldehyde-3-phosphate (GALP) in a reduction reaction, which requires a reducing agent. The reducing agent used in this reaction is NADPH, which is produced during the light-dependent reaction. NADPH donates its electron to BPG, allowing it to be reduced to GALP. An inorganic phosphate group also splits from BPG in this process.

Two of the twelve GALPs produced are then removed from the cycle to make glucose via gluconeogenesis. This is possible because each GALP molecule is three carbons long, and 12 GALP has a total of 36 carbons. When two GALP molecules leave the cycle, six carbon molecules leave in total, leaving 30 carbons remaining. This is equivalent to the number of carbons in six RuBP molecules, as each RuBP molecule contains five carbons.

Regeneration

The light-independent reaction, also known as the Calvin cycle, is a series of biochemical reactions in photosynthesis that do not require light to proceed and ultimately produce organic molecules from carbon dioxide. The energy released from ATP (produced during the light reactions) drives this metabolic pathway. In order to ensure that the cycle continues, RuBP has to be regenerated from GALP. This means we need to add another phosphate group, as GALP has only one phosphate attached to it whilst RuBP has two. Therefore, one phosphate group needs to be added for every RuBP generated, meaning six ATPs need to be used to create six RuBP from ten GALP. The equation for this is:

6 ATP + 10 GAL → 6 RuBP + 10 ADP

RuBP can now be used again to combine with another molecule, and the cycle continues! Overall, the entire light-independent reaction looks like this:

6 CO2 + 12 NADPH + 18 ATP → C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi

What are the products of the light-independent reaction?

The light-independent reaction, also known as the Calvin cycle, is a self-sustaining cycle that converts carbon dioxide into glucose. It does not depend on light to occur and takes place in the stroma of the plant cell, which is a colorless fluid that surrounds the thylakoid discs in the chloroplast. The reactants of the light-independent reaction are carbon dioxide, NADPH, and ATP, while its products are glucose, NADP+, ADP, and inorganic phosphate.

Glucose is formed from 2ALP, which leaves the cycle during the second stage of the light-independent reaction. Glucose is used to fuel multiple cellular processes within the plant. NADP+ is formed after the reduction of NADPH, and it is reformed into NADPH during the light-dependent reactions. ADP is produced in the light-independent reaction alongside inorganic phosphate, and it is converted back to ATP to be used again in the Calvin cycle during the light-dependent reactions.

The overall equation for the light-independent reaction is:

6 CO2 + 12 NADPH + 18 ATP → C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi

The light-independent reaction has three overall stages: carbon fixation, phosphorylation and reduction, and regeneration. Carbon fixation is the process of converting carbon dioxide into an organic molecule, while phosphorylation and reduction involve the conversion of ATP and NADPH into organic molecules. Regeneration is the process of converting organic molecules back into RuBP to continue the cycle.

Light-Independent Reaction

What is the light-independent reaction?

The light-independent reaction is the second stage of photosynthesis. The term refers to a series of reactions that result in the conversion of carbon dioxide to glucose. The light-independent reaction is also referred to as the Calvin cycle as it is a self-sustaining reaction.

Where does the light-independent reaction take place?

The light-independent reaction occurs in the stroma. The stroma is a colourless fluid found in the chloroplast, which surrounds the thylakoid discs.

What happens in the light-independent reactions of photosynthesis?

There are three stages to the light-independent reaction: carbon fixation, phosphorylation and reduction, and regeneration. Carbon fixation: Carbon fixation refers to the incorporation of carbon into organic compounds by living organisms. In this case, the carbon from carbon dioxide and ribulose-1,5-biphosphate (or RuBP) is going to be fixed into something called 3-phosphoglycerate, or G3P for short. This reaction is catalysed by an enzyme called ribulose-1,5-biphosphate carboxylase oxygenase, or RUBISCO for short.Phosphorylation and reduction: G3P is then converted into 1,3-biphosphoglycerate (BPG). This is done using ATP, which donates its phosphate group.BPG is then converted into glyceraldehyde-3-phosphate, or GALP for short. This is a reduction reaction, so NADPH acts as the reducing agent. Two of these twelve GALPs produced are then taken away from the cycle to make glucose via a process called gluconeogenesis.Regeneration: RuBP is then generated from the remaining GALP, using the phosphate groups from ATP. RuBP can now be used again to combine with another CO2 molecule, and the cycle continues!

What do the light-independent reactions of photosynthesis produce?

The light-independent reaction of photosynthesis produces four main molecules. These are carbon dioxide, NADP+, ADP and inorganic phosphate.

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